Atlas Image mosaic, covering
6.0´ × 6.0´ on the sky, of the Hickson Compact Group
40 (HCG 40, aka Arp 321 and VV 116; Hickson 1982, ApJ, 255, 382).
This isolated ensemble of seven galaxies (five of which are clearly seen in
the 2MASS Image), at a redshift z=0.022,
like the other HCGs, provides an interesting laboratory for
studying the effects of close proximity and possible interaction on the
evolution of galaxies. For instance, how these factors influence the
presence of active galactic nuclei (e.g., Coziol et al. 1998, ApJ, 493, 563).
HCG 40 is dominated by a giant elliptical galaxy (seen toward the center of
the image). The giant elliptical and the two smaller spiral galaxies all
show some levels of nuclear activity. The HCGs also offer the possibility
to measure the amount of dark matter in the groups through their relative
motion and the inferred mass-to-light ratios (M/L) for the groups. A
kinematical study of the group
suggest that it is a dynamical and compact septet of galaxies, covering
only 749 kpc (Ribiero et al. 1998, ApJ, 497, 72). HCG 40's self-gravitating
M/L~33, along with the ratios of other HCGs, imply
a cosmological density parameter
Atlas Image, covering
5.0´ × 5.0´ on the sky, of the planetary nebula
NGC 3242.
This nebula has an elliptical shape and several interesting structural
features,
including the inner bright elliptical ring, the two extensions to the ring,
or ansae, that are placed roughly along the major axis of
the elliptical emission, and the larger faint halo that envelops the inner
structure (Hora, Latter, & Deutsch 1999, ApJS, 124, 195). The bright ring
is pure continuum emission, while the faint halo is (dust) scattered light.
Planetary nebulae are formed as low-mass stars, like our Sun, reach the end of
their lives and lose their outer envelopes (for this nebula, what are now the
ring and halo structures) to the interstellar medium.
The bright source within the bright elliptical ring is the hot central star of
the planetary nebula, originally the core of the dying star, which will
eventually become a white dwarf and cool off over billions of years.
These data are included in the Second Incremental Release!
Atlas Image mosaic, covering
10.0´ × 10.0´ on the sky, of the infrared source
LDN 1641 S 4.
This object, also known as GGD 7 (Gyulbudaghian, Glushkov, & Denisyuk 1978,
ApJ, 224, L137), at a possible distance of about 500 pc, is part of a group of
nebulae in Lynds dark cloud (LDN) 1641,
in the Orion complex. The region was previously imaged in the near-infrared
by Carballo, Eiroa, & Mampaso (1988, MNRAS, 232, 497); they identified five
infrared sources (IRS 1-5) in this field. These sources are the five brightest
clustered toward the center of the 2MASS image. Four of the sources (IRS 1-4)
are clearly embedded in nebulosity. A dark dust lane runs between IRS 3 and 4.
Carballo et al. suspected that IRS 1-3 were variable; comparison of their
JHK photometry with the 2MASS values comfirms this, as well as possible
variability for IRS 5. Carballo et al. concluded that IRS 2 and 3 are likely
pre-main-sequence stars. Clearly, from the 2MASS image, there are many more
stellar sources embedded in this dark cloud of young pre-stellar objects, some
(the reddest ones) obscured by more than 20 visual magnitudes of extinction.
Image mosaic by S. Van Dyk (IPAC). These data are included in the Second
Incremental Release!
Atlas Image mosaic, covering
10.0´ × 10.0´ on the sky, of the star cluster Lyngå 7.
The true nature of this low-galactic-latitude cluster is unclear.
The cluster was listed as an open cluster by Lyngå (1987, Lund Catalog
of Open Cluster Data, 5th ed.), but Ortolani et al. (1993, A&A, 273, 415),
based on a BVI optical color-magnitude diagram, conclude that Lyngå 7
would have to be among the oldest known open clusters. They conclude that
it is likely a very young metal-rich globular cluster. Tavarez & Friel (1995,
AJ, 110, 223), based on an integrated optical spectrum of the cluster, find
that its location (galactocentric distance R0=4.4 kpc;
7.2 kpc from us),
kinematics (6±15 km/s), and metallicity ([Fe/H]=-0.62±0.15 dex)
make it consistent with the disk globular cluster population, with an
age
Atlas Image mosaic, covering
13.0´ × 13.0´ on the sky, of the spiral galaxy NGC 7331
and its apparent neighbors. The recessional velocity of NGC 7331 is only 816
km/sec, while the velocities for the other galaxies in the 2MASS mosaic range
from 6315 to 8800 km/sec for NGC 7335, 7336, 7337, and 7340 (in increasing
eastward distance from NGC 7331). The highly-inclined NGC 7331, at a
distance of 15.1 Mpc, determined using Cepheid variables by Hughes et al.
(1998, ApJ, 501, 32), is of Hubble
type Sb and has a weak LINER nucleus. "LINER" is low-ionization nuclear
emission region and is a characteristic of low-luminosity active galactic
nuclei, which are thought to harbor supermassive black holes. Evidence for a
nuclear black hole in NGC 7331 was recently reinforced by a ROSAT X-ray
detection of a bright source (Stockdale, Romanishin, & Cowan 1998, ApJ, 508,
L33). The spiral arms, bulge, and extended disk are evident in the 2MASS
image. The disk, in fact, contributes only in a minor way to the mass
content of NGC 7331 (Bottema 1999, A&A, 348, 77).
Molecular hydrogen is the dominant mass contributor to the
interstellar medium in the galaxy's bulge and in a ringlike zone at a distance
of ~3.5 kpc from the center; far-infrared emission from dust peaks inside the
ring at 100 µm (warm dust), and in the ring at 85 µm (colder dust;
Israel & Baas 1999, A&A, 351, 10; Bianchi et al. 1998, MNRAS, 298, L49).
The overall far-infrared luminosity of NGC 7331 is similar to that of our
Milky Way Galaxy. Image mosaic by E. Kopan (IPAC). These data are
included in the Second Incremental Release!
Atlas Image mosaic, covering
8.0´ × 8.0´ on the sky, of the ultracompact HII region
RAFGL 5173, also known as G192.16-03.82 and IRAS 05553+1631. This
region, at a distance of about 2 kpc (6520 light years) from us, has a
luminosity of about 3000 suns, which implies that the region is excited by a
very young mid- to early-type B star with a mass of 5 to 10 times the Sun
(Shepherd et al. 1998, ApJ, 507, 861).
The region is at the center of a very extended pair of Herbig-Haro emission
nebulae, HH 396/397. Herbig-Haro (HH) objects are shock-excited visible
nebulae powered by outflows from young stellar objects; the emission from
HH 396/397, one of the most spatially extended HH complexes, is not
nearly as luminous in the near-infrared. Devine et al. (1999, AJ, 117,
2919) conclude from a
recent combined optical/near-infrared study of this region that its dynamical
age is about 105 years and that it is in a relatively late state of
the evolution of a moderate-mass young stellar object, which will indeed become
an early B-type star. Image mosaic by S. Van Dyk (IPAC).
These data are included in the Second Incremental Release!
Atlas Image mosaic, covering
5.0´ × 5.0´ on the sky, of the planetary nebula
NGC 6369. The nebula,
noted for its peculiar fishtail-like outer loop morphology in
the light of optical emission lines (also seen faintly in this near-infrared
image, outside the bright ring of the nebula). This planetary nebula
is associated with fast low-ionization
emission regions, or FLIERs, which are thought to be discrete, collimated,
and highly supersonic ejection events from the very hot central star
(Hajian 1997, ApJ, 487, 304), which can be seen at the center of the ring.
Planetary nebulae are formed as low-mass stars, like our Sun, reach the end of
their lives and lose their outer envelopes to the interstellar medium.
The pinkish color in the bright ring of the nebula may arise from emission by
atomic and molecular hydrogen primarily in the 2.17 µm wavelength
(Ks) band, or dust scattering in a combination of all three bands.
These data are included in the Second Incremental Release!
Atlas Image mosaic, covering
6.7´ × 6.7´ on the sky, of comet 52P/Harrington-Abell,
which was serendipitously observed by 2MASS on 1998 Oct 8 UT
and is among twenty comets included in the
2MASS Second Incremental Data Release. Comets, of course, are within our
Solar System and orbit the Sun, and therefore do not move sidereally (that is,
as the stars do). Astronomers, however, have determined the
ephemerides, or orbital parameters, for large numbers of known comets, so
their positions in the sky along their orbits can be accurately predicted.
Occasionally, comets, asteroids, and even planets can "accidentally" wander
into the Survey. Comets reflect sunlight, yet near-infrared photometry can
provide useful compositional and structural information about these objects.
This comet was seen optically to experience a significant outburst in 1998
July (IAUC 6975) and became unusually bright by August (IAUC 6994), exceeding
visual magnitude 11 by 1999 February (IAUC 7113). Its coma diameter was
1-2´ and tail length of about 2´ in 1998 (IAUC 6975).
Atlas Image mosaic, covering
11.0´ × 16.8´ on the sky, of NGC 205 (Messier 110), a
dwarf elliptical companion of the giant spiral Andromeda Galaxy (Messier 31;
M31), in the
Local Group of galaxies. NGC 205 is peculiar in that evidence exists for
recent star formation in what nominally should be a galaxy composed of old
stars. The evidence is in the form of centrally-concentrated bright early
type stars, neutral and molecular gas, and dust, much of it in clouds and some
of it very cold (Haas 1998, A&A, 337, L1). From HST imaging,
Cappellari et al. (1999, ApJ, 515, L17) determined that
the quite modest star formation episode occurred over the last 100 million
years; Welch, Sage, & Mitchell (1998, ApJ, 499, 209) found that it ended
only a few million yr ago. Where the star-forming gas originally came from
is a mystery, possibly from interaction with M31 or from the environment of
the M31 system. Image mosaic by S. Van Dyk (IPAC). These data are included
in the Second Incremental Release!
Atlas Image mosaic, covering
15.3´ × 17´ on the sky, of IRAS 20306+4005, an object
in the Infrared Astronomical Satellite (IRAS) catalog of sources. Little is
known about this object, but it appears to be a region of current massive star
formation in the Galactic plane and near the Cygnus 0B2 stellar association.
This is the first known near-infrared image of this source. Infrared-bright
filaments of gas surround a young cluster of stars and young stellar objects
still embedded in their natal dusty molecular cloud.
A large patch of heavily-obscuring dust is seen to the
north of the nebula. 2MASS is ideal for investigating the nature of many IRAS
objects and other very young star-forming regions throughout the Galaxy.
Image mosaic by S. Van Dyk (IPAC). These data are included in the Second
Incremental Release!
Atlas Image mosaic, covering
7´ × 7´ on the sky, of a small, stellar cluster in the
constellation Cygnus, consisting of the optically-bright Herbig Be star
BD +40° 4124 (V1685 Cygni=MWC 340; center of image), V1686 Cyg
(LkH
Atlas Image mosaic, covering
14.3´ × 14.3´ on the sky, of the Tarantula, or 30 Doradus,
Nebula (also known as NGC 2070). This nebula, in the Large Magellanic Cloud,
is the closest example to us of a giant ionized hydrogen (H II) region,
covering several hundred parsecs in diameter. The closest analog in our
Milky Way Galaxy is the H II region NGC 3603.
Clusters of hundreds of young, massive O and B stars, particularly the
dense central "super star" cluster, R136, provide the
ultraviolet photons which ionize and photoevaporate the large filamentary
cloud. A number of
other stellar populations, including red supergiants and Wolf-Rayet stars,
coexist in 30 Doradus. Detailed studies in the optical of the nebula and its
stellar contents shows a complex history of recent star formation. In the
near-infrared, Rubio et al. (1998, AJ, 116, 1708) point out that
pre-main-sequence objects are also found, particularly along the
Ks-bright molecular hydrogen (H2) line-emitting
filaments in the nebula's periphery, which can be seen in the 2MASS image.
What emerges is a scenario
of new generations of stars triggered by the energy input from the massive
stellar clusters, which is likely a characteristic picture for star-forming
regions of this scale in galaxies.
Image mosaic by E. Kopan (IPAC).
These data are included in the Second Incremental Release!
Atlas Image mosaic, covering
9.2´ × 9.2´ on the sky, of the Sombrero, Messier 104
(M104).
The galaxy derives its nickname from its appearance, as a nearly edge-on
disk system with a "rim" of dark, obscuring dust. The "rim" are the dust
lanes within the disk of this early-type spiral galaxy, which are still
particularly visible, even in this 2MASS near-infrared image. As is true
for other spiral galaxies as seen by 2MASS, the overall apparent structure
of the galaxy is much smoother in the near-infrared than in the optical.
M104 shows an extended halo of emission above and below the plane of the
galaxy's disk. The faint, purplish "stars" emanating both due north and
due south of the galaxy's nucleus are known persistence artifacts.
Image mosaic by S. Van Dyk (IPAC).
These data are included in the Second Incremental Release!
Atlas Image mosaic, covering
7.5´ × 7.5´ on the sky, of the globular cluster Palomar
6, which is in the direction of the Galactic bulge. The loose cluster is
optically obscured, but more prominent among the surrounding bulge stars in the
near-infrared. Ortolani et al. (1995, A&A, 296, 680) determined that the
cluster is seen through about four visual magnitudes of extinction, at a
distance from us of about 8.9 kiloparsecs (29000 light years), placing it
nearby the Galactic center. They determined that
the cluster is relatively metal-rich, higher in metals than
47 Tucanae.
Although the spatial resolution of 2MASS is insufficient to overcome the
extreme stellar crowding within the cluster, 2MASS is well suited for revealing
obscured clusters in the Galaxy and for
studying the cluster environment. Image mosaic by S. Van Dyk (IPAC).
These data are included in the Second Incremental Release!
Atlas Image mosaic, covering
0.5° × 0.5° on the sky, of the galaxy cluster Abell 3558,
also known as Shapley 8. This is a rich cluster, which has been the subject
of study at a number of wavelengths. The cluster is dominated by a central
giant cD galaxy (the brightest galaxy near the center of the 2MASS image),
which, interestingly enough, also lies near the center of the larger
Shapley concentration, or supercluster, of 25 clusters of galaxies, including
Abell 3558. Dantas et al. (1997, ApJ, 485, 447) recently found that the
cluster is actually composed of a collection of several groups
of galaxies, suggesting that it is a dynamically complex, young cluster of
galaxies. One of the data products released by 2MASS is an Extended Source
Catalog, much of which consists of galaxies. The 2MASS Processing Pipeline
measures magnitudes, sizes, and positions for extended sources through a large
number of possible apertures. To see a representative color-color diagram for
a 1°-radius area
centered on Abell 3558, click here;
shown on the diagram are the Bessell & Brett (1988, PASP, 100, 1134) and
Koorneef (1983, A&A, 128, 84) tracks for dwarf and giant stars, as well as
K-correction tracks for elliptical and spiral galaxies (in steps of
redshift z=0.1) and the reddening vector from Rieke & Lebofsky (1985,
ApJ, 288, 618). Abell 3558 is at z=0.048.
A radius vs. magnitude diagram for the cluster can also be
found here. Finally, a source count diagram
can be found here (the magenta crosses,
shown for reference, are deep, narrow-field generalized galaxy counts from
Gardner et al. (1997, ApJ, 480, L99).
For a larger 0.58° × 1.33° view of Abell 3558, click
here; this image is 9.7 Mb in size!
(A meteor streak can be seen through both the large and smaller view.)
Image mosaic by E. Kopan (IPAC). These data are included in the
Second Incremental Release!
Atlas Image mosaic, covering
0.64° × 1.15° on the sky, of a portion of Baade's Window
centered at about l=1.0°, b=-3.9°. (The downloadable
image mosaic has been shrunk by 50%, but is still 2.2 Mb in size.)
This "window" through our Milky Way
Galaxy, discovered by the German-American astronomer Walter Baade early in
the last century, is an important region of the sky, because the interstellar
extinction, due to intervening dust in the Galactic plane, is substantially
lower than other regions of the Galaxy nearby to it. Stanek (1996, ApJ, 460,
L37) estimates from optical data that the extinction ranges from 1.26 to 2.79
visual magnitudes. As a result, this window allows astronomers to more easily
view and study stars in the Galactic bulge. The chemical composition, ages,
masses, and kinematics of these stars can be determined, the distance to the
Galactic center can be measured, microlensing events can be discovered and
analyzed, and the presence of a stellar bar in the bulge can be inferred, all
important aspects to understanding the nature of the Milky Way. The 2MASS
near-infrared color-color diagram and
color-magnitude diagram for the stars within
1° of (l,b)=(1.0,-3.9) also demonstrate the relatively low
extinction for the stellar populations in a region at this general position in
the Galaxy. (Shown on the color-color diagram are the Bessell & Brett 1988,
PASP, 100, 1134, and Koorneef 1983, A&A, 128, 84, tracks for dwarf and giant
stars, as well as the reddening vector from Rieke & Lebofsky 1985, ApJ, 288,
618). Image mosaic by S. Van Dyk (IPAC).
These data are included in the Second Incremental Release!
Atlas Image mosaic, covering 33.3´ × 46.1´ on the sky, of the
reflection nebulae NGC 2068 and 2071, comprising Messier 78 (M78).
This complex is located in the LDN 1630 (Orion B)
cloud, at a distance of about 400 pc (1300 light years).
Reflection nebulae, particularly in the optical, are dusty regions
that scatter and reflect the light from typically bright, hot, blue stars
within the regions. They are commonly associated with molecular clouds and
the dusty sites of active star formation.
The illuminating stars for NGC 2068 (to the south) are
HDE 38563A and 38563B, while the illuminating star for NGC 2071 (to the north)
is HDE 290861. The two nebulae appear to contain small clusters of stars.
A number of embedded, young stellar objects are located
throughout this region, highlighted by the large number of dramatic Herbig-Haro
outflow sources; Zhao et al. (1999, AJ, 118, 1347) brought the complement of
HH objects in M78 to 17. Lada et al. (1991, ApJ, 371, 171) conducted a 2.2
µm survey of L1630, complete to K<13 (the 2MASS data go deeper than
this), and concluded that much of the young, embedded star formation is
occurring in clusters. Image mosaic by S. Van Dyk (IPAC).
These data are included in the Second Incremental Release!
Atlas Image, covering 5.0´ × 5.0´ on the sky, of the supernova
remnant (SNR) N49 in the Large Magellanic
Cloud (LMC). This appears to be the first near-infrared image of this SNR.
The optical, X-ray, near-infrared and radio
morphology for N49 all are in general agreement, with an incomplete shell
covering nearly 1´ in diameter.
Emission due to the hydrogen Paschen beta line and lines of forbidden
singly-ionized iron ([Fe II]) in the J and H bands likely dominate what we
see. Strong Ks-band emission, as in the
case of the SNR IC 443 in our Galaxy, due
to molecular hydrogen (H2) line emission excited by
shock-molecular cloud interaction, is surprising lacking, given the apparent
interaction with an extended dense cloud to the southeast, as deduced by the
optical (Vancura et al. 1992, ApJ, 394, 158) and X-ray (Hughes et al. 1998,
ApJ, 505, 732) observations. N49 is also intriguing, with its possible
association with the soft gamma-ray repeater SGR 0525-66 near the SNR's
northern edge, however, this association has not yet been firmly established.
SGRs are linked to isolated neutron stars, so if the association holds, this
is further possible evidence of a massive stellar progenitor for this SNR.
These data are included in the Second Incremental Release!
Atlas Image, covering 8.4´ × 17.1´ on the sky, of the region
of the sky containing the red nebulous objects GGD 12-15
(Gyulbudaghian, Glushkov, & Denisyuk 1978, ApJ, 224, L137).
These objects are part of an active star-forming region located in the
Monoceros molecular cloud, about 1 kpc (3260 light years) away. The many
Ks-bright objects in the region appear to be members of a still-forming
star cluster. Associated with this cluster are a strong water maser, a
compact H II region, and a bipolar molecular outflow, all signatures of active
ongoing star formation. Many stellar objects are detected in the 2MASS Atlas
Image. To see a JHKs color-color diagram for the detected point
sources, click here. (The green stellar track is
for dwarfs, the blue track is for giants; Bessell & Brett 1988, PASP, 100,
1134. The reddening vector is from Rieke & Lebofsky 1985, ApJ, 288, 618.) The
embedded objects are obscured by up to at least 10 visual magnitudes of
extinction. These data are included in the Second Incremental Release!
The planetary nebula NGC 6818. (Caption lost in Y2K frenzy....)
These data are included in the Second Incremental Release!
Atlas Image mosaic, covering
23.3´ × 20.0´ on the sky, of the young open cluster IC 348.
This cluster, at a distance of ~320 pc, is still embedded in its parental
molecular gas cloud, which is part of the larger Perseus Molecular Cloud.
Thus, many of the cluster members are obscured by dust and would go
relatively undetected in optical imaging of the cluster. For such very young
embedded clusters as IC 348, near-infrared imaging, such as by 2MASS, provides
a more complete stellar census and determination of the stellar and cluster
properties and histories. The bright star near the top center of the 2MASS image mosaic is
omicron Persei. The main portion of IC 348 is toward the image mosaic center,
just south of the star. From their JHK imaging, Lada & Lada (1995, AJ, 109, 1682) found
~380 members of the cluster, that the stellar density is consistent with
other rich embedded clusters, and that ~20% of the members are infrared-excess
objects, suggesting the presence of circumstellar disks.
For a near-infrared color-color diagram for sources in the IC 348 field drawn
from the release Point Source Catalog, click
here. Image mosaic by S. Van Dyk (IPAC).
These data are included in the Second Incremental Release!
Atlas Image mosaic, covering
5.0´ × 5.0´ on the sky, of the famous Seyfert galaxy Messier
77 (NGC 1068). The nucleus of this galaxy is so bright in the infrared,
particularly at longer wavelengths, that in the 2MASS mosaic, it has created
persistence artifacts, seen as the two red "spots" due north and south of
the galaxy center. (A pair of artifacts are seen, due to the mosaicing of
images made by the two different scan directions.) The bright galactic bar is
apparent in the image, with two or more dusty arms extending outward along the
galaxy disk. Although classified as Seyfert 2, it has been recently speculated
that the nucleus of this well-studied galaxy is actually the prototypical
dust torus-obscured Seyfert 1 nucleus, where a powerful supermassive black
hole is thought to reside. The extinction toward the nucleus may be as high
as 40 visual magnitudes (Lumsden et al. 1999, MNRAS, 303, 209).
The bar is the site of very active star formation, out to
10 kpc radius, and a relationship may exist between the spectacular activity of
the nucleus and the copious gas and new stars around it. Image mosaic by S.
Van Dyk (IPAC). These data are included in the Second Incremental Release!
Atlas Image, covering
6.0´ × 6.0´ on the sky, of the Quintuplet star cluster
near the Galactic Center. The Quintuplet is a young (~4 Myr old), very massive
cluster of stars, formed during one of the recent bursts of star formation
around the Milky Way's center. The cluster, seen behind about 29 magnitudes of
visual extinction, actually consists of more than just five stars.
The five bright stars that earned the cluster its name, however, have high
luminosities in the near-infrared and are likely young, dust-enshrouded stars.
Figer, McLean, & Morris (1999, ApJ, 514, 202) recently produced a census of
the massive stars in the cluster, based on near-infrared photometry and
spectroscopy. In addition to O- and B-type main-sequence and supergiant
stars, a number of post-main-sequence decendants of massive stars, i.e.,
several Wolf-Rayet stars and Luminous Blue Variables (LBVs), have also been
identified. The most famous of these LBVs is the
Pistol Star (seen ~0.5´ due south of the main cluster in the center of
the 2MASS image), imaged recently with HST/NICMOS and studied by Figer
et al. (1998, ApJ, 506, 384).
The Pistol Star appears to have 100-200 times the mass of the Sun, making it
one of the most massive and luminous stars in the Milky Way!
Considering the mass and number densities of stars in the cluster, the
Quintuplet, also recently imaged with
NICMOS, can be considered a small "super star cluster," examples of
which are found in starbursts occurring in many other galaxies. Super star
clusters are thought to be young globular clusters in the making.
These data are included in the Second Incremental Release!
Atlas Image mosaic, covering
11.7´ × 25.0´ on the sky, of the Herbig Be star MWC 297
(IRAS 18250-0351), also known as the variable star NZ Serpentis. In the
near-infrared 2MASS image one can see an extended reflection nebula and dust
clouds around the star. Based on optical-to-near-infrared photometry, the
star is behind ~8 magnitudes of visual extinction, at a distance of ~450 pc
(1470 ly). Bergner et al. (1988, Astrofizica, 28, 529) found from UBVRIJHK
photometry that the star is variable at all wavelengths, likely due to changes
in dust obscuration in the nebula in which the star is still embedded.
Optical spectra by Andrillat & Jaschek (1998, A&AS, 131, 479) show
no absorption lines, only emission lines from the young star's extended
atmosphere, which indicate an underlying spectrum of a massive late O- or
early B-type star. Image mosaic by S. Van Dyk (IPAC).
Atlas Image mosaic, covering
15.0´ × 29.9´ on the sky, of Abell 426, better known as the
Perseus Cluster, a nearby rich galaxy cluster. The cluster is notable with
its prominent grouping of bright galaxies near the core and an exceptionally
strong deficiency of spiral galaxies. The cluster is dominated by NGC 1275,
the strong radio source Perseus A, ther bright galaxy seen near the center
of the 2MASS image mosaic; the bizarre galaxy has an active nucleus, with
bright, extended radio lobes, and evidence that the galaxy is the remnant of
a recent merger. The x-ray emission from the cluster,
and the fact that the cluster hosts several galaxies with active nuclei,
pronounced radio structures, enhanced far-infrared emission, and signs of
strong gravitational interactions, have been interpreted as signs of a
merger of galaxy clusters. Studies of the Perseus cluster are hampered by its
low galactic latitude (l=150.6°, b=-13.3°), resulting
in a high density of Galactic foreground stars and appreciable extinction in
the optical. In the 2MASS near-infrared image the extinction has been
substantially reduced, and the galaxy light appears much smoother than it
does in the optical.
Atlas Image mosaic, covering
34.6´ × 33.1´ on the sky, of the southern star-forming
region RCW 38. (N.B.: The mosaic is 3.7 Mb!) Zoom in on
the central cluster (IRS2; Frogel & Persson
1974, ApJ, 192, 351) in the inner 6´ × 6´ of the nebula;
a number of massive O stars are embedded within a dense, heavily obscuring
cloud. Ligori et al. (1994, MemSAI, 65, 815) find an age of ~2 Myr for the
stars in IRS2, showing that the star formation is very recent and still
ongoing. RCW 38 is at a distance of ~1.7 kpc (~5500 light years) from us,
near the famous Vela supernova remnant and the Gum nebula. As seen in the
2MASS near-infrared Image mosaic, the nebulosity associated with RCW 38 is
extensive across a large area, with dust lanes and patches running throughout.
Other, smaller obscured star-forming regions are nearby, with less-obscured
young, massive stars and associated reflection nebulae also in the field. A
high-resolution view in the near-infrared of IRS2 was recently
obtained by ESO's Very Large Telescope. Image mosaic by S. Van Dyk (IPAC).
Atlas Image mosaic, covering
21.0´ × 21.0´ (in full mosaic) on the sky, of the nearby
spiral NGC 253, which is a prototypical starburst galaxy. Starburst
galaxies are those that are experiencing intense recent star formation, often
concentrated at a galaxy's nucleus or along the galaxy's spiral arms. The
nucleus of NGC 253 as seen in the 2MASS Image mosaic is a very bright source
of near-infrared emission. Compare the 2MASS image mosaic to an equivalent
large-scale
optical image
of the galaxy. In the near-IR the very prominent and patchy dust lanes seen
in the optical image are far less obvious; the overall light distribution in
the near-IR is smoother, even though the galaxy disk is significantly
inclined. What is obvious in the near-IR is the barred nature of this galaxy
(Forbes & DePoy 1992, A&A, 259, 97; Scoville et al. 1985, ApJ, 289, 129), which
is not as evident in the optical. A number of bright knots of
red supergiant stars and some dusty patchiness are also seen along the arms
and bar. Image mosaic by S. Van Dyk (IPAC).
Atlas Image mosaic, covering
12.0´ × 12.0´ (in full mosaic) on the sky, of GGD 27.
This object is the
center of an active young star formation region obscured optically by dense
molecular gas and dust along our line of sight. In the 2MASS near-infrared
image a number of point sources in this region can be seen, along with the
extended and wispy bipolar reflection nebulosity, which corresponds to a
bipolar molecular outflow, with a dynamical age of about 105 years
(Yamashita et al. 1989, ApJ, 347, 894). Particularly noticeable is the
reddish core region
of the source, where the most embedded star formation is taking place.
Imaging of this core region at slightly longer wavelengths than the 2MASS
bandpasses (3.8 and 4.7 µm) by Aspin et al.
(1994, A&A, 292, L9) reveal several sources so obscured they are not seen
at 2 µm. Image mosaic by S. Van Dyk (IPAC).
Atlas Image mosaic, covering
11.0´ × 11.0´ on the sky, of RCW 103, a young supernova
remnant (SNR) in the Milky Way, less than 1° from the Plane of the Galaxy.
As a result, the interstellar extinction along the line of sight to the SNR is
about 4.5 visual magnitudes (Oliva, Moorwood, & Danziger 1990, A&A, 240, 453).
In the near-infrared 2MASS image, one sees filamentary emission of a
bluish-green color, forming what appears to be an incomplete shell, with some
fainter emission interior to this partial shell, surrounded by reddish
emission, particularly in the southeast. The red color is from molecular
hydrogen (H2) line emission at 2.12 µm. The blue-green color
is [FeII] (forbidden singly-ionized iron) line emission at 1.64 µm.
Oliva et al. (1999, A&A, 341, L75) find that the morphology of the [Fe II]
emission identically traces out the optical line emission, and that the
H2 emission arises from a region outside the SNR, as seen at
optical, radio, and X-ray wavelengths. The H2 emission is from
molecular gas around the SNR which has not yet been reached by the fast-moving
(~1200 km/s) shock wave, but is most likely a dense molecular cloud heated by
the X-rays being emitted by the shock.
The remnant likely has an age of only about 1000 years, arising from the
explosion of a massive star. The nature of the progenitor star can be inferred
from the presence of a 69-millisecond-period radio and X-ray pulsar near
RCW 103 (Kaspi et al. 1998, ApJ, 503, L161; Torii et al. 1998, ApJ, 496, L207);
pulsars are rapidly rotating neutron stars, and neutron stars are the remnant
collapsed cores of stars more massive than about 8 times the mass of our Sun.
The picture that emerges is of a massive star not unexpectedly ending its life
explosively within its natal gas cloud. Image mosaic by S. Van Dyk (IPAC).
Atlas Image mosaic, covering
7.0´ × 7.0´ on the sky, of Tonantzintla 2 (Pismis 26),
which is one of the least-known Galactic globular clusters. At
l=350.8° and b=-3.4°, it is quite close to the
Galactic Center. From V and I photometry using the ESO 3.6-m NTT, Bica,
Ortolani, & Barbuy (1996, A&AS, 120, 153) found that the positions of
red giant branch and horizontal branch stars on the optical color-magnitude
diagram indicate that the chemical composition of the cluster stars are
similar to those in
47 Tucanae. They also found a reddening to the cluster which implies an
extinction of AV~4 magnitudes, and a distance
from us of 6.4 kpc (20,900 light years). Bica et al. conclude that Ton 2
belongs to the bulge population of the Milky Way, but is only moderately
"metal-rich". Image mosaic by S. Van Dyk.
Look also at preliminary 2MASS near-infrared
color-color
and color-magnitude diagrams. (These include
all stars in the Atlas Image Mosaic detected by the 2MASS pipeline processing;
crowding affects and limits the detections and photometry of stars in the
densest regions of the cluster.)
Atlas Image mosaic, covering
1° × 1° on the sky, of Messier 17 (M17), the Omega Nebula.
Image mosaic by E. Kopan (IPAC). (N.B.: The full JPG image is 12.7 Mb in size! A smaller version
[1.4 Mb] can be obtained here.)
Atlas Image mosaic, covering
6.0´ × 6.0´ on the sky, of the planetary nebula NGC 6781.
Planetary nebulae (PNe) are formed as low-mass stars, like the Sun, reach the
end of their lives and lose their outer envelopes to the interstellar medium.
NGC 6781 has a very similar "ring-like" morphology to the famous PN, the
Ring Nebula (M57). The red glow of NGC 6781's
ring in the near-infrared is due to 2.12 µm emission from molecular
hydrogen (H2), which is strong in the 2MASS Ks band.
Although the main ring is quite bright, the fainter H2 filaments
within and halo emission outside of the bright ring, as studied by Kastner et
al. (1994, ApJ, 421, 600), can just barely be seen in the 2MASS image. Kastner
et al. postulate that if all PNe showing axial symmetry, and therefore,
possessing bipolar structure, are H2-bright, as first found by
Zuckerman & Gatley (1988, ApJ, 324, 501), then ring-like PNe, like
NGC 6781, with strong shocked H2 emission may be bipolar nebulae
viewed with an inclined polar axis with respect to the plane of the sky. The
bright ring is the equatorial torus, from a high-density slow wind from the
evolving star, whereas the fainter halo structures are the polar lobes and the
remnants of a faster, low-density wind. For NGC 6781 the lobe geometry may be
a bipolar cylinder with half the radius of and within the bright torus.
Near-infrared observations, such as those by 2MASS, provide very important
information about PNe and the evolution of stars like our Sun.
Atlas Image mosaic, covering
10.0´ × 10.0´ on the sky, of the barred spiral galaxy
Messier 83 (NGC 5236). This bright southern galaxy is seen nearly
face-on
and is one of the nearest large barred galaxies to the Milky Way. The bulge
region of this galaxy is experiencing an intense starburst; the structure of
this burst of circumnuclear star formation at high spatial resolution in the
near-infrared, and other wavelengths, is complex (Gallais et al. 1992, A&A,
243, 309). A prominent dust lane emanates in both directions along the bar
and out along the spiral arms. Although the overall appearance of the galaxy
is quite smooth in the near-infrared (e.g, Adamson, Adams, & Warwick 1987,
MNRAS, 224, 367), compared to its appearance in the
optical, a large number of bright knots are seen along the arms, corresponding
to clusters of red (supergiant) stars in and near the many OB stellar
associations and H II regions. Clearly, star formation is happening today not
only in the nucleus, but all over M83. This galaxy has been host to six
historical supernovae, four of which are thought to arise from young, massive
stellar progenitors, both near the nucleus and in the spiral arms, further
evidence for the vigorous galactic star formation. Image mosaic by S. Van Dyk
(IPAC).
Atlas Image mosaic, covering
12.3´ × 24.2´ on the sky, of the star-forming region
RCW 108. The infrared-bright nebula at the center of the region is a
cluster of
recently-formed stars still deeply embedded within their natal molecular cloud
(Straw et al. 1987, ApJ, 314, 283), only 1.5° below the Galactic Plane.
The cluster may have an age of only ~5×105 years.
Large amounts of dust obscuration are evident throughout the image around the
cluster, with at least 20 visual magnitudes of extinction toward the cluster
itself. A number of other young embedded sources are seen within the cloud
around the cluster. The molecular and dust cloud, at a distance of ~1.3 kpc
(4238 light years), is likely part of the much larger Ara OB stellar
association complex. It is possible that this young region was triggered into
formation by the energy input into the interstellar medium by the hot winds
from the young stars in the older nearby Ara OB1 association.
Atlas Image mosaic, covering
10.0´ × 10.0´ on the sky, of the Herbig Ae/Be star
R Coronae Australis. Herbig Ae/Be stars are the intermediate-mass
(2 to 8 solar masses)
counterparts of T Tauri stars. The emission-line star highlights the Corona Australis
molecular cloud complex and is embedded within it.
The complex, at a distance of only ~130 pc
(424 light years) is one of the nearest star-forming regions. The densest
part of the molecular cloud core, containing about 50 solar masses of gas,
has high visual extinction, about 35 magnitudes. The bright star just to
the southeast of R CrA (at image center) is T CrA, and the two stars to the
northeast, enshrouded in bluish reflection nebulosity near the image edge, are
TY CrA (northern-most) and HD 176386 (southern-most). Stars are forming
throughout the cloud, highlighted by
several young stellar objects (YSOs), the most prominent ten or so of which
surrounding R CrA have been dubbed the Coronet; starting clockwise from north
the brighter reddish YSOs comprising the Coronet are IRS 6,
IRS 5, IRS 2, Herbig-Haro (HH) 100 IR, and T Cr A itself.
(The "sources" emanating due north and south of R CrA, with decreasing
brightness are latent image artifacts, produced by the mode of the survey
scanning; diffraction spike artifacts from R CrA are also seen in the image.)
A number
of other fainter YSOs and HH objects (IRS 10 through 15, HH 99, and HH 104)
are also seen; Wilking et al. (1997, AJ, 114, 2029) recently produced a
similar, yet deeper near-infrared map of this region and identified the various
sources in the cloud. They find reflection or HH nebulae associated with most
of the YSOs in the cloud, suggesting nearly coeval star formation in the
cloud some 3 million years ago. They also find a shallower reddening vector
slope than found for normal interstellar dust, implying the presence of larger
than average dust grains throughout the cloud. Image mosaic by S. Van Dyk
(IPAC).
Atlas Image, covering 5.0´ ×
5.0´ on the sky, of the carbon star V713 Monocerotis.
This star, also known as
AFGL 935 and IRAS 06230-0930, is an example of asymptotic giant branch (AGB)
stars which are surrounded by an expanding shell. The AGB stars have evolved
from main sequence stars in the mass range 1.2-1.6 solar masses (Claussen et
al. 1987), but a significant population come from 2.5-4 solar mass main
sequence stars (Barnbaum, Kastner, & Zuckerman 1991, AJ, 102, 289).
The bright red color of the star in the 2MASS image indicates a general
infrared excess due to carbon-rich dust formation in the shell. (The fainter
red "star" to the south is a latent image artifact, produced by the mode of the
survey scanning; diffraction spike artifacts are also seen in the image.)
The dust is subject to pressure from the star's radiation and is dragged
outward with the atmospheric gas, and is the cause for the mass loss from the
star. The evolved star experiences pulsations and is variable with a long
period; the variability and the amount of mass loss appears correlated for the
carbon stars. Although the dust formation mechanism is not known, it appears
that the dust is induced by the stellar pulsations. LeBertre (1997, A&A, 324,
1059) finds that the infrared colors of carbon stars shed light on the
processes of dust formation and mass loss. V713 Mon, at a distance of 2240
pc (from its 494-day pulsation period), has a shell expansion velocity of 13.7
km s-1 and a mass-loss rate of 2.8×10-6 solar
masses per year. LeBertre finds a correlation between the mass-loss rate for
this and other carbon stars and the stars' near-infrared colors. 2MASS is
particularly well-suited for not only finding many carbon stars throughout the
Galaxy, but can also assist in characterizing the nature of their evolution.
Atlas Image, covering 4.0´ × 4.0´ on the sky,
of the Butterfly Nebula. The Butterfly, also known as M 2-9 and
IRAS 17028-1004, is thought to be a young planetary nebula, which
is the final stage of evolution for stars similar to our Sun. It has
a distinctly bipolar structure, evident in the 2MASS image, with a
bright central star. (The red "star" directly to the south of the
nebula's central star is a known persistence artifact of the infrared-bright
star; diffraction spike artifacts are also seen emanating from the bright
star.) The star has an optical spectrum of late O- or early B-type, hot
enough to ionize the gas in the two bipolar lobes, but it is clear from the
star's high brightness in the Ks band that we are more likely seeing
emission from a region of concentrated light scattering to the infrared,
directly around the star. Within the lobes are a number of knots. The knots
emit strongly in the [Fe II] lines, indicating high temperature shocks; in
the lobes light is also primarily emitted by hydrogen recombination and
continuum light scattering from the central star (Hora & Latter 1994, ApJ, 437,
281). The outer shell structure of the lobes is a well-defined
photodissociation region, as H2 is radiatively excited and emits
light at 2-2.5 µm. Studying fainter more extended lobes in the optical,
Schwarz et al. (1997, A&A, 319, 267) find a distance of ~650 pc and a
dynamical age for the nebula of ~1200 yr; they also argue that the central
source is a hot, compact possible binary star system. The Hubble Space
Telescope has also obtained an interesting
view
of this nebula.
Atlas Image Mosaic, covering 12.0´ × 12.0´ on the sky,
of the infrared-luminous galaxy NGC 4945. Throughout the sky are a
number of galaxies that are very luminous in the infrared
(the luminosity LFar-IR>1010
L
Atlas Image, covering 8.3´ × 17.1´ on the sky,
of the nebula RCW 87. Also known as IRAS 15015-5720 and
misclassified originally as a planetary nebula (Acker et al. 1987, A&AS, 71,
163), the nebula is more clearly an HII region (ionized nebula) and
young star cluster embedded in a dark dust cloud, situated less than 1°
from the Galactic Plane. The paucity of stars around the nebula, even in the
near-infrared, imply a high extinction toward this region. The nebula appears
to have a distinct boundary. The near-infrared emission from the nebula is
likely a combination of light from the bright young stars in the cluster
reflected by dust and possibly 2.12µ emission from the molecular hydrogen
gas from which the stars recently formed and still appear to be forming; along
the northwest edge of the nebula appears to be what could be an embedded young
stellar object. About 4.5´ to the northeast of the nebula's center
is another smaller embedded nebula, which could be the site of ongoing star
formation as well. Little is known about this probable recent star formation
site, which deserves further study. 2MASS is revealing many poorly-studied
or previously-unknown star formation regions, such as this one.
Atlas Image mosaic,
covering 6.0´ × 6.0´ on the sky,
of the source Cassiopeia A. Cas A, for short, has a faint, ghostly
shell-like
appearance in the 2MASS image, with several brighter filaments.
Cas A was first discovered in the late '40s by radio astronomers as the
brightest radio object in the sky at 1 GHz. It is now understood to be
possibly the youngest supernova remnant in the Milky Way Galaxy! No
clear records of the supernova's appearance in the optical sky exist
historically, as is the case for the Crab Nebula (the Chinese 'guest star' of
1054 AD), but it has been linked to a 6th magnitude star, no longer visible
near the constellation Cassiopeia, in a catalogue by Flamsteed from 1680 AD.
The inference is that Cas A
may have been a subluminous supernova, resulting from the explosion of a
Wolf-Rayet
star; this picture is motivated by the chemical composition of the
various fast and slow moving knots in the remnant, which, along with analyses
of the radio and X-ray emission, are consistent with
the explosion of a very massive star with very little hydrogen in its
envelope. Cas A is at a distance of ~3.4 kpc (11,000 ly); at only ~2°
from the Galactic Plane, it is seen behind ~5-6 mag of visual
extinction (Hurford & Fesen 1996, ApJ, 469, 246).
Most of the energy from Cas A is, in fact, radiated in the infrared.
The mid-infrared emission, as seen by the recent ISO satellite, is mostly
thermal dust emission, with some line emission, and is associated primarily
with the fast-moving knots, suggesting that dust is condensing out in the
expanding supernova ejecta (Lagage et al. 1996, A&A, 315, L273).
Image mosaic by S. Van Dyk (IPAC).
Atlas Images of Uranus, Neptune, and their moons. Uranus and Neptune
were observed serendipitously by the 2MASS Southern Facility, during routine
operations on 1998 June 7 and June 11 UT, respectively. Both planets appear
very blue, i.e., they are not nearly as bright in the Ks band as in
the shorter wavelength bands, due to more reflection of sunlight at short
wavelengths and to absorption of light by methane gas in their atmospheres.
The moons, or satellites, of both planets have icy surfaces with no (or very
little) atmospheres. They
simply reflect sunlight by various amounts, depending on the albedo, or
reflectivity, of each moon's surface. Umbriel has a surprisingly dark, icy
surface. Miranda is the innermost and smallest of the five large Uranian moons.
Triton is odd, in that it is in a highly-inclined retrograde orbit around
Neptune, leading planetary scientists to infer that Triton was captured by
Neptune's gravity. For more information about these and other planets, see
NASA/JPL's Welcome to the Planets
and NASA/GSFC's
Planetary Fact Sheets.
Moon identification in and further analysis of these 2MASS images by
B. Nelson (IPAC).
Atlas Image mosaic,
covering 6.7´ × 6.7´ on the sky, of the Herbig Ae/Be star
LkH
Atlas Image mosaic,
covering 15.3´ × 13.6´ on the sky, of the heavily
reddened young open cluster Trumpler 27. This relatively unstudied cluster
lies about 5° from the Galactic Center at a distance of 1.65 kpc from us
(Bakker & The 1983, A&AS, 52, 27), behind a visual extinction of 4 mag or more.
The brightest star in the 2MASS image is a M0 supergiant; the next brightest
star, to the northwest, is thought to be a Cepheid variable, due to its
proximity to the instability strip on the H-R Diagram. The bright reddish star
to the northeast of the image center is a WC9 Wolf-Rayet star with a very large
infrared excess, due to thermal reemission of the star's ultraviolet photons
by dust grains local to the star. The cluster, however, is dominated by young,
massive O- and B-type main sequence stars. An optical photometric estimate
of the cluster's age is about 107 yr (Battinelli et al. 1994,
A&AS, 104, 379).
Atlas Image, covering 5.8´ × 5.8´ on the sky, of the spiral
galaxy NGC 3877, taken as part of routine operations on 1998 Dec 24 UT
at the 2MASS Northern Facility on Mt. Hopkins, AZ. What is most interesting
about this image is that the supernova SN 1998S was caught in the act
in the near-infrared, about
49´´ southwest of the galaxy's nucleus, along the spiral arm! SN
1998S (Li, Filippenko, & Moran 1998, IAUC 6829) is of type II-"narrow"
(Schlegel 1990, MNRAS, 244, 269). Supernovae of this subtype tend
to show broad emission lines of hydrogen (and other elements) in their optical
spectra (thus, the Type II classification), due to the rapid expansion of the
hydrogen-rich supernova ejecta. But, atop the broad lines are narrow
lines, likely due to emission from the interaction region of the SN shock with
slower-moving very dense circumstellar matter lost by the progenitor star in
the late stages of its evolution prior to explosion. SN 1998S emits strongly
in the near-infrared, primarily due to the broad hydrogen recombination lines,
but also due to the first and second overtone bands of CO (the carbon monoxide
molecule) in the H and Ks bands (Gerardy et al. 1998, BAAS, 30,
1324). Gerardy et al. find from their near-IR spectra a CO overabundance of
~2 and a temperature in the CO-forming region of 4000 to 4500 K. Their
spectra may indicate that molecule formation is common in Type II supernovae;
this is important, since molecular emission can be a strong coolant and a
first step toward dust formation.
The SN had Ks magnitude 13.16, indicating a possible brightening
in this and the other near-IR bands since observations by others on 1998 Nov
10 UT (Garnavich et al. 1998, IAUC 7058), which already indicated a large
infrared excess had developed then.
Atlas Image,
covering 6.8´ × 6.8´ on the sky, of a new globular cluster
candidate seen near the Galactic Plane, only 10° away from the Galactic
Center. This object was serendipitously discovered in the 2MASS data, and
is primarily seen only in the H and Ks bands; hence, the rather
reddish color for the cluster stars. The characteristics of the stars and
the total extinction to the cluster have not yet been determined, however,
further analysis, including follow-up non-2MASS observations, will be
undertaken. To date there are 147 known globular clusters associated with our
Milky Way Galaxy; if confirmed to be a globular, this new cluster would be
number 148. Because these clusters are know to exist in larger numbers towards
the center of our Galaxy it is likely that a number are hidden behind
dust clouds found in that direction, many of which may be discovered by
2MASS. For more about this discovery, see the abstract
for the poster presentation at the 194th Meeting of the American Astronomical
Society.
Atlas Image Mosaic,
covering 6.7´ × 6.7´ on the sky, of the barred spiral galaxy
NGC 2903. These data are part of the online
Spring
1999 Incremental data release.
Atlas Image Mosaic,
covering 9.8´ × 9.8´ on the sky, of the H II region complex
Sharpless 252. Optically, this complex is dominated by HD 42088,
the blue star east of center in the mosaic, at the heart of the nebula NGC
2175. H II regions are areas of ionized gas, where electrons have been
stripped away from once-neutral hydrogen (H) and other atoms, by the intense
ultraviolet radiation from hot, young stars. In the near-infrared we see the
young, massive embedded and infrared-bright stars, forming in the other parts
of the complex: the bright clusters Sh 252A in the west, Sh 252C north of
center in the mosaic, and Sh 252E to the east. The blue star HD 252325
excites the gas that is Sh 252B (between A and C). Faint, extended
Ks-bright emission is seen throughout the center of the mosaic.
These data are part of the online Spring 1999 Incremental data release.
Atlas Image, covering 6.3´ × 8.2´ on the sky, of the prototype
pre-main sequence star T Tauri, which appears in the
2MASS Image Gallery of young stellar objects. T Tau is actually a
0´´.6-separation binary (or possible multiple) star system. However,
neither of the fainter red "stars" immediately east of north and west of south
of T Tau is the binary companion, but both are actually known Ks
filter glint artifacts. (The reddish "stars" trailing to the south of T Tau
are also latent image artifacts, produced by the mode of the survey scanning.
The diffraction spikes around this
bright object are characteristic of the 2MASS optics.) Hind's reflection
nebula (NGC 1555) can also be clearly seen to the west, showing a complex of
structure. Any extended emission from the inner Burnham's nebula (HH 255) is
overwhelmed by the brightness of the central star in this rendition. T Tau,
and many other young stellar objects in Taurus and elsewhere, are part of the
Spring 1999 data release.
Atlas Image mosaic, covering 15.0´ × 15.0´ on the sky, showing
the infrared sources RAFGL 5180 and 5182 (IRAS 06058+2138 and 06061+2151,
respectively). These are both embedded dense clusters of massive young stars
in the Gemini OB1 molecular cloud complex (Carpenter, Snell, & Schloerb 1995,
ApJ, 450, 201), near the HII region Sharpless 247. These data are part of the
Spring 1999 data release. Image mosaic by S. Van Dyk (IPAC).
Atlas Image, covering 5.4´ × 5.4´ on the sky, showing the
NGC 2392. This is a planetary nebula, showing a double
ring structure. Planetary nebulae are formed as low-mass stars, like the Sun,
reach the end of their lives and lose their outer envelopes to the interstellar
medium. From an analysis of the nebula's kinematics, O'Dell, Weiner, &
Chu (1990, ApJ, 362, 226) proposed a model where the observer is looking
into a bipolar stellar wind flow from the hot central star (seen in the image
brightly at the nebula's center). The star first lost mass during the extended
red giant envelope stage from the equator of the precursor star, forming the
outer disk, or ring, ~5300 yr ago;
the inner disk, or ring, with an age of ~1000 yr, represents a more recent,
strong, ongoing wind. The central star will eventually evolve to a
white dwarf, as the nebular gas dissipates. Latter et al. (1995, ApJS, 100,
159) previously imaged NGC 2392 in the near-infrared; in the case of this
nebula, the near-IR emission is dominated by reradiated light from the central
star by nebular dust
likely formed during the precursor star's asymptotic giant branch phase.
These data are part of the Spring 1999 data release.
Atlas Image, covering
4.4´ × 6.2´ on the sky, showing the Solar System asteroid
2 Pallas, one of the most studied and the second brightest of these
minor planets. These data are part of the Spring 1999
data release, which will contain a number of other detections of known
asteroids. Identification of the objects takes place as part of the 2MASS
pipeline processing. This image appears in the
2MASS Image Gallery of Solar System objects.
Near-infrared photometry of asteroids can tell much
about the Sun-reflecting surface composition, for example, the amount and
nature of any organic solids or ices coating the asteroid's rocky interior.
The properties of this regolith affect the albedo, or reflectivity, of the
asteroid. Using thermal emission models for asteroids, for instance, a number
of parameters, including diameter and thermal history, can be derived. The
observed colors and inferred compositions of asteroids can place constraints
on the thermal environment in the early Solar System, providing clues to the
formation of the major planets, including the Earth.
An image mosaic, covering
14.2´ × 14.2´ on the sky, of the star formation complex and
young stellar outflow Cepheus A, at 725 pc (2400 light years) distant.
The infrared bright core of the radio source Cepheus A, to the northwest in the
mosaic, contains a number of highly obscured young, massive stars and
molecular gas. A complex molecular outflow extends from the core. The core
itself is obscured by more than 100 magnitudes of optical extinction!
Reflection nebulae, bright Herbig-Haro objects, bow shocks, and jets are also
seen in the complex (Hartigan et al. 1996, AJ, 111, 1278). The bright nebula
to the southeast in the mosaic is IRAS 22551+6139, also a probable young
stellar object. These data are part of the Spring 1999 data release.
Image mosaic by R. Cutri (IPAC).
This image of the nearby barred spiral galaxy Maffei 2, covering
12.0´ × 14.4´ on the sky, appears in the
2MASS Image Gallery of extragalactic objects. Maffei 2 is located
near the Galactic Plane, at galactic longitude l=136.50 and
latitude b=-0.33, and suffers from ~5 magnitudes of visual
extinction. As such, it wasn't recognized as a galaxy until it was initially
detected in the near-infrared as an anomalous source by Maffei (1968. PASP, 80,
618) and its morphological type was identified by Spinrad et al. (1971, ApJ,
163, L25). Maffei 2, at a distance of about 5 Mpc (16.3 million light years),
and its nearby elliptical companion, Maffei 1 (see the
Image Gallery), highlight the importance of the near infrared, and,
specifically, 2MASS, as a window
on galaxies behind the Milky Way disk in the so-called "Zone of Avoidance",
where they are hidden due to Galactic dust. Although its gas
kinematics are consistent with those of other barred galaxies, Maffei 2 in
the infrared, radio continuum and H I emission appears markedly disturbed.
The structural asymmetries, as well as the nuclear starburst, are possibly
driven by an ongoing merger with a small satellite companion galaxy (Hurt,
Turner, & Ho 1996, ApJ, 466, 135).
These data are part of the Spring 1999 data release.
Image mosaic by R. Cutri (IPAC).
The galactic HII
regions Sharpless 254 through 258, covering 21.5´ × 14.5´ on
the sky. The molecular cloud associated with Sharpless 254 through 258
is an active, ongoing star formation location. This cloud is part of the
larger Gem OB1 complex. These regions of ionized gas are at a distance of 2.5
kpc (8150 light years) from us and are located near the Galactic anticenter.
These regions were previously imaged at K-band by Hodapp (1994, ApJS, 94, 615).
A complex of nebulosity is seen in the 2MASS image.
The bright blue, hot ionizing stars in the HII regions are surrounded
by bluish reflection nebulae (dust scattering the stars' blue light). Also
seen throughout the field and, particularly, in the central cluster of
Sharpless 255 are a large number of infrared bright, reddish, presumably
high-mass young stars and stellar objects, still embedded in the cloud. A
number of OH and H20 masers are also associated with these regions,
indicating recent and ongoing star formation.
These data are part of the Spring 1999 data release.
Image mosaic by S. Van Dyk (IPAC).
The star OH 345.0+15.7
(AFGL source GL 1822; IRAS 16029-3041) is an OH (hydroxyl) line emitter and
has no optical counterpart. It is likely a late M-type star
with a very dense dusty circumstellar envelope at a distance of about 6 kpc
(19600 light years) from us. It is losing its mass at a
rate of 1.4 × 10-4 solar masses per year, implying that it
is in a superwind phase at the tip of the asymptotic giant branch (Persi et
al. 1990, A&A, 237, 153). The wind is moving out from the star at about 13
km/sec. Persi et al. in 1985 June found a K magnitude of 6.82 for the star
(the 2MASS Ks magnitude is 6.87 ± 0.02; the 2MASS color
is J-Ks=8.72).
The OH/IR star is likely nearing the end of its life.
The red "stars" trailing to the south in decreasing brightness are
ghost artifacts of the bright OH/IR star.
The relatively nearby (redshift z=0.016), very rich galaxy
cluster Abell 3627. The image covers 21.7´ × 41.7´ on
the sky. The cluster, which lies at the core of the Great Attractor,
is at galactic longitude l=325°, but, more
importantly, is at the low galactic latitude b=-7.2°, i.e.,
near the Galactic Plane, where extinction from Galactic dust is significant.
The cluster has an internal velocity dispersion which implies a gravitational
mass comparable to that of the Coma cluster, another rich nearby cluster.
But, most
intriguingly, Abell 3627's distance and direction puts it near the
predicted location of the center of the Great Attractor, implying that the
cluster may sit at the bottom of the Attractor's gravitational potential well.
The Great Attractor, so dubbed, is a position in the sky toward which a bulk
flow of galaxies appears to be moving (Lynden-Bell et al. 1988, ApJ, 326, 19).
In effect, the Attractor represents the center of a large mass concentration
of galaxies and galaxy clusters, of which our own Local Group is included,
known as the Local Supercluster. 2MASS is less susceptible
to the extinction toward Abell 3627 and the Great Attractor than observations
at optical wavelengths, and therefore will provide valuable clues as to its
true nature. Image mosaic by S. Van Dyk (IPAC).
The barred Seyfert galaxy NGC 1097.
This three-color JHKs composite image mosaic of NGC 1097 shows
the prominent bar in this LINER (now Seyfert 1) galaxy. The presence of the
bar may perhaps be due to tidal interaction with the fainter small elliptical
companion galaxy, NGC 1097A, also seen in the image.
In the inset one
can see the very bright well-known circumnuclear ring of gas, dust, and
stars, where intense star formation is occurring around the active nucleus
of NGC 1097.
The ring, both in the optical and in the near-infrared, resolves into
two-armed spirals, which appear to be common in barred galaxies and is in
accord with simulations of bar-driven inflow of gas (Barth et al. 1995, AJ,
110, 1009). Extremely compact young star clusters were found by Barth et al.
along the ring in Hubble Space Telescope imaging. One of the important
questions is the connection between circumnuclear rings and the active nucleus.
Are the gas and stars in the ring feeding the putative supermassive black
hole at the nucleus? The broad optical emission line profiles seen in
1996 by Storchi-Bergmann et al. (1997, ApJ, 489, 87) from the galaxy's nucleus
is consistent with attributing their origin to an accretion disk that has
formed abruptly from the tidal disruption of a star by the black hole.
The globular cluster 47 Tucanae. This mosaic of one of the brightest
Milky Way
globular star clusters, 47 Tuc (NGC 104), covers 19.2´ × 23.3´
on the sky. This cluster, seen near the Small Magellanic Cloud in the sky,
and at a distance of 4.6 kpc (15000 light years) from us and 7.3 kpc (23800
light years) from the Galactic Center, likely contains about 1 million stars.
Its optical half-light radius is 2.79´, or only 3.7 pc; the stars in
globular clusters, such as 47 Tuc, are clearly densely packed. This cluster
is known to be typically metal-rich, relative to many other globular clusters.
Globular clusters formed early in the Galaxy's history and, therefore, must
have been chemically enriched by massive short-lived stars. The luminosity
functions for globular clusters vary, with metal-rich clusters having flatter
function slopes than metal-poor clusters; recent indications are that a
cluster's interaction with the Galactic disk could strip lower-mass stars from
the cluster, leading to a relative
overabundance of higher-mass stars, which would lead to greater enrichment of
elements, such as oxygen. In the near-IR, globular cluster stars look very
homogeneous, as can be seen in this image, with very little in the way of
color or population gradients, particularly in the central regions (see also
Montegriffo et al. 1995, MNRAS, 276, 739). The near-IR light is dominated by
the old red giants and asymptotic giant branch stars in the cluster.
The high-mass star formation region K3-50.
The red nebular region directly above the brightest star in this
three-color composite image, which covers 14.6´ × 13.4´ on the
sky, is the high mass star formation complex known as K3-50. The cluster is
located in the constellation Cygnus at a distance of about 8700 pc (28000
light years). The region is quite young. While the Sun is presently about 5
billion years old, this cluster is only around 10 to 100 thousand years
old. Stars are generally considered massive if they are 10 times the mass
of the Sun or larger. In the case of this complex good evidence exists
that at least three of the stars are larger than 60 times the mass of the Sun
(Howard et al. 1996, ApJ, 460, 744). In addition to these stars, at
least four other massive stars are present in this region. High mass stars
typically form in complex clusters, and this region is no exception. K3-50
is comprised of at least five different regions of high mass star formation.
The southernmost region is visible at optical wavelengths and is one of
the more evolved regions in the complex. Progressing northward, the regions of
star formation are located deeper in the parent molecular cloud. Most of
the nebular emission seen here is only detectable by observing in
near-infrared, or even longer, wavelengths.
Is it the Black Hole of Calcutta? No, it's a dark, obscuring cloud of dust
near the Galactic Plane (the cloud is at RA=17h35m46.84s Dec=-25d33m10.8s,
J2000; in galactic
coordinates, l=1.62, b=+3.77; this image covers 8.2´
× 12.5´ on the sky). The cloud is the entry
``1-457'' in the catalog by Feitzinger & Stuwe
(1984, A&AS, 58, 365) of dark
nebulae. Based on the (J-H, H-Ks)
color-color diagram, we
estimate that the extinction toward the center of the cloud is
AV>20 mag.
This mosaic image of our dwarf irregular satellite neighbor, the Large
Magellanic Cloud (LMC), covers 6.9° × 6.1° on the sky.
(It has been binned from 1´´ pixels to 5´´ pixels.)
The LMC is at a distance from us of 50 kpc (or 163,000 light years).
The nebulosity and bright recent star formation seen so prominently in
the optical are much diminished in the near-infrared. Only 30 Doradus
(the Tarantula Nebula) is still relatively recognizable in this image.
The galaxy's bar region, however, is quite prominent, as seen by 2MASS,
as it is dominated by intermediate-age red giants. Also detected in
the galaxy are bright red and blue supergiants, and a large number of
asymptotic giant branch (AGB) stars. In particular, 2MASS is sensitive
to the large quantity of dust-obscured red AGB stars and carbon stars, which
are spatially distributed throughout the galaxy.
The molecular cloud and embedded star cluster Monoceros R2.
The 2MASS image of a stellar cluster in the Mon R2 chain of reflection nebulae.
At a distance of 830 pc (2700 light years), the Mon R2 cluster is one of the
closest massive star-forming regions to the Sun that remains embedded in a
molecular cloud. The cluster contains at least a few hundred pre-main sequence
stars within a 0.4-pc diameter region, and likely has formed within the past
few million years (Carpenter et al. 1997, AJ, 114, 198). The two blue patches
of luminosity on either side of the Mon R2 cluster are illuminated by early
B-type stars, and are part of a chain of about 30 such reflection nebulae that
define the Mon R2 association (Herbst & Racine 1976, AJ, 81, 840). (The
full field in the image covers 20.8' × 12.5'.)
The Galactic Center.
This 2.2° × 3.9° image mosaic shows the Galactic Plane (the Plane
of the Milky Way). The Galactic Center is the very luminous
Ks-bright (reddish) source south of the image
center. (The image is centered at RA=17h46m22.0s Dec=-27d58m20s [J2000].)
This mosaic contains more than 1 million stars. Visible, even in the
near-infrared, are the obscuring dust lanes that fill the Plane between us and
the Galactic Center, located about 8.1 kpc (26400 light-years) away. However,
with 2MASS, we are able to see farther through this dust than can be
accomplished at visible wavelengths.
NGC 2024, also known as the Flame Nebula, is located
at a distance of 400-500 pc and is part of the
Orion Molecular Cloud Complex (Orion B). See Meyer &
Lada (1999, The Orion Complex Revisited, ASP Conference
Series, in press) and references therein for an overview
of the region. To the south
of the NGC 2024 region lies NGC 2023 (a well-studied
photo-dissociation region) and the Horsehead Nebula
(cf. David Malin's
optical image). At near-infrared
wavelengths, a dense stellar cluster is revealed in the
dark lane separating the two halves of the flame.
The age of the cluster is thought to be <1 Myr and the
distribution of stellar masses appears to be consistent
with that characterizing the solar neighborhood.
Between 40-70% of the stars in this cluster are
surrounded by circumstellar accretion disks, commonly
associated with young stellar objects. Such disks may
be sites of planet formation and could give rise to solar
systems not unlike our own.
Find a "movie" of the optical versus 2MASS view of the Horsehead
Nebula, near NGC 2024,
here.
The Wolf-Rayet galaxy NGC 4214.
The globular star cluster Messier 13.
The bipolar planetary nebula NGC 2346.
The nearly edge-on Sb galaxy NGC 891, with its conspicuous dust lane, is often
compared to the Milky Way Galaxy, due to its suspected resemblance.
Thus, studying
this galaxy provides insights on the nature of our own Galaxy. Imaging this
galaxy in the near-IR allows us to look deeper through the extinction caused
by the dust and also better map the light and mass distribution of the galaxy,
since
the light from spiral galaxies in the near-IR is dominated by lower-mass stars,
which comprise most of a galaxy's visible mass. This galaxy was previously
imaged in the near-IR by Aoki et al. (1991, PASJ, 43, 755) and Xilouris et al.
(1998, A&A, 331, 894). Aoki et al. found
that the patchy reddish color of the galaxy's disk is not only due to the dust
lanes, but also to the light from late-type supergiants, with a z scale
height of ~350 pc (assuming a distance of 9.8 kpc). From modelling of the
optical and near-IR light, Xilouris et al. find an overall scale height for the
stars of ~400 pc and a scale height for the dust of ~260 pc; the scale length
for stars in the disk is 5.1 kpc, while for the dust, it is 7.5 kpc.
Xilouris et al. conclude that if seen face-on, NGC 891 would be optically
thin, and that from its estimated dust mass and extinction law, this galaxy
is indeed very much like our own Milky Way.
This 1°.18 × 0°.72 Atlas Image mosaic shows one of the most
luminous star-forming complexes in the Galaxy, W51. The environment here is
rich in newly-forming massive stars,compact and extended H II regions, and
molecular clouds, including the
Ks-bright, arc-like region, G49.5-0.4, to the northeast.
The extended emission in Ks (2.17 µm) is thermal emission from
ionized gas associated with the H II regions (Goldader & Wynn-Williams 1994,
ApJ, 433, 164). W51 is
along the Sagittarius arm in our Galaxy at about 7.5 kpc.
The visual extinction in this
region is very high, from AV ~ 25 to 1000! Hot, main-sequence OB
stars can be detected in these near-IR colors, which are consistent with the
distance and extintion to W51 and are likely members. A number of embedded
massive stars probably comprise the famous IRS sources
(Wynn-Williams, Becklin, & Neugebauer 1974, ApJ, 187, 473) in G49.5-0.4.
The supernova
remnant W51C is not detected in this image.
The Wolf-Rayet ring nebula M1-67. Several Wolf-Rayet (W-R) stars,
which represent the final evolutionary stages
of very massive stars, have surrounding them thick shells of matter called
"ring nebulae." The W-R star 124 (van der Hucht et al. 1981, SpSciRev, 28,
227) has a relatively
young ejection nebula (M1-67). In this 2MASS image, the bright
Ks-band emission seen around the bright W-R star (near the center
of the image) is likely to be molecular H2. The molecular gas
excitation is due to either ultraviolet flourescence, as photons from the
star are intercepted by the ring, or to shocks interacting with surrounding
gas. The nebula was recently imaged in H-alpha by the Hubble Space Telescope
(Grosdidier et al. 1998, ApJ, 506, L127). Grosdidier et al. find unprecented
structure never seen before, including what appear to be hot, dense clumps in
the wind coming from the star.
The face-on spiral galaxy NGC 5247.
NGC 416 (Lindsay 83) is one of several populous star clusters in the
Small Magellanic Cloud (SMC), which are analogous to
globular clusters in our own Galaxy, but significantly younger. This cluster
was recently observed with the Hubble Space Telescope by Mighell, Sarajedini,
& French (1998, ApJ, 494, L189). From their deep photometric observations,
they derive an
age for the cluster of 6.6±0.5 Gyr. For comparison, the Galactic
globular cluster 47 Tuc is 13 Gyr old. NGC 416, however, is intermediate
in age, relative to other populous (or, globular) clusters in the SMC. The
metallicity, or heavy element content, is less for NGC 416 and the other
SMC clusters, relative to Galactic clusters, such as 47 Tuc, due to the
very different star formation history of the SMC, as compared to the Milky
Way.
The embedded young star cluster IRAS 20050+2720,
the bright agglomeration of stars near the center of
the 2MASS three-color image. This cluster has been recently
studied by Chen et al. (1997, ApJ, 475, 163). The cluster appears to consist
of several subclusterings, three of which were studied by Chen et al.; there
appear to be several other subclusterings not included in their near-IR
imaging. The IRAS source itself is the
brightest, reddest southern subclustering ("Subcluster A").
Some reflection nebulosity is
associated with the cluster stars. Chen et al. find an extinction to
the cluster of AV ~ 10, although the extinction is clearly higher
for subcluster A. (It is also clear from the 2MASS image that
significant extinction is distributed throughout this field.)
More than half of the point sources in the
cluster appear to show an infrared excess beyond this extinction,
indicating that we may be seeing thermal dust emission from circumstellar disks
around classical T Tauri stars. These sources are therefore likely pre-main
sequence objects. Chen et al. find that the slope and turnover of the K-band
luminosity function imply an overall age for this cluster of ~1 Myr, although
the subclusters are of somewhat different ages. The IRAS source
"subcluster A" is also associated with dense molecular gas as traced by
millimeter radio observations and coincides with the center of
a multipolar outflow with a dynamical age of only 103 to
104 yr.
Three-color composite Atlas Image Mosaic, covering 0.83° × 0.97°
on the sky, of the intermediate-age Galactic supernova remnant IC 443,
which is at a distance of about 1.5 kpc. What can be seen over this large area
are two regions of near-infrared emission from the remnant. The bright bluish
arc to the northeast appears to be line emission from excited iron in these
remnant filaments, bright in the J band (Rho et al., in preparation). Along
the south, from east to west, is the interaction of the remnant with the nearby
molecular cloud. IC 443 offers a unique laboratory for studying such
interactions. The supernova shock is exciting 2.12-micron H2
molecular line emission, very bright in the Ks band (Richter,
Graham, & Wright 1995, ApJ, 454, 277). Some H2 emission is also
seen weakly to the north. The emission
from the partially dissociative "J-type" shock-cloud interaction is complex and
clumpy. The overall structure of the remnant, as seen by 2MASS, follows both
the radio and X-ray emission contours, showing that the radiation in all these
wavelength regimes seems to arise from the same regions in the remnant.
Analysis of this large-area 2MASS mosaic will provide important insight into
the interaction of supernova remnants with their immediate environment, and
therefore into the probable nature of the supernova remnant's stellar
progenitor. These data are part of the 2MASS Spring 1999 Incremental data
release. Image mosaic by E. Kopan (IPAC).
Messier 8: The Lagoon Nebula.
The red emission in the optical image is from hydrogen, after recombination, in
this famous H II region. Note the absence of the nebular hydrogen emission in the
2MASS image. The Hourglass Nebula (see the 2MASS
Image Gallery) is clearly visible in both images, toward the center.
Extremely Young Stellar Objects in L1551.
The Lynds 1551 (L1551) dark cloud region contains several very young low-mass stellar
objects in the process
of forming stars. At the earliest phases of formation, nascent stars are
deeply embedded in dense dust cores and molecular gas. Their emission is
reprocessed into long-wavelength far-IR and sub-mm radiation. Even at near-IR
wavelengths, the young stars are not detectable as point sources, but
associated nebular and reflected emission can be detected, as in this
2MASS composite three-band image. The early evolution of young stars is
accompanied by at least one episode of jet-like mass loss (Edwards et al.
1986, ApJ, 307, L65).
The bright yellow shrouded source toward the center of the image is the
well-known IRS 5; the reddish object to the northeast is L1551-NE. Both
sources are almost certainly deeply-embedded forming stars. The
blue-greenish emission fanning out to the southwest of IRS 5 is a gas outflow
from that source.
IRS 5 is evolved enough that its jet-like outflow has possibly excavated a
cavity with large opening angle in the ambient medium (Hodapp & Ladd 1995,
ApJ, 453, 715). The oblate shape of
IRS 5 may be due to a circumstellar disk, which is perpendicular to the
outflow (Strom et al. 1985, AJ, 90, 2575).
L1551-NE is likely younger and also has
fan-shaped nebulosity extending to the southwest; but much of the
nebulosity is likely
reflected light, possibly tracing out an outflow-blown cavity.
NGC 7419, a heavily-reddened Cepheus open star cluster.
The cluster in the near-IR is highlighted by the five bright red supergiants.
Beauchamp, Moffat, & Drissen (1994, ApJS, 93, 187) derived an age of
14 ± 2 million yr and a distance of 2.3 kpc from
theoretical evolutionary tracks, and found that the cluster is dynamically
relaxed.
Beauchamp et al. also found that there is at most one blue supergiant member,
which contradicts theoretical expectations. The brightest star in the
2MASS image is the M7.5 OH/IR star MY Cephei.
The yellowish bright star to the southeast is the carbon star MZ Cephei, which
has Ks=3.7, J-H~1.7, H-K~1.6.
The red "stars" just to the northeast of the brightest stars are filter glint
artifacts, and the "fuzzy" bluish "stars" due north of these bright stars are
latent image ("ghost") artifacts, produced by the mode of the survey scanning.
The Crab Nebula, or Messier 1, is one of the most spectacular and intensively
studied objects in the sky. It is the remnant of a supernova in AD 1054,
observed as a "guest star" by the Chinese in today's constellation Taurus.
It is among the brightest remnants across a broad wavelength spectrum.
The Crab Nebula is probably the best-known synchrotron emission nebula. The
synchrotron is what is primarily seen in the 2MASS image. In addition,the
central power-law source, the Crab pulsar, is photo-exciting line
emission. The gas in the nebula has not yet mixed with the interstellar
medium, and so study of the line-emitting gas gives us insight on the
progenitor star. The blue-green
[Fe II] 1.644 µm emission is excited in the optically thick filaments in
the Crab Nebula by the power-law photoionization source within; H2
seen as K-band emission in the filaments could not normally survive in these
nebular conditions and must have formed early in the
remnant's expansion when densities were higher
(Graham, Wright, & Longmore 1990, ApJ, 352, 172).
Return to the 2MASS Home Page
0~0.2-0.4
(Ribiero et al.). The compactness of HCG 40 implies that it will evolve
through strong galaxy merging on a timescale of a few group crossing times.
Most of the HCGs have been observed by 2MASS so far; many of these, included
HCG 40, are included in the Second Incremental Data Release. HCG 40
was previously imaged in the near-IR by Bushouse & Stanford (1992, ApJS, 79,
213). The near-IR light of galaxies is dominated by the major mass
components and, therefore, can best be used to study the relationship between
galaxy interaction, activity, and morphology. Image mosaic by S. Van Dyk
(IPAC).
12 billion years. Tavarez & Friel find a reddening to
the cluster of E(B-V)=0.73; here we show
the 2MASS color-color diagram for the cluster. It is difficult to exactly
determine the reddening from this diagram, but it does appear to be modest.
Here we show the 2MASS color-magnitude diagram,
with Padua theoretical isochrones (Bertelli et al. 1994, A&AS, 106, 275)
overlaid. An age of ~12 billion years (green line) is consistent with
the near-infrared data, however, with a lower reddening, E(B-V)~0.60.
(The diagram also shows that ages of 10 billion years [red line]
and 14.5 billion years [blue line] are also consistent with the data,
since these age tracks become somewhat degenerate in the near-infrared colors;
scatter in the datapoints, due to crowding, is also present;
the other bluer stars on the diagram are foreground stellar contamination.)
At least one likely dust-enshrouded AGB star, the reddish star
(Ks=8.03, J-Ks=4.55, not shown on the diagram) is
evident in the cluster (the other reddish
"star" just south of the brightest star in the image is a known artifact).
Lyngå 7 represents an important
link between the oldest open clusters and the youngest globular clusters in
the Galaxy. Image mosaic by S. Van Dyk (IPAC). These data are included in
the Second Incremental Release!
224; southeast of center), and V1318 Cyg (LkH 225; just east of V1686 Cyg). Herbig Ae/Be stars are
young, intermediate-mass stars exhibiting mass loss, as seen from their
optical line emission. This stellar group is about 1 kpc (3260 light years)
from us, along the Cygnus spiral arm. V1686 Cyg and V1318 Cyg are also
optical emission-line objects. V1318 Cyg, as seen in the 2MASS image, is
actually two objects; Aspen, Sandell, & Weintraub (1994, A&A, 282, L25) suggest
that the pair form a possibly-interacting binary of pre-main-sequence
objects. A water maser source and bipolar molecular outflow are associated
with V1318 Cyg (Palla et al. 1995, A&A, 293, 521).
Many more highly-embedded objects are seen around this
group of stars, which can be seen to be surrounded by infrared-bright
nebulosity; the cluster of objects shows both a significant spread in stellar
mass (both low and high, forming simultaneously) and
age (~3 Myr; Hillenbrand et al. 1995, AJ, 109, 280). The
infrared-bright yellowish star further southeast is unidentified.
Image mosaic by S. Van Dyk (IPAC).
These data are included in the Second Incremental Release!
, based on IRAS measurements).
These galaxies are now understood as experiencing vigorous star formation,
from follow-up observations at optical and other wavelengths. Understanding
this star formation at both high and low redshifts, and its relation
to possible nuclear activity in these galaxies, is currently an important
astronomical problem. The nuclear activity in galaxies is thought to be due
to accretion by a supermassive central black hole. A prime example of a nearby
luminous galaxy is the nearly edge-on barred NGC 4945. Prominent dust lanes
can be seen in the 2MASS image, along the inner bar in the bottom half and along
the outer spiral arm in the top half. This apparent asymmetry is a geometric
effect, as the dust lanes lie along the leading edge of the bar and along the
trailing edge of the spiral arms; at the point of galactic corotation, where
the arms join the bar, the dust lanes switch sides. The nucleus of NGC 4945 is
the brightest infrared source in the south, after the Magellanic Clouds.
Spectra of the nucleus in the near-infrared show a mixture of Seyfert and
starburst phenomena (Moorwood & Oliva 1994, ApJ, 429, 602), as well as evidence
for activity at a range of wavelengths. Lipari, Tsvetanov, & Macchetto (1997,
ApJS, 111, 369) recently made a
comparison of the nucleus' optical and near-IR appearance, and find a distinct
spatial offset; their conclusion is the active nucleus is dust-obscured and
possibly fed by a nuclear starburst, which is driving a superwind, seen in
emission at optical wavelengths. Image mosaic by R. Hurt (IPAC).
198, at a distance of 600 to 900 pc.
The Herbig Ae/Be stars are intermediate-mass pre-main sequence objects,
showing emission lines in their optical spectra and appearing with associated
nebulosity (they are more massive than their lower-mass counterparts, the
T Tauri stars).
LkH 198 is the bright object at the center of the image;
35´´ north of this star is another
Ae/Be star, V376 Cas. Both stars can be seen in the near-infrared to
be embedded in associated nebulous clouds. (Diffraction spike artifacts can
be seen emanating from both bright objects; latent image artifacts, produced by the mode of the survey scanning, show a
similar pattern of "red stars" both due north and due south of the two stars.)
Both stars are thought to drive a low-velocity bipolar molecular outflow.
LkH 198 also has an embedded infrared companion (unseen
in the 2MASS image). Also seen in the 2MASS image is a blue elliptical loop
associated with LkH 198 and a similar "sickle-shaped"
nebula (weakly) seen to the west of V376 Cas; this extended emission is
consistent with light scattering by small dust grains. A diagram of this
complex region, showing the relationship of its various components, is
provided by Koresko et al. (1997, ApJ, 485, 213; their Figure 7).
Image mosaic by S. Van Dyk (IPAC).
(The full JPG image above is 2.5 Mbytes. For a smaller version
[449 kbytes], click here.)
Image mosaic by S. Van Dyk (IPAC).
Image mosaic by E. Kopan (IPAC).
Image mosaic by S. Van Dyk (IPAC). Caption provided by E. Howard (UMass).
Image mosaic by E. Kopan (IPAC).
Image mosaic by E. Kopan (IPAC).
Image mosaic by E. Kopan (IPAC).
Image mosaic by E. Kopan and R. Hurt (IPAC).
Caption provided by M. Meyer (Steward Obs, U of A).
Image mosaic by E. Kopan (IPAC).
(The very red star seen in the SMC field, to the northeast of NGC 416 and
toward the edge of the image, is
likely to be a dust-obscured asymptotic giant-branch or carbon star in that
galaxy.)
Image mosaic by E. Kopan (IPAC).
The two bright stars to the north are the unusual T Tauri stars HL and XZ Tau.
HL Tau has a circumstellar accretion disk (Close et al. 1997, ApJ, 478, 766).
Both stars are associated with HH30 and other Herbig-Haro objects.
The star to the southwest of those two stars is LkH358.
All of these objects are about 105 years old and at a distance
of about 140 pc (456 light years).
