xi. How to Use the XSC
The 2MASS Extended Source Catalog (XSC) is comprised of near-infrared sources that are clearly resolved and extended beyond the beam/PSF. Sources range in angular size from 10´´ to 2°: from small, faint galaxies to the largest Local Group galaxies. By their very nature, galaxies are complex and, therefore, require characterization measurements which are much more detailed than that for point sources. As such, the XSC includes many different kinds of measurements and various flavors of, e.g., position, orientation, size, symmetry, surface brightness and integrated flux. The user should take care to understand these parameters and their limitations. Here we answer some "Frequently Asked Questions" and give some brief guidelines on how to use the XSC to maximum effect.
Fundamental: The XSC is an extended source catalog. Although galaxies dominate the total numbers (and we frequently refer to the XSC as a galaxy catalog), the XSC is also comprised of Milky Way entities, such as globular and open clusters, planetary nebulae, HII regions, ISM emission and nebulosity, young stellar objects and compact star-formation regions, and even Solar System comets. These Galactic sources tend to be confined to the plane of the Milky Way. Refer to the "All-Sky" maps in II.3c2 to see how these sources are distributed across the sky.
| Fast links to: IRSA astrometry photometry large galaxies Milky Way objects completeness/reliability outliers references/links |
IRSA includes many services to retrieve data and to compare across datasets. The two most important services relevant to the XSC are:
The absolute astrometric accuracy of 2MASS is better than 100 milliarcsec for
point sources. For extended sources, you can expect 0.5´´ accuracy
for the peak-pixel coordinates and 0.3´´ accuracy for the centroid
coordinates. For a comparison of the 2MASS peak-pixel
coordinates with the FIRST radio survey, see II.3c5.
For most applications, the elliptical isophotal aperture
is a good choice, both in terms of capturing most of the integrated
flux (~80-90%) and providing accurate colors for galaxies of all sizes.
Choose the "total" apertures (e.g., Kron or the Extrapolation Surface
Brightness Profile)
if you need integrated fluxes that reflect the total flux of the source.
Beware: these apertures are vulnerable to stellar contamination
and surface brightness irregularities.
The photometry based on the extrapolation of the surface brightness profile
seems to be the more robust of the two methods (primarily because
stellar contamination is minimized by averaging over the azimuthal
isophote[s] used to derived the median surface brightness profile.)
The most robust aperture in the
XSC is the circular, 7´´ radial aperture. This is a good choice if
you are focused on faint or small compact sources. Beware: for larger
galaxies the small apertures are not a good choice -- they are subject
to a "bulge" color bias. Explanations of the
aperture photometry are available:
That said, if the user so desires to recapture the "default"
magnitudes which were used in the Incremental Releases, then select
the circular (Ks-band fiducial) isophotal magnitudes, which are available
in the XSC.
The 2MASS survey acquired images of the sky using relatively small arrays.
A "tile," or scan, is 8.5´ in angular width, and the typical overlap
between scans is 50´´.
Galaxies that are smaller than this overlap are guaranteed to be fully sampled
in at
least one survey scan. Larger galaxies may be truncated based on their proximity
to a scan edge. Therefore it was necessary to construct an atlas of large
objects made from "pieces" of adjoining scans. The net outcome is that we will
fully recover galaxies that are currently "lost" or misrepresented in the
2MASS extended source pipeline.
The resultant set of mosaics and corresponding source characterizations
are collectively called the
2MASS Large Galaxy Atlas (LGA). This LGA information has been incorporated
into the XSC for the largest 550 galaxies in the sky. LGA galaxies are easily
identified in the XSC with the
parameter cc_flg; they are tagged with
cc_flg = "Z".
We have also identified sources which are in close proximity to large galaxies,
whose photometry has been eliminated from the Catalog, due to their unreliable
nature, i.e., cc_flg = "z".
The 2MASS Large Galaxy Atlas is described in detail in
Jarrett et al. (2003).
[Last Updated: 2002 Oct 27; by T. Jarrett]
Astrometry: Which Position Do I Use and How Accurate Is It?
Each XSC source is tagged with two different sets of photometry. (1) The first
set is based on the J-band peak pixel of the source. Since each pixel is
1´´
in size (while the PSF beam is closer to 2.5´´),
the relative accuracy of this measurement
is between 0.3´´ and 0.5´´. The source
name, or designation (see
Table 1),
is derived from this position.
(2) The second set is based on the intensity-weighted centroid of the
combined J+H+Ks image. The relative accuracy of this measure is
nearly twice
as good as the peak-pixel method, due to the improved SNR and centroiding
method.
Photometry: Which Should I Use?
Because extended sources are complex by nature,
a variety of apertures are used to compute the integrated flux.
Choose the aperture or method that best matches your science
goals. Since there is no "one-size-fits-all" aperture, the XSC
does not provide "default" magnitudes (unlike the PSC). See note below.
We do have some guidelines.
Photometry: Why Are There No Default Magnitudes for the XSC?
Although "default" magnitudes were provided in the Incremental XSC Releases,
they are not provided in the All-Sky XSC Release (which supercedes all
Incremental Releases). It was decided that, since extended sources
are complex entities and no one aperture (or method) satisfies
all science requirements, the notion of "default" in the XSC is
misleading, at best. The user should decide which aperture
best matches the desired science goals. We do offer some guidelines as
to what photometry works best for common cases. See the above
"Photometry: Which Should I Use?"
Photometry: What About Total Magnitudes?
"Total" magnitudes refer to apertures in conjunction with corrections
that account for the "total" flux of a source. We have determined
that between 10% and 20% of the flux (galaxy morphology-dependent)
of an object is lost in
the formidable background noise. This flux is recovered using
the median (elliptical) surface brightness profile. Beware:
"total" magnitudes are vulnerable to stellar contamination and to irregularities
in the surface profile (e.g., asymmetries). "Total" magnitudes are not
a good choice when colors are desired (since one is effectively
root-square-summing two large uncertainties to compute a color). For colors,
use a smaller, more robust aperture (e.g., isophotal magnitudes;
See "Photometry: Which Should I Use?" above.)
Large Galaxies: Why Are Large Galaxies Special Objects in the XSC?
The XSC is complete for all galaxies larger than ~10´´-15´´
in diameter, including the largest galaxies in the sky. However, due to their
proximity
to a survey "scan" edge, galaxies larger than ~1´ or 2´ will have
photometry
that is systematically incomplete. We have rectified this situation.
Milky Way: How Do I Distinguish Milky Way Fuzz from Background Galaxies?
Completeness: Why Is My Favorite Galaxy Missing from the XSC?
Reliability: What Is a Star Doing in the XSC?
Duplicity: Why Do Some Galaxies Appear to be Duplicates?
Contamination: Why Do Some Galaxies Appear to have Unphysical Colors or Brightnesses?
Z. Helpful References
Return to Section IV.5a.