5. 2MASS Extended Source Objects
2MASS resolved or ‘extended’ sources are, by and large, located beyond the confines of the Milky Way – that is to say, they are galaxies. Large-angular size galactic objects, such as HII regions, stars with nebulosity, planetary nebulae, reflection nebulae, etc, are relatively rare (compared to stars and background galaxies) and generally confined to the Galactic plane and star formation nurseries. The extended source catalog is predominantly composed of galaxies and, at the 1 to 20% level depending on the stellar number density, double and triple stars (see figures below). Examples of galaxies found in low stellar density fields (glat ~ 20), moderate to high density fields (glat ~ 10) and very high density fields (glat ~ 5) are given below. H II regions and molecular cloud nebulosity are predominately found in the Galactic plane; a few examples are given below.
The final component to the extended source catalog is a class of objects that are collectively referred to as "artifacts". These false signals generated by bright stars, transient phenomenon, such as meteor streaks, and upper atmospheric "clouds" (or "airglow" in the infrared) which occur at all spatial frequencies. Most artifacts associated with bright stars are easily identified within the 2MASS database (from which the catalogs are constructed) using simple geometric rules, but the occasional false detection slips by, for example, due to a very bright star that defies formulation, see example below. Meteor streaks are more difficult to identify using automated rules, but in general their frequency is low. Airglow not only generates false detections (especially under severe conditions), but it also significantly affects the photometry of real sources. Overall, artifacts generally add less than 1 - 5% to the total contamination under normal conditions. Examples of 2MASS galaxies and various kinds of artifacts are given below. More information on data anomalies and image artifacts can be found in Appendix H, Data Anomalies and Artifacts.
The 2MASS extended source catalog contains galaxies ranging in brightness from K=9th to 14th mag. This flux range is constrained by the sensitivity of the survey and size limitations fixed by the scale of the Atlas Image (coadd) and the background removal process. Galaxies as large as 3 or 4¢ in diameter may be processed (if they are predominantly located near the center of the coadd) but only the inner 2¢ or so is examined in detail. For very large galaxies (>5¢ in size), such as the Messier objects, the only processing is to extract pieces of coadd images associated with the galaxy. At the faint end of the flux spectrum, the size of the galaxies is limited by the resolution of the survey, ~2’, and the elongation or asymmetry in the PSF. Isophotal radii are generally accurate down to about 10² .
In Figures 25-27 a representative sample of galaxies from low stellar number density fields is shown with their K-band postage stamp images. The data come from scans passing through the Abell 3558, Hercules, & Abell 2065 clusters, as well as random non-cluster fields. A wide range in morphology, surface brightness and integrated flux comprise the sample. Figure 25 shows the brightest galaxies, ranging in total K-band flux from 9th to 13th mag. Each image is 60² ´ 60² , and we show examples of several morphological classes: elliptical (E), lenticular (S0, SA0), spiral (S), and complex, including double nucleus, interacting and pre-merger systems.
The next set of galaxies, Fig. 26, represent the faint limit at which the extended source catalog is both reliable (>98%) and complete (>90%), with K mags ranging from 13th to 13.5 mag. The size of each image is 30² ´ 30². Although it is possible to discern with the eye, automated morphological classification is a very difficult procedure (with ultimately limited fidelity), relegated to the brightest galaxies, highly inclined spirals and low surface brightness (LSB) galaxies; see Galaxy Morphology Gallery.
The final set of low density galaxies (Fig. 27) represent the faintest galaxies resolved with 2MASS, with K mags ranging from 13.5 to 15th mag, corresponding to a SNR range between 5 and 10. Each image is 20² ´ 20² in width. The lowest surface brightness galaxies belong to this set, which are generally detected only in J-band due to the blue color of most LSB-type. For example, the last four galaxies in the set are the result of J-band detections.
When the source density is high, the confusion noise approaches the level of the atmospheric thermal background noise (see appendix C). The probability of +triple stars is significant and the ability to distinguish galaxies from multiple groupings of stars is limited. Nevertheless, a reliability of >80% is possible for most of the galactic plane corresponding to K-band source densities less than ~10,000 point sources deg-2 brighter than 14th mag. Figure 28 gives examples of galaxies found in the galactic plane. The approximate galactic coordinates are (240° , +4.5° ), corresponding to a density of 4500 stars deg-2 brighter than 14th mag, and a differential confusion noise equivalent of 0.7 mag in a 10² aperture (see appendix C). The integrated K-band fluxes range from 11.8 to 13.8 mag. The estimated J-K reddening is 0.15 mag. Closer to the galactic center, coordinates (12° , +5.0° ), the density of stars is over 30,000 per deg2, resulting in an equivalent differential confusion noise of nearly 2 mags, galaxies are still detected by 2MASS (Fig. 29a). The estimated reddening is now 0.3 mag. Note the significant stellar contamination to the local environment of the galaxies. The integrated K-band flux ranges from 11.0 to 12.7 mag, indicative of confusion noise limits on the faint end discovery spectrum. False detections are dominated by multiple stars (mostly triples and quadruples), a representative set is shown in Figure 30.
5.2 Galactic Extended Sources
Nebulosity associated with bright stars (e.g., H II regions, PNs) and with molecular clouds (reflection nebulae, YSOs) typically appear as very bright and large extended sources (Figure 29b). Since these objects are primarily located in the Galactic plane, confusion from foreground stars is significant.
5.3 Bright Stars and Artifacts
Bright stars are historically a major nuisance to any image-based survey. Off-axis stray light can land just about anywhere on the focal plane, while dense concentrations of light (e.g., diffraction spikes) are distributed geometrically with respect to the optical axis. Features referred to as "glints" and "ghosts" are focused or semi-focused reflections of light that appear as slightly asymmetric point sources or flattened (low surface brightness) extended sources. Not only do bright 2MASS stars (K < 9th) produce diffraction spikes, halos, glints and ghosts, but for the brightest stars (K < 5th mag, which is approximately the saturation limit of the 2MASS survey), they generate horizontal stripes that persist along the entire cross-scan (east-west axis) of the scan, or a total of 8.5’ in length. Finally, bright stars induce another feature unique to infrared arrays: latent residual or persistence ghosts. The central core of a bright star leaves a residual signal after the array has been readout. The residual persists for several seconds (and for the brightest stars, many tens of seconds). What this means for 2MASS survey data is that a bright star will leave a ‘trail’ of persistence ghosts as the telescope is in declination. All of these bright star artifacts, many of which look like perfectly reasonable galaxies, must be removed within the pipeline reduction process to minimize false detections. GALWORKS removes halos, stripes and spikes by measuring their surface brightness constrained by a priori boundary conditions based on the estimated total flux of the star and the expected confusion noise as traced by the stellar number density.
A sequence of bright stars, ranging from 10th to 4th mag in J band, is shown in Fig. 31. Each image is 120² ´ 120² in size. The sequence demonstrates two kinds of artifacts: diffraction spikes (along the north-south-east-west cardinal axis) and ‘halo’ emission. The diffraction spikes extend several arcminutes for very bright stars; see for example Fig. 32, which shows a 4th magnitude star in a J-band coadd. Note the three horizontal stripes extended and flaring across the entire 8.5’ of the field. Also note the persistence ghosts trailing to the south of the bright star. An even more dramatic example of spikes, ghosts, halo and stripes is seen in Fig. 33, which shows two adjacent J-band coadds with a K = -1 mag star (b Pegasus ) straddling the boundary. The vertical spikes extend well beyond the coadd boundaries, while the halo emission completely dominates both coadds. The persistence ghosts (trailing to the south of b Peg) appear nearly as bright as field stars. The influence of b Peg extends across scan boundaries as well. Internal telescope reflections produce stripe features extending over 1 degree in radius from the center of b Peg (see figure below), presenting a major clean-up challenge for artifact removal. It may be that in the vicinity of the brightest stars (K < 0 mag) in the infrared sky, the reliability will inevitably suffer. Fortunately, there are only a handful of these dreadful stars.
Many meteor streaks have the unfortunate property of high surface brightness coupled with severe elongation – similar to large highly inclined spiral galaxies. Figure 34 demonstrates transient streaks in two different J-band coadds. Note the sharp boundaries for the bright streak and the episodic flaring for the fainter streak. The latter is, in fact, associated with b Pegasus (Fig. 33), discussed above. The one critical difference that meteor streaks have with real edge-on spiral galaxies is that multiple detections (in some cases several hundred sources) occur along the streak which can in principle be easily identified with simple database queries and cleaned from the catalogs accordingly.
False extended source detections, "artifacts", are produced directly and indirectly by bright stars, meteor streaks, background gradients (for example, airglow "bumps" that are not removed) and bright to moderately bright stars on the edges of coadds. Figure 35 illustrates some of the kinds of artifacts found in the extended source database. The first two (reading left to right) are the result of a "ghost" or "glint", most prominent in J band, to the southwest of the 8th – 9th mag progenitor star. The third column shows a false detection due to a flared diffraction spike from a star on the edge of coadd. The 4th and 5th columns are examples of faint stars or faint galaxies located on or within the boundary of a horizontal stripe or meteor streak. The final column is a faint star boosted up by background airglow (note the prominent H-band emission). The airglow artifact is probably the most insidious class of false detection since it is so difficult to discriminate from real galaxies or real interstellar nebulosity. The only way to minimize their effect is to avoid data with significant airglow (generally correlated with the overall background level). Because the airglow at H-band shows the most severe variations, we therefore exclude sources identified in the H-band only.