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 where most of the molecular gas (or cites of star formation) is located. 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.

The final component to the extended source catalog is a class of objects that are collectively referred to as "artifacts". These are generally not real astronomical sources, but rather 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 presence is minimal. Airglow not only generates false detections (especially under severe conditions), but it also significantly affects the photometry of real sources. All in all, artifacts generally add less than 1 - 2% to the total contamination under normal conditions. Examples of 2MASS galaxies and various kinds of artifacts are given below.

The 2MASS extended source catalog contains galaxies ranging in brightness from K=9^th to 14^th 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 7" or so.

In Figures 1- 3 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 (and evolution sequence), surface brightness and integrated flux comprise the sample. Figure 1 shows the brightest galaxies, ranging in total K-band flux from 9^th to 13^th mag. Each image is 60" in angular width. Easily identified are the morphological types: elliptical (E), lenticular (S0, SA0), spiral (S), complex, including double nucleus, interacting and pre-merger systems. The next set of galaxies, Figure 2, represent the faint limit at which the extended source catalog is both reliable (>98%) and complete (>90%), with K mags ranging from 13^th to 13.5 mag. Now, the size of each image is 30". Morphology classification is considerably more difficult than that for the preceding set, yet highly inclined spirals are still discernable, as are low surface brightness (LSB) galaxies. The final set of low density galaxies (Figure 3) represent the faintest galaxies resolved with 2MASS, with K mags ranging from 13.5 to 15^th mag. Each image is now 20" in width. The lowest surface brightness galaxies belong to this set, which are generally detected only in J-band due to the intrinsic color of this type (LSB) of galaxies (they are mostly blue) and the superior J-band array sensitivity. 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 (see appendix ?). The probability of triple-plus 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 densities <10,000 sources per deg² brighter than 14^th mag. Figure 4 illustrates the kinds of galaxies found in the galactic plane. The approximate galactic coordinates are (240° , +4.5° ), corresponding to a density of 4500 stars per deg² brighter than 14^th mag, and a confusion noise equivalent of 0.7 mag. The integrated K-band fluxes range from 11.8 to 13.8 mag. Closer to the galactic center, coordinates (12° , +5.0° ), the density of stars is over 30,000 per deg², resulting in an equivalent confusion noise of nearly 2 mags, galaxies are still detected by 2MASS (Figure 5). 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 6.

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 < 9^th) produce diffraction spikes, halos, glints and ghosts, but for the brightest stars (K < 5^th mag, which is approximately the saturation limit of the 2MASS survey), they generate horizontal stripes that persist along the entire cross-scan (equatorial 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 stepped (i.e., drift scanned) along the declination axis. All of these bright star ‘features’ or artifacts must be removed within the pipeline reduction process to prevent (or at the worst, minimize) false detections, many of which appear as perfectly reasonable looking galaxies. GALWORKS removes halos, stripes and spikes by measuring their surface brightness constrained with apriori 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 10^th to 4^th mag in J band, is shown in Figure 7. Each image is 120" in angular width. 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 Figure 8 , which shows a 4^th magnitude star in a J-band coadd. Note the three horizontal stripes extended and flaring across 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 Figure 9, which shows two adjacent J-band coadds with a -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. Reflection stripes and angular streaks extend over 1 degree in radius from the center of b Peg (see figure below), presenting a major cleanup challenge for artifact removal. It may be that for the brightest stars in the infrared sky (K < 0), the reliability will inevitably suffer. Fortunately, there are only a handful of these dreadful stars.

Meteor streaks have the dubious property of high surface brightness coupled with severe elongation – similar to large highly inclined spiral galaxies. Figure 10 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 the super bright star b Pegasus (Figure 9) located nearly an angular degree away from the coadd field. 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 11 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 8^th – 9^th 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 4^th and 5^th columns are examples of stars (or even 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 (and real nebulosity). The only way to minimize their affect is to avoid data with significant airglow (generally correlated with the overall background level).

Figure 1—Bright 2MASS galaxies as seen in the K-band. The sequence is arranged in order of integrated K-band flux, reading left to right. 1^st panel: K mags range from 9^th to 10.5; 2^nd panel: 10.5 – 11.0; 3^rd panel: 11.0 – 11.6; 4^th panel: 11.6 to 12.1; 5^th panel: 12.1 – 12.3; 6^th panel: 12.3 – 12.6; last panel: 12.6 – 12.9. Each image is 60" in angular width.

Figure 2--2MASS galaxies at the K-band sensitivity requirement limit, K~13.5 mag. The sequence is arranged in order of integrated K-band flux, reading left to right. 1^st panel: K mags range from 13^th to 13.1; 2^nd panel:13.1 – 13.3; 3^rd panel:13.3 – 13.4; last panel:13.4 – 13.5. Each image is 30" in angular width.

Figure 3—Faint 2MASS galaxies as seen in the K-band. The sequence is arranged in order of integrated K-band flux, reading left to right. 1^st panel: K mags range from 13.5 to 13.7; 2^nd panel:13.7 – 13.9; 3^rd panel:13.9 – 14.1; 4^th panel:14.1 14.2; last panel:14.3–15th. Each image is 20" in angular width.

Figure 4—Galaxies found in the galactic plane (approx coords: 240° , +4.5° ), corresponding to a density of 4500 stars per deg² brighter than 14^th mag. The sequence is arranged in order of integrated K-band flux, ranging 11.8 to 13.8 mag (reading left to right). The upper row corresponds to the J-band postage stamp image, middle row the H-band and bottom row the K-band images. Each image is 50" in angular width.

Figure 5—Galaxies found near the galactic center bulge (approx coords: 12° , +5.0° ), corresponding to a density of 30,000 stars per deg² brighter than 14^th mag. The sequence is arranged in order of integrated K-band flux, ranging 11.0 to 12.7 mag (reading left to right). The upper row corresponds to the J-band postage stamp image, middle row the H-band and bottom row the K-band images. Each image is 50" in angular width.

Figure 6—Triple (& multiple) stars found near the galactic center bulge (approx coords: 12° , +5.0° ), corresponding to a density of 30,000 stars per deg² brighter than 14^th mag. The upper row corresponds to the J-band postage stamp image, middle row the H-band and bottom row the K-band images. Each image is 25" in angular width.

Figure 7—Bright star sequence as seen in the J-band. The sequence is arranged in order of integrated J-band flux (mag units), reading left to right: 9.8, 8.7, 7.9, 7.4, 6.8, 6.4, 5.9, 5.5 & 4.1. Each image is 120" in width.

Figure 8—J-band (coadd) image of a 4^th magnitude star. The image size is 8.5 ´ 16’. Features associated with the bright star: halo emission, N-S-E-W diffraction spikes, three horizontal stripes, glints/ghosts, and persistence ghosts (trailing to the south of the star).

Figure 9—J-band (coadd) images of b Pegasus, a –1 mag star. The star lands on the in-scan boundary of two coadd images. The total area is approximately 8.5’ ´ 25’. Note the prominent halo emission, N-S-E-W diffraction spikes, three horizontal stripes, glints/ghosts (particularly to the northeast), and persistence ghosts trailing to the south of the star.

Figure 10—Meteor and bright star streaks as seen in the J-band. The images are 8.5’ across. The meteor streak is the left image and the streak associated with b Pegasus, a –1 mag star located nearly a degree away, the right image.

Figure 11—Example of "artifact" or false extended source detections. The upper panels show J-band, middle panels H-band and the bottom panels K-band. Each image is 30" in width. The first two columns are the result of a "ghost" or "glint" to the southwest of the progenitor star; 3^rd column shows a false detection due to a flared diffraction spike from a star on the edge of coadd; 4^th and 5^th columns are examples of located on or within the boundary of a horizontal stripe or meteor streak; last column shows s a faint star boosted up by background airglow emission.