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The extended source catalog (XSC) contains over 1.6 million spatially resolved astronomical sources: primarily > 98% galaxies, and to a lesser extent Galactic diffuse nebulae, H II regions, stellar clusters, planetary nebulae and young stellar objects. The galaxy catalog was constructed to satisfy the survey science requirements (Jarrett et al 2000), most important being the reliability (~ 99%) and completeness (> 90%) for unconfused regions of the sky, |b| > 20 deg. These requirements are achieved for sources brighter than Ks = 13.5 mag (~ 2.7 mJy) and resolved diameters larger than ~ 10-15 arcsec. The differential source counts, Figure 3, illustrate the depth and areal coverage of the 2MASS galaxy catalog for unobscured regions of the sky. The Ks-band counts are steady up to ~14 mag, with the faint end supported by the more sensitive J-band observations (complete down to 15.3 mag).

Figure 3

Figure 3. Total number of galaxies per deg2 per mag internal for |b| > 25 deg (total area: ~ 24,000 deg2). The J, H and Ks source counts are represented with blue, green and red lines, respectively. For comparison, the narrow (but deep) galaxy counts of Glazebrook et al (1994) and Gardner et al (1997) are shown in black.

Morphology - As a function of Hubble or morphological type, 2MASS is most sensitive to early-type spirals and ellipticals (whose light is dominated by the older population of stars emitting in the near-infrared), and less sensitive to late-type spirals (whose light is dominated by the younger, hotter disk population), dwarfs (low surface brightness) and compact objects (resolution limitations of 2MASS); see Jarrett (2000) and Jarrett et al (2003). Consequently, the 2MASS galaxy catalog is partial to normal "old" galaxies, which are typically lower in mass-to-light compared to gas-rich spiral galaxies (see section 4 below).

Zone of Avoidance - A primary science driver of 2MASS was to penetrate the dust mask of the Milky Way. At near-infrared wavelengths, the opacity of dust is significantly smaller (~ 1/10) compared to that of optical wavelengths, and thus amenable to penetrating observations. The Galactic "zone of avoidance" (ZoA) is still, however, a formidable barrier due to the sheer number of stars that block background light as well as produce a foreground (confusion) "noise." Near the center of the Milky Way the confusion noise is extreme, blocking nearly 100% of the background light; whereas far from the Galactic center the confusion noise is minimal and the veil of the Milky Way is lifted at near-infrared wavelengths. As is shown in Figure 4, 2MASS penetrates deep into the ZoA, extracting galaxies well down to |b| ~ 5 degrees, where detection completeness declines by ~ 0.5 mag (Jarrett et al 2000).

Figure 4

Figure 4. Cumulative number of galaxies per deg2 with Galactic latitude. Three Ks-band flux limits are shown: 12.5 (black), 13.0 mag (green) and 13.5 mag (blue). All longitudes are used to draw the galaxy sample. For comparison, the integrated counts per deg2 for a field deep in the ZoA (l = 50°, b = 2°) is indicated with a green triangle.

All Sky Galaxy Distribution - Simply by counting the number of galaxies along the line of sight it is a straightforward exercise to create a crude map of the local Universe. Spatial over-densities from galaxy clusters trace the large scale structure - see for example the beautiful maps of Courtois et al (2004) who constructed the extragalactic sky using galaxies archived in LEDA. A more constructive way to map the surface density of galaxies is to integrate the 2µm flux from all galaxies along the line of sight, thereby weighting the nearest structures and producing contrast between the Local Supercluster (cf. Tully 1982; Tonry et al. 2000) and the more distant "cosmic web" structures; see Figure 5. This technique mitigates the biasing effects of non-uniform incompleteness due to surface brightness differences and galaxy morphology (see above). The figure illustrates how 2MASS creates a uniform view of the local Universe, except for the extreme Galactic Center, bridging the two hemispheres above and below the plane of the Milky Way (center region of figure).

Figure 5

Figure 5. Equatorial view of the 2MASS galaxy catalog (6hr RA at center). The grey-scale represents the total integrated flux along the line of sight - the nearest (and therefore brightest) galaxies produce a vivid contrast between the Local Supercluster (center-left) and the more distant cosmic web. The dark band of the Milky Way clearly demonstrates where the galaxy catalog becomes incomplete due to source confusion. Some well known large-scale structures are indicated: P-P = Perseus-Pisces supercluster, H-R = Horologium-Reticulum supercluster, P-I = Pavo-Indus supercluster, GA = "Great Attractor", GC = Galactic Center, S-C = Shapley Concentration, O-C = Ophiuchus Cluster, Virgo/Coma/Hercules = Virgo-Coma-Hercules superclusters. A 3-dimensional, Galactic projection, version of the 2MASS galaxy catalog is shown in Figure 1.

A further enhancement to the all sky maps is to color-code the galaxies according to their total integrated flux. Since the integrated flux is strongly correlated with the distance to the object (assuming 2MASS galaxies have roughly the same luminosity; see the next section), the color-coding effectively adds depth to the surface density maps. This is a "poor man's" redshift, which conveniently provides a qualitative view of the 3-D galaxy distribution, illustrated in Figure 6 using a Supergalactic projection. This crude but effective method delineates real large scale structure in the local Supercluster and beyond (Figure 6). But we can go one step further by combining the integrated flux with the measured galaxy near-infrared colors to derive a photometric redshift, described in the next section.

Figure 6

Figure 6. Supergalactic projection of the 2MASS galaxy catalog. Sources are RGB color-coded according to their Ks-band integrated flux: brightest galaxies (Ks < 10) are blue, moderately bright galaxies (10 < Ks < 12.5) are green, and the faintest (Ks > 12.5) are red. The Local Supercluster as defined by de Vaucouleurs et al (1976) extends along the equator crossing through the Virgo galaxy cluster (bright blue cluster, left-center of image). The Milky Way is shown with a cyan color scale, crossing the center of the image and outer boundary. The Perseus-Pisces supercluster extends from the center right (northern Galactic hemisphere) down into the ZoA and out the other side to the southern Galactic hemisphere.

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