iii. 2MASS XSC Source Counts
Differential source counts (logN vs. logS) for the XSC are presented below. The sky is divided into four Galactic zones: |glat| > 25°, 10° < |glat| < 25°, 5° < |glat| < 25°, and |glat < 5°.
Density vs. XSC
| density min | density max | %XSC | |Glat| min | |Glat| max |
|---|---|---|---|---|
| 0.0 | 3.2 | 73.4% | ~25 | 90 |
| 3.2 | 3.6 | 19.0% | ~10 | ~25 |
| 3.6 | 4.0 | 6.9% | ~5 | ~10 |
| 4.0 | 4.5 | 0.7% | 0 | ~5 |
The first zone represents the clearest view of the extragalactic Universe, subject to minimal stellar confusion noise and mostly free of Galactic (Milky Way) contamination. The final three zones comprise the traditional "Zone of Avoidance," characterized by significant confusion noise (and, hence, lower sensitivity for detection) and contamination from Milky Way stars. See the discussion on confusion noise and stellar number density in IV.5c.
The source counts for zones 1 and 2 (i.e., |glat| > 10°) are a direct measure of the galaxy counts in the local Universe. But only Zone 1 (i.e., |glat| > 25°) is subject to the Level-1 Science Requirements for galaxy completeness and reliability. For zones 3 and 4 (i.e., |glat| < 10°), the source counts are a mixture of galaxies and Milky Way extended objects: Extended sources associated with the Galaxy are highly confined to the Galactic Plane (see, for example, the all-sky distribution of XSC sources in Figure 1 of II.3c2), and include HII regions, nebulosity, star formation regions (with young stellar objects) and other deeply embedded Milky Way objects. See also the visual inspection of extended sources in IV.5e for examples of the kinds of extended sources within the XSC.
The photometry used for constructing the source counts is the fiducial elliptical isophotal (Ks=20 mag arcsec-2) integrated fluxes. These magnitudes represent approximately 85% of the total flux of a galaxy; see the discussion of large aperture photometry, profile fitting, isophotal and total magnitudes in IV.5a1.
Figure 1 shows differential sources counts for |glat| > 25°, where the total number of sources= 1,096,398. Figure 2 shows differential sources counts for 10° < |glat| < 25°, where the total number of sources= 404,654. Figure 3 shows differential Sources Counts for 5° < |glat| < 10°, where the total number of sources= 92,166. Figure 4 shows differential sources counts for |glat| < 5°, where the total number of sources= 54,089.
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| Figure 1 | Figure 2 | Figure 3 | Figure 4 |
See Figure 5 for an all-sky map, showing the locations of various clusters. Figure 6 shows the source counts for the Virgo Cluster of galaxies, with z = 0.004. Figure 7 shows the counts for the Abell 262 Cluster of galaxies (Perseus-Pisces Supercluster), with z = 0.016. Figure 8 shows the counts for the Abell 3627 Cluster of galaxies (Great Attractor Field), with z = 0.016. Figure 9 shows the Coma Cluster of galaxies, with z = 0.023. Figure 10 shows the counts for the Abell 2052 Cluster of galaxies, with z = 0.035. Figure 11 shows the counts for the Hercules Cluster of galaxies, with z = 0.037. Figure 12 shows the counts for the Abell 3558 Cluster of galaxies, with z = 0.048. Figure 13 shows the counts for the Abell 2670 Cluster of galaxies, with z= 0.075. Finally, Figure 14 shows the counts for the Abell 2721 Cluster of galaxies, with z= 0.114.
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| Figure 5 |
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| Figure 6 | Figure 7 | Figure 8 |
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| Figure 9 | Figure 10 | Figure 11 |
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| Figure 12 | Figure 13 | Figure 14 |
[Last Updated: 2003 Feb 25; by T. Jarrett]