2MASS XSC Source Counts

Differential source counts (logN vs. logS) for the XSC are presented below. The sky is divided into four Galactic zones:

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.

The source counts for zones 1 and 2 (i.e., |Glat| > 10 deg) are a direct measure of the galaxy counts in the local Universe. But only Zone 1 (i.e., |Glat| > 25 deg) is subject to the Level-1 Science Requirements for galaxy completeness and reliability. For zones 3 & 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), and include HII regions, nebulosity, star formation regions (with Young Stellar Objects) and other deeply embedded Milky Way objects. See also Visual Inspection of Extended Sources for examples of the kinds of extended sources within the XSC.

The photometry used for constructing the source counts is the fiducial elliptical isophotal (20 mag arcsec-2 in the Ks band) integrated fluxes. These mags represent approximatetly 85% of the total flux of a galaxy; see Large Aperture Photometry: Profile Fitting, Isophotal & Total Mags.

a. All Sky Source Counts for XSC


b. All Sky Source Counts for XSC Galaxies

Here the known Galactic sources have been removed, leaving only galaxies in the differential source counts.

c. Cluster Source Counts

Here the known Galactic sources have been removed, leaving only galaxies in the differential source counts.

[Last Updated: 2003 Jan 29; by Tom Jarrett]