II. User's Guide to the 2MASS All-Sky Data Release

3. Extended Source Catalog (XSC)

a. Extended Source Catalog Format

b. How to Use the 2MASS XSC

c. XSC Catalog Selection Criteria:

The 2MASS All-Sky Release covers >99% of the sky (see II.3d1). The Extended Source Catalog (XSC) contains 1.65 million sources which are resolved with respect to the observed 2MASS point spread function. The Catalog sources were selected to have the following properties:

• SNR(band) >= 7    (SNR=signal to noise ratio)
  where two sets of photometry were used:
  
  1. K_s-band fiducial elliptical isophotal apertures and
     
  2. 7´´ circular apertures
  the source need only satisfy the SNR criteria in one or more bands;
• e_score < 1.4    or    g_score < 1.4;
• A single apparition of multiply-detected sources in Tile overlap regions.

Users are recommended to review the Cautionary Notes for the XSC in Section I.6.c for known features of and problems in the Catalog.

d. General Properties

Approximately 97% of XSC sources are galaxies, with the other ~3% belonging to the Milky Way. The following subsections discuss some of the basic properties of the XSC, with an image gallery to follow. More information on the algorithms, derived parameters, and the catalog/database, can be found in IV.5.

• Extended Source Spatial Coverage Maps

• All-Sky Images of the XSC

• Galaxy Counts

• Colors

• Astrometry: comparison with the FIRST radio catalog

e. Photometric Properties

Differential source counts (logN vs. logS) for the entire XSC is shown in Figure 1. In Figure 2 is shown the J-H vs. H-K_s color-color Hess diagram for all XSC sources. For reference, we also include the stellar main-sequence and evolved (giant) tracks (green line), and the redshift K-correction (magenta line, assuming SAB-type galaxies with no evolution cosmology; adapted from the models by Bruzual & Charlot 1993, ApJ, 405, 538, and McLeod & Rieke 1995, ApJ, 454, 611).

Figure 1	Figure 2

f. Size Distribution

Extended sources range in size from 10´´ to nearly 2° in diameter (e.g., M31; for the parameters of additional large galaxies, see Table 1 of Jarrett et al. 2003, AJ, 125, 525). Most 2MASS XSC galaxies are small, settling near the resolution limit of 2MASS. The distributions for extended sources of the isophotal and total radii are shown in Figure 3 (galaxies) and Figure 4 (Galactic sources).

Figure 3	Figure 4

g. Relevant Images

Figure 5: The 30 Largest Galaxies in the Extended Source Catalog. Galaxies larger than ~5´ were reconstructed using large mosaics and source characterization optimized for large extended objects. A description of the methods used to build the Large Galaxy Atlas and science results using the Atlas is given in Jarrett et al. (2003, AJ, 125, 525).


Figure 6: Bright 2MASS galaxies as seen in the K_s-band. The sequence is arranged in order of integrated K_s-band flux, reading left to right. first panel: K_s magnitudes range from 9 to 10.5; second panel: 10.5 to 11.0; third panel: 11.0 to 11.6; fourth panel: 11.6 to 12.1; fifth panel: 12.1 to 12.3; sixth panel: 12.3 to 12.6; last panel: 12.6 to 12.9. Each image is 60´´ in angular width.


Figure 7: 2MASS galaxies at the K_s-band sensitivity requirement limit, K_s~13.5 mag. The sequence is arranged in order of integrated K_s-band flux, reading left to right. first panel: K_s magnitudes range from 13 to 13.1; second panel: 13.1 to 13.3; third panel: 13.3 to 13.4; last panel: 13.4 to 13.5. Each image is 30´´ in angular width.



Figure 8: Galaxies found in the Galactic Plane (approximate coordinates, in decimal degrees: 240°, +4.5°), corresponding to a density of 4500 stars deg^-2 brighter than K_s=14 mag. The sequence is arranged in order of integrated K_s-band flux, ranging from 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_s-band images. Each image is 50´´ in angular width.

Figure 9: Galaxies found near the Galactic Center bulge (approximate coordinates: 12°, +5.0°), corresponding to a density of 30,000 stars deg^-2 brighter than K_s=14 mag. The sequence is arranged in order of integrated K_s-band flux, ranging from 11.0 to 12.7 mag (reading left to right). The upper row corresponds to the J-band postage stamp image; the middle row, the H-band; and, the bottom row, the K_s-band images. Each image is 50´´ in angular width.

Figure 10: Galactic extended sources. JHK_s three-color images of H II regions (upper panel), clusters and nebulosity (middle panel; far right image corresponds to a piece of the Crab Nebula), reflection nebulae, and young stellar objects (bottom panel).


[Last Updated: 2003 Mar 13; by T. Jarrett and R. Cutri]