II. Contents of the 2MASS Second Incremental Data Release

3. Extended Source Catalog (XSC)

a. Extended Source Catalog Format

b. XSC Catalog Selection Criteria:

• J <= 15.1, or, K_s <= 13.5
• SNR(J) >= 7, or, SNR(K_s) >= 7 (SNR=signal-to-noise ratio)
• 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.

c. General Properties

Approximately 90% of XSC sources are galaxies (particularly as measured with the G_score). Approximately ~2-5% of the catalog are non-galaxies in low confusion areas, consisting of either double stars or artifacts (e.g., pieces of bright stars). The following plots illustrate some of the basic properties of the XSC. An image gallery is given at the end of this page. More information on the algorithms, derived parameters, and the catalog/database, can be found in Section IV.5.

Figure 1 shows the sky distribution of sources in the Second Incremental Release XSC. The map shows the density of XSC sources averaged in 30' bins in a galactic aitoff projection, with an equatorial coordinate grid overlaid.

Figure 2 shows the fractional distribution of 2MASS XSC sources with galactic latitude. Note that the source counts are not normalized by the areal coverage.

Figure 3 shows representative differential extended source counts from low stellar confusion fields, <1000 stars deg^-2 brighter than mag=14 at K_s. The error bars represent sqrt(N) uncertainty measures. The sources comprise a total areal coverage of ~2350 square degrees. Most of the area is free of known nearby clusters (redshift z < 0.1), but a small percentage does include the Abell 262 cluster in Perseus and the outer fringes of the Virgo cluster. For comparison, deep but narrow-field counts from Glazebrook et al. (1994, MNRAS, 266, 65), denoted with a dotted line, and Gardner et al. (1997, ApJ, 480, L99), denoted with "+" symbols, are shown.

Figure 1	Figure 2	Figure 3

d. Photometric Properties

Figure 4 is the color-color distribution for extended sources located within low stellar confusion fields, <1000 stars deg^-2 brighter than mag=14 at K_s. Galaxies are indicated by symbols coded according to the J-K_s SNR. The magenta solid line denote the main sequence locus (giants diverge as the magenta dashed line at larger J-H). The cyan dashed line and triangle points denote the K-correction curve for a spiral galaxy (no evolution), where each triangle represents 0.1 in redshift (z).

Figure 5 is the color-color distribution for extended sources located within moderate stellar confusion fields, 1200 - 4000 stars deg^-2 brighter than mag=14 at K_s.

Figure 6 is the color-color distribution for extended sources located within moderate stellar confusion fields, >4000 stars deg^-2 brighter than mag=14 at K_s. Extinction from obscuring dust in the Milky Way is a significant reddening component. Approximately half of the sources are double or triple star clusterings.

Figure 4	Figure 5	Figure 6

e. Size Distribution

Figure 7 shows the angular size of extended sources as a function of K_s. The top panel shows the size distribution (in arcsec) of the semi-major axis radius corresponding to the best fit ellipse of the 20 mag arcsec^-2 K_s-band isophote. The minimum radius corresponds to 7´´ (horizontal dashed line). The middle panel shows the size distribution of the semi-major axis radius corresponding to the best fit ellipse of the 21 mag arcsec^-2 J-band isophote. The bottom panel shows the semi-major axis radius corresponding to the 3 K_s-band isophote, corresponding to roughly 18.8 mag arcsec^-2 surface brightness. The minimum 3 radius is ~5´´. Note: for the 3 distribution, only about half of the sources are plotted.

Figure 7

f. Relevant Images

Figure 8: Examples of large 2MASS Virgo galaxies as seen in composite JHK_s. Each image is 101´´ in angular size, the maximum size for 2MASS extended source "postage-stamp" images. Reading left to right: (top panel) Messier 100, Messier 98, NGC 4340, NGC 4189; (bottom panel) NGC 4305, NGC 4344, IC 769, NGC 4165.


Figure 9: 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. 1st panel: K_s mags range from 9 to 10.5; 2nd panel: 10.5 to 11.0; 3rd panel: 11.0 to 11.6; 4th panel: 11.6 to 12.1; 5th panel: 12.1 to 12.3; 6th panel: 12.3 to 12.6; last panel: 12.6 to 12.9. Each image is 60´´ in angular width.


Figure 10: 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. 1st panel: K_s mags range from 13 to 13.1; 2nd panel: 13.1 to 13.3; 3rd panel: 13.3 to 13.4; last panel:13.4 to 13.5. Each image is 30´´ in angular width.



Figure 11: Galaxies found in the Galactic Plane (approx coords: 240°, +4.5°), corresponding to a density of 4500 stars deg^-2 brighter than K_s mag=14. 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 12: Galaxies found near the Galactic Center bulge (approx coords: 12°, +5.0°), corresponding to a density of 30,000 stars deg^-2 brighter than K_s mag=14. 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 13: 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: 2000 March 6; by T. Jarrett and R. Cutri]