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

3. Extended Source Catalog (XSC)

d. General Properties

iv. 2MASS XSC Colors

The fiducial elliptical isophotal (K_s=20 mag arcsec^-2) magnitudes are used to construct the colors J-H, H-K_s, and J-K_s. The XSC sources (1.65 million total) are subdivided in Table 1 according to the stellar number density (see IV.5c), which measures the surface density of stars against which the extended object is projected.

Table 1: Density vs. XSC Statistics

density_min density_max % of 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°

Table 1: Density vs. XSC Statistics
density_min	density_max	% of 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°

J-K_s Colors

Here we demonstrate the distribution in J-K_s color as a function of source brightness. Figure 1 shows the distribution in this color for the entire XSC. Figure 2 shows the distribution for the magnitude bin K_s<10.0. Figure 3 shows the distribution for the magnitude bin 10.0 < K_s < 11.0. Figure 4 shows the distribution for the magnitude bin 11.0 < K_s < 12.0. Figure 5 shows the distribution for the magnitude bin 12.0 < K_s < 12.5. Figure 6 shows the distribution for the magnitude bin 12.5 < K_s < 13.0. Figure 7 shows the distribution for the magnitude bin 13.0 < K_s < 13.5. Figure 8 shows the distribution for the magnitude bin 13.5 < K_s < 14.0. Figure 9 shows the distribution for the magnitude bin K_s > 14.0.

Figure 1 Figure 2 Figure 3

Figure 4 Figure 5 Figure 6

Figure 7 Figure 8 Figure 9

J-H vs. H-K_s Distribution vs. Density

Here we demonstrate the color distribution of XSC sources as a function of the stellar density. 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, McLeod & Rieke 1995). Figure 10 shows the J-H vs. H-K_s color-color Hess diagram for the entire XSC. Figure 11 shows this diagram for density < 3.2. Figure 12 shows this diagram for 3.2 < density < 3.6. Figure 13 shows this diagram for 3.6 < density < 4.0. Figure 14 shows this diagram for density > 4.0.

Figure 10 Figure 11 Figure 12

Figure 13 Figure 14

To further illustrate the dependence of source color on density, but to avoid source saturation in the diagrams, we reduce the number of points shown by fixing the number to 2000 per mag bin interval defined as:

where we have selected the points shown at random. These figures, below, should be an adequate representative of the XSC as a whole. Figure 15 shows the color-color diagram for density < 3.2. Figure 16 shows the diagram for 3.2 < density < 3.6. Figure 17 shows the diagram for 3.6 < density < 4.0. Figure 18 shows the diagram for density > 4.0. Figure 19 shows the diagram for density > 4.0, with expanded axes to illustrate the extreme reddening.

Figure 15 Figure 16 Figure 17

Figure 18 Figure 19

Discussion of Color Outliers

Sources with colors that significantly deviate from the "norm" are either (1) intrinsically "red", or (2) characterized by one or more inaccurate fluxes.

Intrinsically red sources are mostly confined to the Galactic Plane (see, for example, the all-sky distribution of XSC sources in Figure 1 of II.3d2), and include HII regions, nebulosity, star formation regions (with young stellar objects) and other deeply-embedded Milky Way objects. Galaxies may also be very red, either from foreground dust extinction or associations with active galactic nuclei (AGN) or even from significantly redshifted colors (particularly in the H-K_s color). See the Near-Infrared Galaxy Morphology Atlas for discussion of the different Hubble galaxy types as imaged in the near-infrared.

The following figures show examples of "red" Milky Way objects: Figure 20 shows general nebulosity. Figure 21 shows a young stellar object. Figure 22 shows an HII region.

Figure 20 Figure 21 Figure 22

The following figures show examples of "red" galaxies: Figure 23 is Markarian 1239, Hubble type E/S0 with Sy 1.5 characteristics. Figure 24 is Hurt 1, Hubble type Sd, at very low galactic latitude. Figure 25 is an anonymous galaxy at (ra,dec)=(345.401123, -59.222527). Figure 26 is an anonymous galaxy at (204.782120, -30.252445); note that it is contaminated by a neighboring bright star; see other examples below. Figure 27 is an anonymous galaxy at (219.206711, -16.228071). Figure 28 is an anonymous galaxy at (0.101620, -12.763147).

Figure 23 Figure 24

Figure 25 Figure 26 Figure 27 Figure 28

Examples of "Blue Fuzzies":

Some examples of "blue," "fuzzy" objects are NGC 7793, Hubble Type SA(s)d in Figure 29; a comet at (92.836586, 36.640507) in Figure 30; a comet at (213.217377, -15.222510) in Figure 31; and, a galaxy-star superposition at (192.828003, -30.317444); Figure 32.

Figure 29 Figure 30 Figure 31 Figure 32

Examples of Sources with Inaccurate Colors:

Sources with inaccurate colors fall into the catagories of (1) confused sources, (2) those contaminated by bright star masking (usually the J-band is the worst offender; see II.3d1for discussion of bright star masking); (3) cases where the isophotal apertures were too large (induced by background removal problems, confusion, contamination from bright star artifacts, etc.), and, (4) mis-characterized objects (e.g., pieces of galaxies, artifacts, double galaxies, etc.). See also IV.5e.

Examples include a source at (269.775452, -28.816984) corrupted by the J-band mask of a bright source (to the south) and from neighbor confusion, shown in Figure 33; a source at (346.893738, -0.293708) corrupted by a J-band horizontal stripe mask, in Figure 34; a source at (174.057327, -55.517132) in close proximity to a bright star (where the J-band is affected), in Figure 35; a source at (230.064285, -69.028946) corrupted by a J-band diffraction spike, in Figure 36; a source at (171.853912, -50.672760) corrupted by J-band halo masking, in Figure 37; a source at (175.922165, -52.398312) corrupted by J-band halo/diffraction spike masking, in Figure 38; and, sources with incorrect isophotal photometry, due to far too large an aperture, introducing critical stellar contamination, at (130.537003, 22.751608) and at (139.921173, 80.359772), respectively, shown in Figures 39 and 40.

Figure 33 Figure 34 Figure 35 Figure 36