IV. 2MASS Data Processing

5. Extended Source Identification and Photometry

c. Stellar Number Density Metric: "density"

Point source counts are used as a proxy for confusion noise in 2MASS data processing. The "density" star count metric is defined to be:

The density is an excellent measure of confusion noise for most of the sky (>95%). It breaks down (saturates) for density > 4.5, corresponding to the Galactic center region. The primary reason the density parameter saturates is that the detection threshold rises with increasing confusion, so, for the very highest source density regions, the detection becomes incomplete, even for brighter than 14th magnitude. Also source blending renders the integrated flux unreliable. More information about the 2MASS Confusion Noise is available here.

The density metric is also used to delineate the XSC for analysis purposes into subsets that have common completeness and reliability properties. For example, the Level-1 Science requirements apply to the "density" corresponding to minimal confusion noise from foreground (Milky Way) stars (but see note below). While the XSC with the highest density generally is confined to the Plane of the Galaxy -- populated by extended Milky Way objects (e.g., HII regions) and false positives (e.g., triple stars); see IV.5c1. For the XSC, we typically divide the samples according to the density bins given in Table 1.

Table 1: Density vs. XSC
density mindensity 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

Note: The Level-1 Science Requirements use the galactic latitude (Glat) as a proxy to the confusion noise. This was a convenient assignment at the time of the 2MASS design and development, but is clearly only a rough guide (see Figure 1 below). The "density" parameter is a much better quantitative measure of the confusion noise and therefore should be employed when considering the completeness and reliability of a catalog.

Figure 1 is an Aitoff projection of the XSC in galactic coordinates, coded according to the foreground stellar number "density" for each source:

Figure 1

The LMC/SMC are clearly seen as high density points 30 to 45 degrees from the Galactic Plane; the Galactic Center (0,0 degrees) is mostly devoid of XSC objects due to the extreme density of stars (the white gap centered about the Galactic Center); notice the spur of high density Orion Nebula points at (210,-20); the Great Attractor field, including the galaxy cluster Abell 3627, located at (325.3, -7.2), corresponds to a very high source density: density ~4 to 4.3; the traditional "Zone of Avoidance" for extragalactic work is |Glat| > 30 deg, corresponding to a density boundary between 3.0 and 3.2.

[Last Updated: 2002 Aug 5; by Tom Jarrett]

Return to Section IV.5.