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:

Log [number of stars deg^-2] brighter than 14th mag in the Ks band (2.2 microns).

The density is an excellent measure of confusion noise for most of the sky (>95%). It breaks down (saturates) for densities greater than 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 brighter than 14th magnitude. Also source blending renders the integrated flux unreliable. Read more about the 2MASS Confusion Noise 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 Visual Inspection of Extended Sources. For the XSC, we typically divide the samples according to the following density bins:

Density vs. XSC

density min	density 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 & development, but is clearly only a rough guide (see Density vs. MW image 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.

a. Density vs. Milky Way

• Aitoff Projection of the XSC in Galactic Coordinates

  

  showing the XSC marked on the sky and coded according to the foreground stellar number "density" for each source:

  density < 3.2 (light gray points) corresponds to roughly |Glat| > 25 deg
  3.2 < density < 3.6 (gray points) corresponds to roughly 10 < |Glat| < 25 deg
  3.6 < density < 4.0 (dark gray points) corresponds to roughly 5 < |Glat| < 10 deg
  density > 4.0 (black points) corresponds to roughly |Glat| < 5 deg
  
  Tidbits: 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]