The extended source catalog must ultimately be very reliable (>98% for a large fraction of the sky) while also maintaining maximum completeness. For a description of the level-1 specifications for extended sources, see T. Chester's memo: Brief Summary of 2MASS Facts. The database, on the other hand, should be optimized for completeness since the catalog will be a subset of this data set. Our goal, that is, the goal of GALWORKS, is to extract a complete set of galaxies, while minimizing contamination from non-extended sources (since we are limited in disk space and cpu power, both hampered by false galaxies).
Generally, to increase one measure, it is only possible at the expense of the other measure (analogous to the Heisenberg "uncertainty principle" ). In addition, the value of "reliability" is a bit of a misleading measure since it pits galaxies vs. stars -- but galaxies have a spatial correlation, while stars are smoothly distributed in space (according to the disk and spheroid populations in the Milky Way). Thus, to maximize the reliability measure, one should look at a galaxy cluster, while also looking toward a low stellar density region -- Coma for example. The eight Coma repeat scans provide an excellent measure of our reliability because of the large number of galaxies of all shapes and sizes. And of course they represent a direct measure of our detection repeatibility -- which approximately reflects completeness.
Reliability
The Coma extended source database consists of 1784 objects, most of which are real galaxies (a lot of them!), some are false detections (e.g., stars, meteor streaks) and the remainder are unknown sources (generally too faint to classify). Each source has been visually inspected and classified as such. The results are given below.
The reliability is measured as follows:
Reliab = Ngalaxies / (Nfalse + Ngalaxies)
The bright false detections arise from the transient seeing change in scan 052; see Coma Seeing and the relevant seeing plot Seeing, scan 052. There is not much we can do about these sources other than throw out the offending scan -- the recommended policy for the survey.
There are far more galaxies than stars or artifacts, and the galaxies also are much more numerous than the "unknown" (faint) sources. The reliability reflects this, >95% to 99% for J brighter than 14, H brighter than 14, and K brighter than 13.8 or so. The reliability can probably be increased another 1% or 2% by increasing some of the key scoring thresholds, for example the "wsh" (see Star - Galaxy Discrimination) without sacrificing completeness to an unsatisfactory extent (see below).
The normalized galaxy counts (number per sq. degree) is given below.
Completeness
The detection repeatibility can provide a good estimate of the completeness since our sample consists of galaxies with a variety of shapes, morphology and size. Since there are eight scans, we expect at most eight detections (although it is possible for more detections, since there is also in-scan overlap between coadds, but this area is small). A small fraction of the total area is lost near the edges of the field since the scans have a slight cross-scan position difference (a few arc seconds -- the largest departure is about 25 arcsec) -- this affect has been accounted for.
The brightest galaxies, J10 < 13, consist of the giant ellipticals and several large satellites thereof, near the Coma center (see images below). As such, the repeatibility can be less than 100% because giant elliptical sometimes "eat" their smaller (but luminous) companions. If you ignore the problems with dense clustering of galaxies near the Coma core, the galaxy detections repeat 100% of the time for J10 < 14.5, H10 < 14.1, K10 < 13.5 or so. The level-1 specification requires better than 90% completeness for K < 13.5 or so. It appears that we achieve this goal -- at least with this set of "database" galaxies. We can probably sacrifice some completeness for reliability to satisfy the goals of the extended source catalog. Finally, note that the repeatibility drops quickly when we reach J = 14.6, H = 14.3 and K = 13.9 to 14.0. These limits represent the sensitivity limits of the 2MASS survey for galaxy detection.
Postage Stamp Images of the Brightest Coma Galaxies
The following images show the JHK + DSS stamp images of bright Coma galaxies, including the ellipticals near the Coma core and some isolated big spirals. It is some of the satellite galaxies that repeat less than 100% of the time (because they are included in the flux of their bigger companion friends).
