Each extraction in the Survey Extended Source Reject Table, and the 6x and calibration scan extended source Working Databases (XSWDBs) is assigned a reliability score, rel, that is related to the probability that the extraction is a detection of a real astrophysical source at the time of the 2MASS observation and that the source is truly extended with respect to the instantaneous point spread function. The reliability score is a single character with values A-F that maps into a probability range according to Table 1.
The algorithm for assigning the
reliability score to extended source
extractions is based on a large, empirically-developed
"truth table"
of WDB extractions whose images have been visually examined
to assess whether or not they are valid source detections
and truly extended rather than single or multiple stars.
The empirical relationships that describe how the fractional reliability
of the ensemble changes
with source parameters are converted
into a matrix of scoring rules that are used
to assign reliability scores to each extended source extraction.
| rel | Probability Range |
|---|---|
| A | P>90% |
| B | 80<P<90% |
| C | 70<P<80% |
| D | 50<P<70% |
| E | 20<P<50% |
| F | P<20% |
An internal (to 2MASS) "truth table" for developing the extended source reliability scoring algorithm was built by visually examining and classifying over 700,000 extended source extractions from the Survey, 6x and Calibration WDBs that span diverse range of source parameters. The examination and classification process is described in IV.5.a.iv
Visual examination consisted of viewing a J+H+Ks
three-color composite image of the extraction, tabular data that summarizes
source size, shape and photometry, and when available an optical of the object
from the Digitized Sky
Survey. Based on the images and tabular data, each entry
was classified as fitting into one of several categories that include
galaxies, Galactic nebulae, single or multiple point sources,
artifacts or not unambiguously classifiable. The results
of the visual classification are included in the
vc column in the Survey XSRT,
6x XSWDB/XSC and
Calibration XSWDB.
A set of reliability matrices were generated by compiling the empirically determined net reliability of the truth table extractions as a function of galaxy brightness and e_score/g_score extended source "pseudo-probabilities" in several foreground source density ranges. Separate matrix sets were generated for the Survey and 6x truth tables. The Survey reliability matrices were used to assign reliability scores to the Calibration XSWDB entries.
Tables 2-13 and 14-22, linked below, contain the reliability matrices for the Survey and 6x XSWDB samples, respectively. These tables give the number of objects in each classification category in 0.5 mag brightness bins, and the for three ranges of e_score/g_score. Within each e_score/g_score range, tables are presented for four (Survey) or three (6x) ranges of foreground stellar density environments. Reliability was computed as a function of J-band magnitude in the fiducial Ks=20 mag arcsec-2 elliptical isophote (j_m_k20fe) for the Survey extractions, and as a function of Ks-band fiducial elliptical isophotal magnitude (k_m_k20fe) for 6x extractions. The stellar foreground density ranges in the Survey reliability matrices are: low - den < 3.0, low-to-moderate - den < 3.6, moderate-to-high, and high - den > 4.0, where den is the stellar number density metric equal to the logarithm of the number of point source extractions with Ks<14.0 mag in the field. For the 6x matrices, the stellar density ranges are: low - den < 3.0, moderate - 3.0 < den < 3.6, and high - den > 3.6.
The last two columns in each reliability matrix summarize the reliability in each magnitude bin:
Reliability (R) = 1.0 - (N_false / N_false + N_true)
where N_false is the total number of "false" extended sources
(e.g.stars, artifacts, double/triples) per mag bin interval, and
N_true is the total number of "catalog" extended sources per
mag bin interval.
Adjusted Reliability (Radj) = 1.0 - (N_false_expected / N_true_expected)
This is the reliability after factoring in knowledge of "false" sources
based on the visual examination; where
N_false_expected is total number of "expected" false sources minus the
total number of known false sources, and N_true_expected is the total
number of "expected" true extended sources. The rate of true extended sources
is the total number of known true extended sources divided by the
total number of sources (both true and false). Note that for mag intervals
in which the visual inspection is fully complete, one can in principle
construct a catalog that is nearly 100% reliable.
The reliability score for each extraction in the Survey XSRT, 6x XSWDB/XSC and Calibration XSWDB is determined as follows:
 |
| Figure 1 - Examples of rel="F" "galaxy chaff" associated with M31 and M32 in the Survey Extended Source Reject Table. Extractions of knots and fragments of these two large galaxies are shown by the green circles. The red circles denote the entries for the two galaxies in the All-Sky XSC. |
[Last Update: 2006 October 2, by T. Jarrett and R. Cutri]