I. Introduction
6. Cautionary Notes
c. Extended Source Catalog (XSC)
i. Definition of Extended Sources
2MASS pipeline processing attempts to identify all sources that are not well-fit by just a single point-spread-function (PSF). Therefore the database from which the XSC was selected includes objects made out of multiple stars that are close together, such as double and triple stars; artifacts around bright stars due to non-flat background around such bright stars; meteor and plane streaks; as well as true extended objects.
We have attempted to reject objects that are not truly extended sources, but the algorithms we have used so far are not perfect, and some of these objects remain.
The XSC is the union of two separate catalogs: an "extended source catalog"
("E" sources) using only spatial extent measures to select (and reject)
objects, plus a "galaxy catalog" ("G" sources) that used the
observed source color as an additional discriminant between galactic
and extragalactic extended objects.
For each candidate extended source, a decision tree algorithm was used to
operate on nine parameters (seven spatial extent measures, the total
brightness, and a measure of source symmetry) in each band separately to
classify a source as either "extended" (
e_score = 1) or "not extended"
(e_score = 2). The scores for each band were weighted by SNR and averaged to
produce a final single e_score per source that ranges from 1.0 to 2.0.
The same process was repeated, with the addition of the source colors to
the other parameters used for the "E" score, to result in a "galaxy"
classification, with a final single
g_score per source that ranges from 1.0
("galaxy") to 2.0 ("not a galaxy").
The criteria for the catalog was then "e_score <= 1.4 OR g_score <=
1.4". 85% of sources pass both criteria, but 15% pass only one of them. Users
can work with only the "E" or "G" subset of the XSC by selecting on those
scores.
The parameters of the xsc were tuned to meet the Level 1 Science
requirements of
99% reliability for "G" sources above point source densities representative
of glat ~ 20° and 80% reliability for "G" sources at glat ~ 10°.
Reliability is defined here as the percentage of sources which are truly
extended sources, counting multiple point sources as "unreliable" extended
sources.
For higher source densities, the emphasis shifted to completeness, at the
expense of reliability. Hence the reliability of "G" sources may be as low
as 30% at the highest source densities.
No requirements were placed on the reliability of "E" sources, and hence a
higher percentage of them are composed of multiple point sources. However,
as noted by the 85% overlap between "E" and "G" sources, the reliability of
"E" sources is only slightly lower than "G" sources.
The problems of bright stars mentioned above (primarily scattered light and
diffraction spikes) create vast numbers of spurious extended source detections.
Hence it was necessary to reject more area around bright stars for the XSC
in order to meet reliability requirements. Perhaps ~5% of the entire sky
is excluded from the XSC due to bright star problems.
As mentioned above, the parameters for some bright stars were not known for
this processing (due to saturation and bright sources from neighboring
so-far-unprocessed scans), and hence some bright star artifacts remain in
this interim release.
The size of the excluded regions was shrunk in high source density regions
due to the increased confusion noise and to meet our goal of completeness
in those regions. Hence bright stars may be the source of artifacts in
high source density regions that may have been excluded from the XSC if the
sources had been in a region of lower source density.
When in doubt about a given source, consulting the Atlas Image will usually
immediately reveal whether a source is an artifact or not.
Some other artifacts, such as a discontinuity in the electronic background
on one side of an Atlas Image, a streak across an Image due to a data
error, and emission variation caused by insects on the camera lens remain
in this release, although most of them have been identified and removed
prior to catalog generation.
As these artifacts become identified, the user can check the
"anomaly list"
to reject them. Artifacts in general, and sources that are not truly
extended, can be rejected through the "feye" flag, which is the result of
human classification of the image. Currently, 1.5% of the sources are
identified as artifacts or not truly extended sources, and 12% are
identified as being truly extended sources, with the remaining sources not
yet classified by a human.
This flag is totally separate from any aspect of the catalog generation,
and can be used as a means of quickly assessing the reliability of any
given selection criteria imposed on the catalog.
Seeing variations causes the observed PSF to change, and the pipeline
attempts to follow such PSF changes in order to properly discriminate
extended sources from point sources. However, it is not possible to follow
rapid seeing changes, which usually results in some point sources falsely
identified as extended sources.
Post-processing analysis estimates how well the seeing has been tracked.
We have eliminated the worst cases of untracked seeing from the catalog,
but less egregious cases were allowed in this release because analysis has
not yet been done to refine the threshold for rejection. We estimate that
this may contribute an extra ~1-2% unreliability to the XSC, mostly for the
"E" sources.
Extended sources are sensitive to a wider spectrum of noise sources than
point sources, which are affected only by high spatial frequency noise.
Some of the known noise sources are mentioned below.
Atmospheric Airglow Emission
Most of the time, the extended source background-removal algorithm removes
any background variation at J and Ks such that the residual noise
in the Images is usually consistent with the measurement error. However,
OH airglow emission variations
contribute extra noise in the H-band roughly equal to the measurement
error.
The photometric error due to airglow noise at H varies strongly with time
and spatial position and with the total brightness and size of an extended
source. A statistical analysis of galaxies with H = 13.8 mag shows that
about 25% of all sources have a measurable increased uncertainty which is
correlated with the measured sigma of the background-removed pixel
distribution.
We have therefore eliminated scans from this release that have large
measured sigmas. Statistically, sources from Atlas Images with a residual
background sigma of just under 1.20 DN, the highest value allowed in this
release, have an extra H photometric error equal to their Poisson
uncertainty of ~0.10 mag, making the total photometric error ~0.15 mag.
No correction of the photometric uncertainties has been made for this
statistical analysis result.
Electronic Noise
Electronic noise with spatial periods of 50-75" is sometimes present in the
Atlas Images. Preliminary analysis shows that the noise can sometimes
resemble a square-wave distribution in the Images, producing a bias in
extended source photometry that is either full-amplitude positive or
negative. These biases can be as large as 17, 7 and 11% at J, H and
Ks, respectively, for
galaxies with Poisson errors of less than 10%, for perhaps ~1% of all
galaxies.
Elliptical Parameters for Extended Sources Smaller Than r=7"
The elliptical-fit parameters (b/a, position angle) for extended sources
with radii less than ~7" are not reliably determined due to the small
source size coupled with the variation in focus of the telescope and the
PSF asymmetries. We intended to blank out the elliptical parameters for
such sources, but did not have time to do so before freezing this release.
Hence users should view those parameters with considerable caution.
Sources with Negative Radii
A few sources have some negative radii reported in the Catalog
that should be nulls. These are due primarily
to artifacts that were not caught prior to the generation of the XSC.
[Last Update: 1999 May 5; T. Chester, T. Jarrett, R. Cutri]
ii. Selection Criteria
iii. Lower Reliability for "E" Sources and at Low Galactic Latitudes
iv. Unreliability and Incompleteness Due to Bright Stars
v. Other Artifacts
vi. Untracked Seeing
vii. Additional Photometric Uncertainty
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