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."
90% of sources pass both criteria, but 10% pass the "e_score"
criteria only. Users
can work with only the "E" or "G" subset of the XSC by selecting on those
scores. A small number of point sources near large galaxies were
incorrectly assigned e_score = "1"; these are discussed below in
Section I.6.c.x.
iii. Lower Reliability for "E" Sources and at Low Galactic Latitudes
The parameters of the XSC were tuned to meet
as closely as possible
the
Level 1 Science requirements of
99% reliability and completeness for "G" sources above point source
densities representative of galactic latitude glat ~ 20° and
80% reliability for "G" sources at glat ~ 10°.
Reliability is defined here as the percentage of sources which are truly
extended (e.g., galaxies and nebulae), with multiple point sources,
artifacts around bright stars and meteor trails counted 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 allowed in the catalog
(cf. VI.4).
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 90% overlap between "E" and "G" sources, the reliability of
"E" sources is still quite high.
iv. Unreliability and Incompleteness Due to Bright Stars
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. About 5% of the entire sky
is excluded from the XSC due to bright star confusion (cf.
IV.5).
The parameters for some bright stars were not known during scan-pipeline
processing (due to saturation and bright sources from neighboring
unobserved and/or unprocessed Tiles), 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.
v. Other Artifacts
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
during Quality Assurance prior to Catalog Generation.
Some artifacts are identified in this release with the cc_flg (see
below). As more of these artifacts become identified, the user can check
the XSC 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.
The feye 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. 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.
As by-eye classification continues, the feye flag
will be updated periodically in the on-line version of the
Incremental Data Release Catalogs.
vi. Untracked Seeing
Atmospheric seeing variations cause the observed PSF to change,
and the scan processing 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 and Quality Assurance
estimates how well the seeing has been tracked in each scan of a 2MASS
Tile.
As part of the original quality analysis for each scan,
the worst scans with untracked seeing have been rejected, and scheduled for
reobservation. In this second major release of 2MASS data, we have allowed
scans with moderate untracked seeing into the PSC, but have removed the
untracked portion of those scans from the XSC. Thus, there are small
portions of scans that are present in the PSC but are not found in the XSC.
It is not possible to find all scan portions with moderate untracked seeing
due to source density limitations, and hence there may be small portions of
scans that contribute false sources to the XSC. We estimate that this may
contribute at most an extra ~1-2% unreliability to the XSC, mostly for the
"E" sources.
vii. Additional Photometric Uncertainty
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.
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,
atmospheric OH airglow emission variations
contribute extra noise in the H-band roughly equal to the measurement
error.
The H-band photometric error due to airglow noise 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
In the First Incremental 2MASS Data Release, scans that have large
measured background No correction of the photometric uncertainties has been made for this
statistical analysis result.
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.
The elliptical-fit parameters (axis ratio b/a, position angle
viii. Confused and Unreliable Sources: the cc_flg flag
Analogous to the point source "contamination & confusion flag", the extended source
catalog includes a flag that indicates either "confusion" or
"unreliability". Confusion refers to the close proximity of bright stars,
resulting in both poor reliability and large photometric bias and uncertainty.
The confusion flag was set using geometric boundaries that depend on the
brightness of the bright point source. Sources marked as such are to be
treated with extreme care. In particular, the completeness and
reliability goals do not apply to such sources.
Sources flagged as unreliable (cc_flg="U")
include those corrupted by a bright star or those that are
outright false detections of filter glints or ghost
images produced by bright stars.
These sources
are found in a variety of ways, and do not result
exclusively from a uniform processing
of the entire data set. Many of these sources have been found by human
verification of a small subset of the data, and therefore could not be
eliminated from the XSC without imposing selection effects on the catalog
as a whole.
The following table summarizes the possible values in the cc_flg, and
shows the number of sources in the Second Incremental Release
XSC having each cc_flg value.
ix. Large Galaxies Not Processed in the 2MASS
Galaxies that do not fall completely (>75%) on an Atlas Image
cannot reliably be detected
and measured during scan pipeline processing. For these cases,
the galaxy is not processed
in the 2MASS pipeline, but the part of the Atlas image that does
contain the galaxy is extracted as a "postage stamp" image (which can
later be used to construct a "mosaic" of the galaxy from many such pieces
of "postage stamp" images).
A list of known galaxies not processed by
2MASS to date (and a list of large galaxies that have been processed) are found
in Large Galaxies Encountered in the 2MASS.
x. Mis-Classified Galaxies
Special processing was used to
select sources spatially coincident with large galaxies (but not too large;
see I.6.c.ix above) and pass them
into the Extended Source Database. Usually sources so identified capture
at least part of the flux of the large galaxies. However, some large
galaxies have poorly determined positions, and as a result sources that are
not in fact part of these large galaxies were picked up by the same
processing.
All such sources, correct and incorrect positional associations, were
assigned e_scores of "1" and erroneously put into the XSC. The
g_scores are not affected by this error.
This error was caught too late to delete such sources from the XSC.
Some of these source were marked as being "unreliable"
(cc_flg="U"). Users can further identify these sources by their
id_flg giving an association to a large galaxy.
xi. Removal of Sources in the Galactic Center
The Galactic Center region has a stellar number density that is too large
for extended sources to be reliably distinguished from the foreground
population of stars (which, in projection, form clusters and complex
unresolved objects). Therefore, an elliptical area centered on the
Galactic Center is masked from the extended source catalog.
The elliptical mask has a semi-major axis length of 12.8° along the
Galactic Plane and a semi-minor axis length of 6° perpendicular to
the Plane.
Details of the
masked area are given in
Elimination of False GC Sources from XSC.
xii. Resolved Solar System Objects
The 2MASS Second Incremental Release XSC contains resolved detections of
four known comets that were identified during scan pipeline processing
(cf. IV.9).
Because positional
associations with solar system objects are not otherwise indicated
in the XSC, the table, Known Comet
Detections in the Extended Source Catalog, provides a list of the known
comets in the XSC. Included in the table is the comet name and
designation, and the position and source designation from the
XSC.
[Last Update: 2000 March 1; T. Chester, T. Jarrett, R. Cutri]
Atmospheric Airglow Emission
of the background-removed pixel
intensity distribution.
were not
included. For this Second Incremental Release, we have refined our airglow
diagnostic to only reject scans that have measured noise that is
significantly greater than that predicted by the measured H background.
This should allow better rejection of scans truly contaminated by
background structure due to airglow. However, note that we now include in
this release Tiles that have much higher noise at H band than in the
previous release.
Some Tiles undoubtedly still contain airglow structure that is not
extensive enough to trigger our thresholds for Tile rejection. Therefore,
users should still be aware that H band fluxes for a significant number of
sources will have a higher photometric error than the quoted error, which
reflects only the Poisson noise in the background.
The best estimate we can make at this time for the magnitude of the excess
noise comes from a statistical analysis of sources from Atlas Images with a
residual background of just under 1.20 DN.
These sources have an
extra H photometric error equal to their Poisson uncertainty of ~0.10 mag,
making the total photometric error ~0.15 mag.
Electronic Noise
Elliptical Parameters for Extended Sources Smaller Than r=7"
) 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. Therefore the elliptical parameters for sources with radii
less than 7´´ have been set to "null".
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