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 true extended sources, such as galaxies and nebulae, as well as objects made out of multiple stars that are close together, artifacts around bright stars due to gradients in the backgrounds around such bright stars, and meteor trails and airplane streaks.

In constructing the XSC, we have attempted to reject objects that are not truly extended sources (cf. V.3). However, the algorithms used so far are not perfect, and some of these objects remain in the catalog.

The XSC is the union of two separate catalogs: an "extended source catalog" ("E" sources) constructed 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.

ii. Selection Criteria

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.

Atmospheric Airglow Emission

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 of the background-removed pixel intensity distribution.

In the First Incremental 2MASS Data Release, scans that have large measured background 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.

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 (axis ratio 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. Therefore the elliptical parameters for sources with radii less than 7´´ have been set to "null".

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.

cc_flg Value
Nature of Artifact or Confusion
Source is unaffected by known artifacts or confusion
Confusion with nearby stellar source
Unreliable - artifact, false, or seriously corrupted source

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]

Previous page. Next page.
Return to Explanatory Supplement TOC Page.