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.

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". 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.

iii. Lower Reliability for "E" Sources and at Low Galactic Latitudes

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.

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. 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.

v. Other Artifacts

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.

vi. Untracked Seeing

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.

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

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]

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