I. Introduction

6. Cautionary Notes

c. Extended Source Catalog (XSC)

Frequently Asked Questions
i. Definition of Extended Sources

2MASS pipeline processing (see IV.1) 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. Since the algorithms (see V.3) are not perfect, at a much lower level (~1%), the XSC also includes objects made out of multiple stars that are close together and artifacts around bright stars due to gradients in the backgrounds around such bright stars.

ii. Selection Criteria

Extended sources were identified and separated from point sources using a "decision tree" method. The method uses a variety of source attribute measures to separate resolved from the unresolved, including size, central surface brightness and color. In all, we use about 10 different measures (or dimensions), whose values are correlated to one degree or another. Read more about this parameter set in IV.5.a.vi.

Two different decision tree "scores" are generated: (1) G score and the (2) E score. The "G score" is geared towards recognizing galaxies. Not only does it include the standard star-galaxy parameters (e.g., shape, size & SB, ... etc) but it also includes the "color attribute". The "E score" is a simpler version of the G-score: it has only three parameters that are tested in the decision trees, including surface brightness, shape and the double/triple star discriminator. It does not include a color attribute. In this sense, it is less biased than the G-score. It was conceived to capture Milky Way objects that might have unusually blue colors, or shapes distinctly different from galaxies.

It should be understood that: (1) the E score is less reliable than the G score, and (2) although it may be less biased to color, the E score is not necessarily better at finding Milky Way objects (the G-score is more than happy to find HII regions, nebula, YSOs, etc).

These are employed as the final arbiter for star-galaxy separation. It turns out that a value of around 1.4 gives a satisfactory separation, while maintaining satisfactory completeness. Summary: extended objects have scores between 1 and 1.4, while point-like objects have values between 1.4 and 2.0.

iii. Lower Reliability for Sources 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 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 50% at the highest source densities allowed in the catalog (cf. VI.4).

No requirements were placed on the reliability of Galactic Plane sources, and hence a higher percentage of them are composed of multiple point sources.

iv. Incompleteness Due to Source Confusion

The presence of foreground Galactic stars limits galaxy detection both in the sense that the background noise is enhanced (i.e., confusion noise) and the ability to distinguish extended emission from the stellar (point-like) emission is diminished. The rate at which these effects become increasingly important is dictated by the stellar number density, roughly matching the exponential distribution of stars along the the Galactic latitude axis.

The drop in completeness with the source confusion can be clearly seen in the allsky "movie" maps and more quantitatively in the XSC source count "movie" map. Note the large gaps in completeness near the Galactic Center, and note the "reddening" of the XSC sources as seen within (or projected against) the Milky Way.

v. 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 1 to 2% of the entire sky is excluded from the XSC due to bright star confusion (cf. IV.5).

When in doubt about a given source, consulting the Atlas Image will usually immediately reveal whether a source is an artifact or not.

The "visual classification" flag, vc, 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. The visual classification flag was assigned on the basis of examining the image of a source, by eye, comparing across bands and (when needed) with the Digital Sky Survey. The catalog contains 316107 sources with "galaxy" or "extended" visual classifications; 7383 with "artifact" or "unreliable" visual classifications; and 35270 with "unknown" visual classifications (that is, the source defies classification, usually because it is too faint in surface brightnesss).

vi. Artifacts

Visual inspection of the XSC reveals a small fraction (less than 1% of the total) of sources that may be classified as image artifact -- usually associated with bright stars. These "sources" are often possessed with unusual photometry or size attributes. Be wary of sources with unusual colors or other "outlier" attributes. In addition to bright stars, other kinds of artifact inducing phenomena include "airglow" (particularly at H-band), transients (e.g., meteor streaks) and image edges. We have made every effort to minimize these contaminants to the XSC.

vii. False Extended Objects

With projection effects, close groupings of stars come in every permutation imaginable. These "double" and "triple" stars represent a small contaminant to the XSC, mostly confined to the Plane of the Galaxy. Since these sources are stellar in nature, they tend to have blue colors, J-Ks < 1 mag, with respect to galaxies. Refer to the XSC FAQ: Reliability: What is a Star Doing in the XSC?.

viii. Special Sources: the cc_flg flag

The cc_flag is used to highlight XSC entries that have some probability of being artifacts or contaminated by nearby large galaxies, or that are Large Galaxy entries that were processed specially.

Sources flagged as artifact (cc_flg="a") 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 were identified as such during visual inspection operations.

The following table summarizes the possible values in the cc_flg, and shows the number of sources in the All Sky XSC having each cc_flg value.

cc_flg Value
Number Count
Nature of Source
large galaxy
Fragment of a large galaxy. Extracted information is generally not useful.
artifacts or unreliable

ix. Duplicate Sources

Duplicate sources are very rare, but do live within the XSC. The (usual) phenomenon arises from scan-to-scan overlap observations (some 10-15% of the scan), where the same source in two different scans may have slightly (~few arcsec) different centroids. The differing centroids are usually associated with low S/N sources (or nuclei), where noise fluctuations may induce the difference. Another source of mis-measurement of the centroid is the scan edge itself, which might (under certain circumstances) disrupt the source characterization process. Be wary of any two XSC sources that are within a few arcsec of each other in coordinate space. Refer to the XSC FAQ: Duplicity: Why Do Some Galaxies Appear to be Duplicates".

x. Null Photometry and/or Attributes

"Null" values arise from either (1) corruptions in the source characterization process, or (2) Signal-to-Noise limitations. The source characterization process is corrupted (or interrupted) by a variety of circumstances, including the deleterious presence of bright star artifacts, scan edges, high-frequency background gradients ("airglow"), and source confusion. Faint sources may (for example) prove to be too formless to adequately measure the isophotal shape or flux. Consequently you will often find "null" attributes for sources in confused regions and for faint (low S/N) sources. "Null" photometry (or corrupted photometry) may result in bizarre, unphysical associations (e.g., strange colors). Refer to the XSC FAQ: Why Do Some Galaxies Appear to Have Unphysical Colors or Brightnesses?.

There is another class of XSC object that is rife with "null" attributes. These are "sources" associated with very large galaxies, and are believed to be (for the most part) unreliable "junk" spawned by the parent galaxy. They may be real (e.g., H II regions), or they may be bogus (e.g., noise bump on top of the large galaxy), but either way their source characterization is seriously compromised by the confused environment. For this reason, these sources have been identified and their photometry and source attributes artificially "nulled". The cc_flg value is set to "z" (see table above). The only useful information in the XSC is the coordinate position of the object itself. Read more about it in: "Sources in close proximity to large galaxies".

xi. Mis-positioned Galaxies & Foreground Contamination

The close presence of foreground stars to an XSC galaxy (i.e., confusion) will on occasion induce (1) poor central positions (i.e., astrometry) and/or (2) poor integrated fluxes (e.g., see below "Wacky Measurements"). The user should beware of stellar confusion.

xii. Wacky Measurements

In the same vein as corrupted photometry and their resultant strange colors, the occasional source characterization mis-measurement may result in totally unphysical attributes, such as angular size. These measurement gaffes typically arise from image problems (e.g., capricious background variations inherent to ground-based observations) or disorder from source confusion. (See above discussion)

They also arise from software design flaws. For example, the early 2MASS pipeline data (see the incremental releases) featured a set of objects, thought to be associated with large galaxies, but instead were either off-center pieces of large galaxies or nearby stars: Now this problem has been, for the most part, fully rectified in the All Sky XSC. In addition, the largest galaxies are treated separately, the Large Galaxy Atlas (LGA), fully rectifying any problems associated with scan-to-scan edge problems (see below for the exceptions to this rule). There is still the occasional XSC source that has an anomalously large image size (mimicing a large galaxy), but in fact is a small extended object. These sources tend to be associated with NED galaxies that are "known" to be large in angular extent (but not necessarily in the near-infrared!). Here is an example: xiii. Large Galaxies and Survey Scan Edges

The 2MASS survey design did not cater to the needs of large angular extent objects. Not only are these objects clipped by the edges of scans, but the background removal process itself is compromised by the small size of the survey images with respect to nearby (zero redshift) galaxies. The Large Galaxy Atlas (LGA) was created to rectify this problem. The XSC now includes the ~500 largest galaxies in sky. The LGA is a still a work in progress, however. A number of moderate to large (non-LGA) galaxies in the XSC are clipped by scan edges, due to the unfortunate proximity within a survey scan. These galaxies can be expected to have underestimated integrated fluxes, as well as compromised large-scale attributes (e.g., isophotal radii). In time the LGA will include these XSC galaxies, and the fluxes and source characterization will be recovered. Here is an example of an XSC galaxy in close proximity to a scan edge:

xiv. Pieces of Large Galaxies or Clusters

A very small fraction of the XSC sources are pieces of larger objects, including galaxies and clusters.

xv. Little Big Man Galaxies

The Large Galaxy Atlas is, as the name suggests, comprised of large angular-extent galaxies. It was created to fill a large gap in the XSC where these sources were either unmeasureable or mis-measured in the 2MASS pipeline. However, due to rather obtuse circumstances (usually related to the author's interest in these special galaxies), a handful of small galaxies have found their way into the LGA. They were created and processed using the LGA pipeline. As such, their photometry and source characterization is perfectly valid. Their only outstanding quality is that they are much smaller than nearby galaxies, and in fact they are small in comparison to typical XSC galaxies. An example is MRK 897. Refer to the XSC FAQ: Why Are Some "Large" Galaxies in Fact Barely Resolved?"

xvi. Half-Light Radii and Diminishing Returns Near the Resolving Limit of 2MASS

The shape and light concentration are difficult to determine for small galaxies, whose profiles are significantly distorted by the point spread function (PSF). For galaxies smaller than 10 arcsec (representing most of the XSC), the half-light radii and concentration indexes will probably be overestimated due to the rounding and extending effects of the PSF. These attributes should be used with caution. See also Half-Light "Effective" Aperture and Concentration Index (IV.5.e) and Axis Ratio and Small Radii.

xvii. Other Miscellaneous Notes

[Last Update: 2007 January 3; T. Jarrett, T. Chester, R. Cutri]

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