XSC Anomalies

The XSC, like any large catalog, is contaminated with anomalies that include false (artifact) sources, mis-measurements and mysterious features. Although these anomalies are quite rare in number (~1% of the total), the user should be aware of their presence. One should first refer to the XSC Cautionary Notes of the 2MASS Explanatory Supplement.

Artifacts

Visual inspection of the XSC reveals a small fraction (less than 1% of the total) of sources that may be classified as artifact in nature -- 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 contaminents to the XSC.

False Extended Objects

With projection effects, close groupings of stars come in every permutation imaginable. These "double" and "triple" stars represent a small contaminent 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?".

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".

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. While very faint sources may (for example) prove to be too formless to adequately measure the isophotal shape or flux. Hence, 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 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".

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.

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:

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:

Pieces of Large Galaxies or Clusters

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

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?"

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 & Concentration Index and Axis Ratio and Small Radii.

Other Miscellaneous Items

NOTES from Users

Anomalous or Objects of Note, Identified by NED

(notes from Harold Corwin; Jarrett comments noted in blue)

Anomalous or Objects of Note, Identified by CfA

(notes from John Huchra; Jarrett comments noted in blue)
Matches between UZC & 2mass galaxies at 20 arcsec level.
First line is UZC galaxy: name, ra & dec (J2000), vel & verr1 Indent line is 2mass match: name, ra & dec (J2000) & mag The following sources come from Lucas Macri. Jarrett notes are in blue.

Macri;s CfA code is as follows: 22=star, 25=plate flaw, 26/27=star cluster, 30=planetary nebula, 32=HII region, 33=reflection nebula, 40=piece of galaxy.

[Last Updated: 2003 Mar 10; by Tom Jarrett]