Over the course of the 2MASS project, extended sources have been visually inspected during spot checks and systematic checks of completeness and reliability. The inspection consists of viewing a J+H+Ks RGB color-combined image, tabular data (sizes and photometry), and (when available) an optical DSS image. The images are viewed with both linear and logarithmic stretches to inspect both high and low surface brightness features.
About 600,000 total sources have been inspected and classified (see code below). Roughly 25-30% of all XSC sources have been inspected, with close to 100% inspected at the bright end (Ks < 12.5 mag). The XSC includes a visual inspection flag "vc" that encodes the following classification:
Galaxy refers to sources that have shapes and surface brightnesses that are associated with extragactic extended sources.
Star/point source refers to sources that are unresolved or barely resolved from the 2MASS PSF. Most of these sources are foreground (Milky Way) stars.
Unknown source refers to sources that are not easily classified. They tend to be faint and compact. They are most likely extragalactic in nature.
Artifact refers to a suite of false sources that are induced from the camera electronics, optics, array and other mechanical origins, and transitory effects (e.g., meteor streaks). A gallery of the various kinds of artifacts is found here.
Double source refers to point sources that are in close proximity (projected doubles) that mimic the profile of a galaxy or induce a false positive in the star-galaxy discrimination process. Most of these sources are stars, with bright star - faint star pairs a common offender.
Galactic extended refers to Milky Way extended sources, such as nebulosity, HII regions, YSOs, and clusters (open and globular). These sources tend to be complex and ill-defined (hence, their photometry follows accordingly). They are highly confined to the Galactic plane (|glat| < 3 degrees) or giant molecular clouds (e.g., Orion GMC).
Triple source refers to a complex sets of point sources (projected stars) that induces a false positive in the star-galaxy discrimination process. These sources tend to confined to regions of high stellar source density (i.e., the galactic plane).
The following is a very brief gallery of some sources encountered by 2MASS.
Galaxy vc = 1, cc_flg = 0
Star vc = 2, cc_flg = 0
Unknown vc = -2, cc_flg = 0
Artifact
vc = 2, cc_flg = A
Double Star vc = 2, cc_flg = 0
Galactic Fuzz vc = 1, cc_flg = 0
Triple Source vc = 2, cc_flg = 0
[Last Updated: 2002 Aug 16; by Tom Jarrett]