IV. 2MASS Data Processing
5. Extended Source Identification and Photometry
c. Stellar Number Density Metric: "density"
i. Visual Inspection of Extended Sources
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-composite 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.
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," which encodes the following classification:
galaxy: vc = 1
star or point source: vc = 2
unknown: vc = -2
artifact: vc = 2
double source: vc = 2
galactic extended: vc = 1
triple source: vc = 2
- Galaxy refers to sources which have shapes and surface brightnesses
associated with extragalactic extended sources.
- Star/point source refers to sources which are unresolved or barely resolved
from the 2MASS PSF. Most of these sources are foreground (Milky Way) stars.
- Unknown source refers to sources which 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 which 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
- Double source refers to point sources in close proximity
(projected doubles) which 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 (their photometry follows this trend accordingly). They are
confined to the Galactic plane (|glat| < 3°) or giant molecular clouds (e.g., the Orion GMC).
- Triple source refers to a complex set of point sources (projected stars) which
induces a false positive in the star-galaxy discrimination process. These sources tend
to be confined to regions of high stellar source density (i.e., the Galactic plane).
The GALWORKS star/galaxy discrimination techniques were developed to
maximized galaxies and galactic fuzz (vc = 1), and to minimize
contamination from stars, artifacts, doubles and triples (vc = 2).
Gallery of Sources
The following is a very brief gallery of some sources
encountered by 2MASS.
|Figure 1||Figure 2||Figure 3||Figure 4|
|Figure 5||Figure 6||Figure 7|
vc=2, cc_flg= 0
vc=1, cc_flg= 0
[Last Updated: 2002 Aug 16; by T. Jarrett]
Return to Section IV.5c.