2MASS XSC Galactic Extended Sources

The 2MASS XSC includes sources that belong or are in close proximity to the Milky Way (e.g., globular clusters, LMC/SMC, ... etc). A wide range of objects and ISM phenomena: Examples of Galactic fuzzies can be found throughout these documents, and are compiled in the XSC Gallery, and the 2MASS Gallery, and even a few can be found in the 2MASS Large Galaxy Atlas.

Nearly all Milky Way objects are tightly confined to the Plane of the Galaxy: |glat| < 5 degrees. The exception to this rule are the giant molecular clouds that float above the Plane (e.g., Orion, Taurus, Rho Ophiuchus) and the occasional globular hovering in the spheroid of Milky Way. You can see the extended objects quite easily in the XSC Allsky image, lining the Plane in color "red". These objects are both intrinsically red (e.g., HII regions are dominated by emission bands in the 2 micron window) and dust-reddened. Since most are confined to the Plane, the Galactic projection (see below) of the XSC number counts gives the best view of their spread and distribution.

XSC as seen in a Galactic Projection
. The limiting magnitude is 13.5 in Ks, where the number counts are shown with a false-color scale (to enhance the contrast). Sources identified as Milky Way in origin are circled in red. Note the prominent molecular clouds: Rho Oph, Orion and Taurus, and the clustering of extended sources in the LMC/SMC galaxies.

To summarize:

We have identified some 8,000 sources in the XSC that are probably Milky Way in origin, with a smaller subset of 3712 sources highly probably Galactic in nature. The smaller table of Milky Way sources is given here and are denoted in the above projection and the color plots to follow.


The fiducial elliptical isophotal (20 mag arcsec-2 in the Ks band) mags are used to construct the colors J-H, H-Ks, and J-Ks. The XSC sources (1.65 million total) and the XSC Galactic sources are sub-divided according to the stellar number density, which measures the surface density of stars that the extended object is projected against.
Density vs. XSC

density mindensity max %XSC |Glat| min |Glat| max
0.0 3.2 73.4% ~25 90
3.2 3.6 19.0% ~10 ~25
3.6 4.0 6.9% ~5 ~10
4.0 4.5 0.7% 0 ~5

b. J-H vs. H-Ks Color-Color

The following Hess diagrams show the density of points in the color-color plane, with blue representing sparse, green showing moderate density, yellow showing high density and red showing the highest density of points. The Galactic sources are circled in red.

Examples of "Red" Milky Way Objects

  • Nebulosity
  • YSOs
  • HII Regions
  • Source at (0.101620, -12.763147)

    Examples of "Blue" Fuzzies:

  • Comet at (92.836586, 36.640507)
  • Comet at (213.217377, -15.222510)
    See also the XSC Gallery.

    [Last Updated: 2003 Feb 24; by Tom Jarrett]