II. User's Guide to the 2MASS All-Sky Data Release

3. Extended Source Catalog (XSC)

b. How to Use the 2MASS XSC

The 2MASS Extended Source Catalog (XSC) is comprised of near-infrared sources that are clearly resolved and extended beyond the beam/PSF. Sources range in angular size from 10´´ to 2°: from small, faint galaxies to the largest Local Group galaxies. By their very nature, galaxies are complex and, therefore, require characterization measurements which are much more detailed than that for point sources. As such, the XSC includes many different kinds of measurements and various flavors of, e.g., position, orientation, size, symmetry, surface brightness and integrated flux. The user should take care to understand these parameters and their limitations. Here we answer some "Frequently Asked Questions" and give some brief guidelines on how to use the XSC to maximum effect.

Fundamental: The XSC is an extended source catalog. Although galaxies dominate the total numbers (and we frequently refer to the XSC as a galaxy catalog), the XSC is also comprised of Milky Way entities, such as globular and open clusters, planetary nebulae, HII regions, ISM emission and nebulosity, young stellar objects and compact star-formation regions, and even Solar System comets. These Galactic sources tend to be confined to the plane of the Milky Way. Refer to the "All-Sky" maps in II.3c2 to see how these sources are distributed across the sky.

Fast links to: IRSA   astrometry   photometry   large galaxies   Milky Way objects   completeness/reliability   outliers   references/links

Parameters: What is available in the XSC?

Data Mining: Where do I get the data? How do I use IRSA?

Where else is the data archived?

Astrometry: Which Position Do I Use and How Accurate Is It?

Astrometry: Why is the 2MASS Designation Different from the Centroid Positions?

Photometry: Which Should I Use?

Photometry: Why Are There No Default Magnitudes for the XSC?

Photometry: What About Total Magnitudes?

Large Galaxies: Why Are Large Galaxies Special Objects in the XSC?

Large Galaxies: Why are some "large" galaxies in fact globular clusters or barely resolved galaxies?

Star-Galaxy Separation: How do the e_score and g_score work?

Milky Way: How Do I Distinguish Milky Way Fuzz from Background Galaxies?

Completeness: Why Is My Favorite Galaxy Missing from the XSC?

Reliability: What Is a Star Doing in the XSC?

Duplicity: Why Do Some Galaxies Appear to be Duplicates?

Contamination: Why Do Some Galaxies Appear to have Unphysical Colors or Brightnesses?

What is the Spatial Coverage of the XSC?

PSC: Do galaxies appear in the PSC?

What Do the Allsky Maps Show? What are the Streaks in the Allsky Maps?

How Do You Estimate the FWHM of the PSF?

Helpful References

[Last Updated: 2015 Oct 21; by T. Jarrett and R. Cutri]

Return to Section II.3.