IV. 2MASS Data Processing

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5. Extended Source Identification and Photometry

a. Algorithms, Analysis and Results

xi. How to Use the 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?

See Table 1 of IV.5 for a detailed description of all the XSC parameters.

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

The catalog tabular and imaging data is archived within the IPAC science archive IRSA (Infrared Science Archive). Use the GATOR catalog query service to retrieve data. Also each source has a corresponding image cube, which is a small FITS image incorporating the object. The FITS and JPG previews of the object can be obtained with the IRSA XSC image retrieval service.

IRSA includes many services to retrieve data and to compare across datasets. The two most important services relevant to the XSC are:

• GATOR, the catalog query service to retrieve catalog data
  
• XSC Postage-Stamp Image Retrieval Service

Where else is the data archived?

The NASA Extragalactic Database (NED) contains the names, positions and basic photometric information from the XSC. Images, including links to the IRSA Image Retrieval Service and JPG previews are also available in NED. For example, compare the 2MASS images with other (mainly optical) images of the M51 (Whirlpool) galaxy.

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

Each XSC source is tagged with two different sets of photometry. (1) The first set is based on the J-band peak pixel of the source. Since each pixel is 1´´ in size (while the PSF beam is closer to 2.5´´), the relative accuracy of this measurement is between 0.3´´ and 0.5´´. The source name, or designation (see Table 1), is derived from this position. (2) The second set is based on the intensity-weighted centroid of the combined J+H+K_s image. The relative accuracy of this measure is nearly twice as good as the peak-pixel method, due to the improved SNR and centroiding method.

The absolute astrometric accuracy of 2MASS is better than 100 milliarcsec for point sources. For extended sources, you can expect 0.5´´ accuracy for the peak-pixel coordinates and 0.3´´ accuracy for the centroid coordinates. For a comparison of the 2MASS peak-pixel coordinates with the FIRST radio survey, see II.3c5.

Photometry: Which Should I Use?

Because extended sources are complex by nature, a variety of apertures are used to compute the integrated flux. Choose the aperture or method that best matches your science goals. Since there is no "one-size-fits-all" aperture, the XSC does not provide "default" magnitudes (unlike the PSC). See note below. We do have some guidelines.

For most applications, the elliptical isophotal aperture is a good choice, both in terms of capturing most of the integrated flux (~80-90%) and providing accurate colors for galaxies of all sizes. Choose the "total" apertures (e.g., Kron or the Extrapolation Surface Brightness Profile) if you need integrated fluxes that reflect the total flux of the source. Beware: these apertures are vulnerable to stellar contamination and surface brightness irregularities. The photometry based on the extrapolation of the surface brightness profile seems to be the more robust of the two methods (primarily because stellar contamination is minimized by averaging over the azimuthal isophote[s] used to derived the median surface brightness profile.)

The most robust aperture in the XSC is the circular, 7´´ radial aperture. This is a good choice if you are focused on faint or small compact sources. Beware: for larger galaxies the small apertures are not a good choice -- they are subject to a "bulge" color bias. Explanations of the aperture photometry are available:

• XSC Aperture Photometry
  
• Isophotal Photometry
  
• "Total" Photometry
  
• Effective or Half-Light Aperture Photometry
• Photometric Repeatibility

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

Although "default" magnitudes were provided in the Incremental XSC Releases, they are not provided in the All-Sky XSC Release (which supercedes all Incremental Releases). It was decided that, since extended sources are complex entities and no one aperture (or method) satisfies all science requirements, the notion of "default" in the XSC is misleading, at best. The user should decide which aperture best matches the desired science goals. We do offer some guidelines as to what photometry works best for common cases. See the above "Photometry: Which Should I Use?"

That said, if the user so desires to recapture the "default" magnitudes which were used in the Incremental Releases, then select the circular (K_s-band fiducial) isophotal magnitudes, which are available in the XSC.

Photometry: What About Total Magnitudes?

"Total" magnitudes refer to apertures in conjunction with corrections that account for the "total" flux of a source. We have determined that between 10% and 20% of the flux (galaxy morphology-dependent) of an object is lost in the formidable background noise. This flux is recovered using the median (elliptical) surface brightness profile. Beware: "total" magnitudes are vulnerable to stellar contamination and to irregularities in the surface profile (e.g., asymmetries). "Total" magnitudes are not a good choice when colors are desired (since one is effectively root-square-summing two large uncertainties to compute a color). For colors, use a smaller, more robust aperture (e.g., isophotal magnitudes; See "Photometry: Which Should I Use?" above.)
• XSC Aperture Photometry
  
• Isophotal Photometry
  
• "Total" Photometry
  
• Effective or Half-Light Aperture Photometry
• Photometric Repeatibility

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

The XSC is complete for all galaxies larger than ~10´´-15´´ in diameter, including the largest galaxies in the sky. However, due to their proximity to a survey "scan" edge, galaxies larger than ~1´ or 2´ will have photometry that is systematically incomplete. We have rectified this situation.

The 2MASS survey acquired images of the sky using relatively small arrays. A "tile," or scan, is 8.5´ in angular width, and the typical overlap between scans is 50´´. Galaxies that are smaller than this overlap are guaranteed to be fully sampled in at least one survey scan. Larger galaxies may be truncated based on their proximity to a scan edge. Therefore it was necessary to construct an atlas of large objects made from "pieces" of adjoining scans. The net outcome is that we will fully recover galaxies that are currently "lost" or misrepresented in the 2MASS extended source pipeline.

The resultant set of mosaics and corresponding source characterizations are collectively called the 2MASS Large Galaxy Atlas (LGA). This LGA information has been incorporated into the XSC for the largest 550 galaxies in the sky. LGA galaxies are easily identified in the XSC with the parameter cc_flg; they are tagged with cc_flg = "Z". We have also identified sources which are in close proximity to large galaxies, whose photometry has been eliminated from the Catalog, due to their unreliable nature, i.e., cc_flg = "z".

The 2MASS Large Galaxy Atlas is described in detail in Jarrett et al. (2003).

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?

Z. Helpful References

• 2MASS Extended Source Catalog: Overview and Algorithms
  
• 2MASS Large Galaxy Atlas
  
• 2MASS Data Release Frequently Asked Questions (FAQ)
  
• IRSA
  
• NED

[Last Updated: 2002 Oct 27; by T. Jarrett]

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Return to Section IV.5a.