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 |
IRSA includes many services to retrieve data and to compare across datasets. The two most important services relevant to the XSC are:
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.
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:
That said, if the user so desires to recapture the "default" magnitudes which were used in the Incremental Releases, then select the circular (Ks-band fiducial) isophotal magnitudes, which are available in the XSC.
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).
[Last Updated: 2002 Oct 27; by T. Jarrett]