The 2MASS Extended Source Sampler Catalog

The first public release of 2MASS data will occur in the fall of 1998. One full night of data comprises this "sampler" release. About ~1600 galaxies brighter than K~13.5 are part of this data set. A description of the expected completeness and reliability for the extended source catalog can be found here: 2MASS Sampler Catalog.

A subset of previously cataloged galaxies are automatically measured and extracted into the 2MASS database. This set of objects is selected based on the optical diameter, in this case, galaxies with a diameter greater than 1', as listed in the Nasa Extragalactic Database. For the larger Messier objects (and some NGC objects, for example), >5', they are typically too large to process with the 2MASS imaging data, and so are not processed or extracted into the 2MASS database. That leaves the remaining (>99%) of the sky for 2MASS to find and characterize galaxies. Extended sources are identified from point source detections. That is to say, we characterize each point source and decide if it is extended with respect to the point spread function. This is accomplished using a battery of star-galaxy discrimination parameters, including intensity-weighted moments, radial profile extent measures, asymmetry metrics and mean surface brightness flux measures. This set of operations is designed to eliminate point-like objects (stars) and minimize contamination from double stars (the primary reliability obstacle) and other false galaxies (e.g., artifacts from bright stars). An important step that precedes star-galaxy separation is careful removal of the image background, particularly at H-band which is severely affected by atmospheric "airglow" emission. Once a source has been deemed "extended" or a candidate thereof, its flux is measured using a disparate set of apertures, ranging from fixed circular to adaptive elliptical/circular apertures. The extended source information is extracted to a table and a small "postage-stamp" image (typically 30X30" in size) is cut out from the J,H & Ks coadd images. Additional star-galaxy separation is performed as a post-processing step to further refine the reliability and aide in generation of the extended source catalog. The final catalog is expected to meet or exceed the Level-1 Specifications , that include >90% completeness and 99% reliability for most of the sky (free of stellar confusion). The point source sensitivity limits (10) are 15.8 (0.8 mJy), 15.1 (1.0 mJy) & 14.3 (1.3 mJy) mag at J,H, Ks, respectively. The extended source sensitivity (10) is ~1 mag brighter than the point source limits, or 14.7 (2.1 mJy), 13.9 (3.0 mJy) & 13.1 (4.0 mJy) mag at J,H, Ks, respectively.

The extended source catalog contains over 340 fields of information per source, most of which are related to photometry. Below we describe the different measures of galaxy brightness, followed by a brief description of each parameter in the extended source catalog.


Given the diverse shape, size and surface brightness that galaxies exhibit in the near-infrared, a corresponding diverse array of apertures are used to compute the integrated fluxes. The simplest, and therefore most robust, measures come from fixed circular apertures. A set of fixed circular aperture include the following radii: 5, 7, 10, 15, 20, 25, 30, 40, 50, 60, & 70". We report both the integrated flux within the aperture (with fractional pixel boundaries) and the estimated uncertainty in the integrated flux. The magnitude uncertainty is primarily based up the measured noise in the coadd, which includes both the read-noise component and background poisson component, as well as the confusion noise component (only relevant when the source density is high). The detailed formula is given here:
Analysis of Coadd Noise: Gain and Read Noise Derived From Coadd Noise Versus Background . Further information with regard to photometry and expected measurement uncertainty are given below (see URL links below). A contamination or confusion flag is also attached to each flux measurement with the following code:

For most galaxies in the 2MASS catalog, small fixed circular apertures give adequate 'total' flux measurements. In particular, we recommend use of the R=7" aperture for galaxies fainter than K ~ 13 mag (see 2MASS Galaxy Catalog: First Results), corresponding to field names:

Adaptive aperture photometry includes isophotal and Kron metrics. The isophotal measurements are set at the 20 mag per arcsec2 isophote at K and the 21 mag per arcsec2 at J, using both circular and elliptically shape-fit apertures. For purposes of computing colors, two classes of photometry are carried out: individual and fiducial. The latter refers to adaptive apertures per band, while the former refers to using fixed apertures (both size and elliptical shape) based on either the J or K isophotes, respectively referred to as the "J fiducial" and "K fiducial" photometry. For the brighter galaxies in the catalog, K > 13 mag, we recommend the use of the "K" fiducial isophotal elliptical aperture photometry: Kron aperture photometry employs a method in which the aperture is controlled/adapted to the first image moment radius. The Kron radius turns out to roughly correspond to the isophotal radii (see above) under typical observing conditions. More information on the Kron aperture can be found here Kron Aperture Photometry .

The central surface brightness (mag per arcsec2) is computed for the peak pixel and for the central R <= 5" region:

Additional information with regard to 2MASS galaxy photometry can be found here:

Error Tree For 2MASS Galaxy Photometry
Error Analysis For Circular Isophotal Magnitudes
H Photometric Error Due To Airglow
Analysis of Noise In The 2MASS Atlas Images
The Effect of Stellar Contamination on Different Measures of Galaxy Photometry
Analysis of Photometric Noises for 2MASS Galaxies

and more specific studies here:

Hercules Cluster Photometric Repeatibility
Coma Repeatibility from 10 scans of 970521n

Extended Source Catalog Field Parameters

A list of the field parameters can be found here: 2MASS Sampler Extended Source Catalog (ESC) . The user has the option to download pre-selected fields (mini-set, short-set, or standard-set). For user and database convenience, we have defined a set of "default" magnitudes, corresponding to the K fiducial isophotal circular metric (see above). The default mag field names are:

Note, however, we recommend using the fixed R=7" circular aperture photometry and the the "K" fiducial isophotal elliptical aperture photometry (see above).