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 reliablity 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 (re: stars) and minimize contamination from double stars
(the primary reliablity 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 reliablity 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 fainter 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.
Photometry
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 fixex 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
. Furthern 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:
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: