Spitzer IRAC Survey of the Galactic Center (GALCEN)
Column Definitions
Overview
Spitzer/IRAC observations of the central 2.0 deg by 1.4 deg (~ 280 pc by 200 pc) of the Galaxy were obtained at 3.6-8.0 microns in Cycle 1 (GO 3677, PI: Stolovy). These data represent the highest spatial resolution (~2 arcsec) and sensitivity uniform large-scale map made to date of the Galactic Center at mid-infrared wavelengths. A point source catalog of 1,065,565 objects was obtained. The catalog includes magnitudes for the point sources at 3.6, 4.5, 5.8, and 8.0 microns, as well as JHK photometry from 2MASS. The point source catalog is confusion limited with average limits of 12.4, 12.1, 11.7, and 11.2 magnitudes for [3.6], [4.5], [5.8], and [8.0], respectively. We find that the confusion limits are spatially variable because of stellar surface density, background surface brightness level, and extinction variations across the survey region.
More details about the point source catalog can be found at Ramírez et al. (2008).
Column Descriptions
| Catalog Fields |
Each entry in the point source catalog has the following information:
Name -> SSTGC NNNNNNN
Position -> ra, dec, glon, glat
General Flags -> rel_flag, pos_flag
2MASS Information -> tm_designation, j_mag, j_err, h_mag, h_err,
k_mag, k_err, qual
IRAC magnitudes -> chX_id, chX_mag, chX_err, chX_flag
IRAC diagnostics -> chX_back, chX_snr, chX_cov, chX_mnr
where X is the IRAC channel number (1 - 4) (3.6, 4.5, 5.8, and 8.0 microns).
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| Name | The 'name' entry has the form:
|
| Position |
There are four "position" related fields:
ra, dec, glon, glatThe position is given in both Equatorial J2000 (RA, Declination) and Galactic (l,b) coordinates. |
| General Flags |
There are two general flags:
rel_flag, pos_flagThe reliability of the sources in our band merged list can be estimated by applying the "2+1" criterion as defined by the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) v1.5 Data Products Description. The M/N ratio is defined as the ratio between M number of detections over N number of possible observations (coverage value). The "2+1" criterion requires M/N >= 0.6 in one IRAC band, M/N >= 0.4 in an adjacent band. Sources that satisfy the GLIMPSE "2+1" criterion have rel_flag set to "1' in our catalog. The coverage of our survey has some incompleteness due to the fact that the four IRAC cameras do not see exactly the same region of the sky. The coverage value is the same as the number of available BCD frames at the position of each source, and it is also listed in our final catalog for each channel. The location of each of the sources is flagged in our catalog by the field pos_flag.
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| 2MASS information |
There are eight fields related to the 2MASS point source catalog:
The fields j_mag, h_mag, and k_mag correspond to the photometric magnitudes in the J, H, and K bands from the 2MASS point source catalog, respectively. The fields j_err, h_err, and k_err list to the photometric uncertainties of the magnitudes in the J, H, and K bands. The qual field corresponds to the photometric quality flag (ph_qual) from the 2MASS Point Source Catalog. It is composed of three letters, one for each 2MASS filter (JHK). The letters can be:
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| IRAC magnitudes |
The following fields are related to the IRAC magnitudes:
The field chX_id corresponds to the identification number from the point source list of the individual IRAC channel. The field chX_mag store the IRAC magnitude of the point source in the respective channel. The field chX_err lists its uncertainty. The IRAC photometry was determined using the SSC Mosaicking and Point source Extraction package (MOPEX). Magnitudes for each of the IRAC channels were computed using zero point fluxes of 280.9 Jy, 179.7 Jy, 115.0 Jy, and 64.13 Jy, for IRAC Channels 1, 2, 3, and 4 respectively, as provided by Reach et al. (2005). The field chX_flag provides information about the provenance for the IRAC magnitudes. It is set to "1" when flux and hence magnitude comes from PRF fitting algorithm, set to "2" when flux and hence magnitude comes from aperture corrected measurement, set to "3" when the full-array flux is greater than corresponding saturation limit, set to "4" when the photometry comes from the sub-array observations, set to "0" when there is no detection in this IRAC channel. The saturation limits are 7.92, 7.38, 4.79, and 4.84 magnitudes) for IRAC Channels 1, 2, 3, and 4 respectively, as provided by the Spitzer Observer's Manual, Version 7.1. |
| IRAC diagnostics |
The following fields are related to the IRAC diagnostics:
The field chX_back lists the value of the local background in MJy/st as computed by the PRF fitting algorithm. The field chX_snr lists the signal to noise ratio as computed by MOPEX. Note that the signal to noise ratio provided by MOPEX is the SNR of the PRF fitting algorithm. The coverage value for each source is listed by the chX_cov field. The coverage for each source at each channel was determined by measuring the value of the pipeline coverage map at the position of each source. The coverage value is the same as the number of available BCD frames at the position of each source. The field chX_mnr lists the M/N ratio, which is defined as the ratio between M number of detections over N number of possible observations (coverage value). |