This is the description of the 1997 February 27 FIRST catalog. It has been replaced by a newer catalog. Old released versions of the FIRST catalog are made available for archival purposes for the benefit of those who have already started research programs based on those catalogs. The newest version is always preferred for new projects.
See the catalog history for information on changes in the various FIRST catalogs.
The catalog is on-line and can be searched using the FIRST Search Engine. However, for large-scale investigations it will be necessary to obtain the complete catalog. This document describes the catalog format.
The catalog is sorted by right ascension and has the following format:
RA (2000) Dec W Fpeak Fint Rms Maj Min PA Field Name 06 50 44.043 +31 10 00.09 1.16 0.64 0.147 -3.0 -4.0 42.0 06510+31143E 06 51 02.181 +31 11 13.33 1.58 2.66 0.139 5.0 3.9 91.6 06510+31143E 06 51 03.826 +31 13 03.32 W 1.02 1.42 0.137 7.6 -3.3 2.4 06510+31143E 06 51 06.134 +31 19 02.00 7.29 9.64 0.145 4.1 1.8 131.3 06510+31143E 06 51 10.784 +31 11 28.80 72.36 100.79 0.141 4.2 2.5 9.8 06510+31143E
unc(90% confidence) = sqrt(5.4**2 + Size**2) (1/SNR + 1/20) arcsecwhere Size is either the major or minor axis FWHM as given in the catalog and SNR is the peak flux density signal-to-noise ratio:
SNR = (Fpeak-0.25) / Rms(The positional uncertainty is of course elliptical for elliptical sources.) The best possible positional uncertainty is limited to about 0.1 arcsec by our ability to fit source positions in maps with 1.8 arcsec pixels and by various random calibration uncertainties. Systematic errors in the positions are smaller than 0.05 arcsec.
The uncertainty in Fpeak is given by the rms noise at the source position, while the uncertainty in Fint can be considerably greater depending on the source size and morphology. For bright sources the accuracies of Fpeak and Fint are limited to about 5% by systematic effects. Note that for sources that are not well-described by an elliptical Gaussian model, Fint is not an accurate measure of the integrated flux density.
A FITS image giving the rms noise as a function of position on the sky is available. This is a 2200 x 479 pixel image (4.2 Mbytes) with the rms in mJy/beam tabulated on a ~3 arcmin grid in RA and Declination. If there is no source in the catalog at a given position, the source peak flux density (before CLEAN bias correction) is less than 5 times the coverage map rms value at that position.
Noise can cause the fitted values of the major and minor axes (before deconvolution) to be smaller than the beam. To preserve this information in the catalog, the deconvolved sizes are negative when the fitted size is smaller than the beam. The fitted sizes can be recovered by
Maj(fitted) = sqrt(5.4**2 + Maj**2) if Maj > 0 = sqrt(5.4**2 - Maj**2) if Maj < 0and similarly for Min(fitted). For most purposes the best estimates of the source size are max(Maj,0) and max(Min,0).
The uncertainties in the sizes depend on both the brightness and the sizes. Objects at the catalog flux density limit have uncertainties of about 2 arcsec in their sizes (so faint objects with Maj < 2 arcsec are consistent with point sources.) An simple empirical estimate of the uncertainty is
Sigma(Size) = 10 arcsec (1/SNR + 1/75)where SNR is the signal-to-noise ratio defined above.
All field names in the current catalog end with E, indicating that they were constructed using the imaging pipeline that applies a small rotation and scale correction. See the catalog history page for more information.
Previously released versions of the FIRST catalogs are still available for historical purposes. We recommend that the new catalog be used where possible for all projects.