The FIRST Survey Catalog: 08Jul16 Version

See the coverage maps for more details of the area covered. Both the northern and southern areas were chosen to coincide approximately with the area covered by the Sloan Digital Sky Survey.

This version of the catalog has a region along the equatorial strip (RA = 21.3 to 3.3 hrs, Dec = -1 to 1 deg) where the detection threshold is deeper than 1 mJy because two epochs of observation were combined. The typical detection threshold in this region is 0.75 mJy. There are approximately 4,500 sources below the 1 mJy threshold used for all previous versions of the catalog.

This catalog was created by rerunning our source detection routine HAPPY on all the coadded images in order to produce a more consistent source lists. The small changes due to inevitable evolution in the software and hardware being used created corresponding small differences between this catalog and previous versions, even in regions where the images have not changed. The great majority of the sources are identical -- ~80% of the sources have positions identical to those in the last version of the catalog. ~1.5% of the sources from the old catalog do not appear in this version; such sources are generally in images with new data, are very near the detection threshold, or are in complex regions that get decomposed differently in the two 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 decreasing declination and has the following format:

                                                                                                                ------SDSS------ ------GSC2------ -----2MASS----
   RA           Dec        P(S)    Fpeak      Fint    RMS     Maj    Min   PA    fMaj   fMin   fPA Field         #  Sep    i  Cl  #  Sep    F  Cl  #  Sep    K  
14 35 55.835 +64 34  4.69 0.014     1.75      1.83   0.158   1.95   0.00  14.9   5.74   5.29  14.9 14360+64240M  1  4.76 20.60 g  0 99.00 99.00 -  0 99.00 99.00
11 27 20.244 +64 32 34.60 0.576     1.03      0.70   0.143   0.00   0.00 113.3   4.86   4.10 113.3 11300+64240M  1  5.36 18.44 s  1  5.17 19.26 g  1  5.27 15.47
06 57 23.618 +64 32 33.37 0.014     1.21      1.75   0.137   5.80   0.00 147.5   7.93   5.33 147.5 07000+64243J -1 99.00 99.00 -  0 99.00 99.00 -  0 99.00 99.00
13 21 43.254 +64 32 30.72 0.289     1.16      4.31   0.147  10.96   7.04  21.5  12.22   8.88  21.5 13240+64240M  0 99.00 99.00 -  0 99.00 99.00 -  0 99.00 99.00
10 07 16.439 +64 32 30.69 0.014     1.35      0.99   0.148   1.58   0.00   7.6   5.62   3.79   7.6 10060+64240M  1  1.29 20.68 s  1  0.76 99.99 g  0 99.00 99.00

• Names for FIRST sources are derived from the positions according to IAU recommendations.

• RA, Dec are the position (J2000) of the source. The accuracy of the position depends on the brightness and size of the source and the noise in the map. Point sources at the detection limit of the catalog have positions accurate to better than 1 arcsec at 90% confidence; 2 mJy point sources in typically noisy regions have positions good to 0.5 arcsec. An empirical expression for the positional accuracy is

     unc(90% confidence) = Size  (1/SNR + 1/20)  arcsec
  

  where Size is either the major or minor axis fitted FWHM (fMaj or fMin) 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.

• P(S) indicates the probability that the source is spurious (most commonly because it is a sidelobe of of a nearby bright source.) Low values mean the source is unlikely to be spurious. Here is the distribution of P(S) values in the catalog:

  Probability Range	Fraction of sources
  0.00 < PS < 0.05	77.9%
  0.05 < PS < 0.15	6.8%
  0.15 < PS < 0.25	4.2%
  0.25 < PS < 0.35	3.2%
  0.35 < PS < 0.45	1.8%
  0.45 < PS < 0.55	1.5%
  0.55 < PS < 0.65	1.4%
  0.65 < PS < 0.75	1.4%
  0.75 < PS < 0.85	1.0%
  0.85 < PS < 0.90	0.8%

  Sidelobe probabilities for this version of the catalog have been computed using an improved algorithm based on multiple voting oblique decision tree classifiers. The classifiers were trained using deep VLA fields that give reliable assessments of the reality of FIRST sources. The algorithm will be described in detail in a future paper on the final FIRST catalog; we believe it is a substantial improvement over the previous sidelobe flagging approach. Nevertheless, we still recommend checking the images using the FIRST Cutout Server if there is doubt about the reality of particular sources. This is easily done when using the FIRST Search Engine to search the catalog, since each source selected in the search has a link to the Cutout Server.

• Fpeak and Fint are the peak and integrated flux densities measured in mJy. They are derived by fitting an elliptical Gaussian model to the source. To correct for the "CLEAN bias" effect, 0.25 mJy has been added to the peak flux density and the integrated flux density has been multiplied by (1+0.25/Fpeak) (see our Astrophysical Journal paper and our catalog paper for more details.)

  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.

• Rms is a local noise estimate at the source position measured in mJy. Rms is computed by combining the measured noise from all grid pointing images contributing to this coadded map position. Note that the signficance of detection for a source is (Fpeak-0.25)/Rms, not Fpeak/Rms, because of the CLEAN bias correction to the peak flux density. The catalog includes only sources brighter than 5 Rms.

  FITS images giving the rms noise as a function of position on the sky are available for the northern and the southern areas. These images give 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. The sky area covered is displayed in the images below (which are linked to postscript versions of the images):

  

  

  Note that the FIRST Search Engine gives the rms noise estimate at the specified sky position, as derived from these coverage maps. That is the most convenient way to determine the noise at a particular position in the survey (or to determine whether a position has been observed.)

• Maj, Min, and PA give the major and minor axes (FWHM in arcsec) and position angle (degrees east of north) derived from the elliptical Gaussian model for the source. Maj and Min have been deconvolved to remove blurring by the elliptical Gaussian point-spread function. (The fitted parameters before deconvolution are given in the fMaj, fMin, and fPA columns.) In the north the beam is circular 5.4 arcsec FWHM; south of declination +4°33'21", the beam is elliptical, 6.4x5.4 arcsec FWHM, with the major axis running north-south. In the southern Galactic cap (RA = 21hrs to 3hrs), the elliptical beam size increases further to 6.8x5.4 arcsec south of declination -2°30'25".

  Noise can cause the fitted values of the major and minor axes (before deconvolution) to be smaller than the beam. The corresponding deconvolved size is given as zero in that case.

  The uncertainties in the deconvolved 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.

• fMaj, fMin, and fPA give the major and minor axes (FWHM in arcsec) and position angle (degrees east of north) derived from the elliptical Gaussian model for the source. These are the fitted sizes measured directly from the image; the elliptical point-spread function has not been deconvolved.

• The Field Name is the name of the coadded image containing the source. Note that the field name encodes the position of the field center: field hhmmm+ddmmm is centered at RA=hh mm.m, Dec=+dd mm.m. The images are available from several archives and through the FIRST Cutout Server.

  All field names in the current catalog end with a letter E through L, depending on the date of the last catalog release in which the image was modified. The L fields are new in this catalog, while sources extracted from the E-J fields are nearly identical to those in the previous version of the catalog. There are a few additional sources from the earlier fields because we improved the source extraction algorithms.

• The remaining 9 columns give information on optical counterparts from the DR6 release of the Sloan Digital Sky Survey and version 2.3.2 of the Guide Star Catalog-2, and infrared counterparts from the 2MASS catalog. The GSC-2 and 2MASS matches were performed with the assistance of the OpenSkyQuery VO interface, and the SDSS match used the CasJobs web interface.

  For each catalog there is a column giving the number of matches within a fiducial radius and, for the nearest match, the separation from the FIRST position in arcsec, the magnitude, and (for SDSS and GSC2) the morphological classification (s=stellar, g=nonstellar/galaxy). The fiducial radii for the counts are 8 arcsec for SDSS and 2MASS and 10 arcsec for GSC2. A count of zero indicates there are no sources within this radius (which is also indicated by a separation of 99.00 and a classification of '-'). For SDSS, a count of -1 indicates that the FIRST source falls outside the SDSS DR6 survey area so that no SDSS data are available. All of these catalogs have multiple wavebands available; the given magnitudes are for i (SDSS), F (GSC2, the red plate), and K (2MASS). For SDSS, a magnitude of -1 indicates that the magnitude in the DR6 catalog was given as -9999. Note that these new parameters can be used for catalog searches via the "Additional Constraints" field in the FIRST Search Engine web form. See the Additional Constraints Help Page for more information.

  Some information summarizing the frequency of counterparts in the various catalogs is given in the table below:

  Survey	Characteristic	Percentage
  SDSS	1 or more counterparts < 8"	53.8%
  SDSS	Counterpart < 1.5"	28.2%
  SDSS	Covered by DR6	93.6%
  2MASS	1 or more counterparts < 8"	11.7%
  2MASS	Counterpart < 1.5"	7.7%
  GSC2	1 or more counterparts < 10"	36.9%
  GSC2	Counterpart < 1.5"	17.5%

The catalog history page describes previously released versions of the FIRST catalogs, which are still available for historical purposes. We recommend that the new catalog be used where possible for all projects.