The FIRST Survey 95Oct16 Catalog

This is the description of the 1995 October 16 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 It has been replaced by a newer catalog.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.

This catalog covers a much larger area than the initially released FIRST catalog (95jan06) and also is of higher quality. Note that the format has changed slightly, as have the interpretations of some of the columns. This version of the catalog is preferred for all scientific programs.

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 00.14       1.16      0.64   0.147  -3.0  -4.0   0.0 06510+31143B
06 51 02.178 +31 11 13.33       1.58      2.66   0.139   5.0   3.9  90.4 06510+31143B
06 51 03.824 +31 13 03.32 W     1.02      1.42   0.137   7.6  -3.3   0.0 06510+31143B
06 51 06.135 +31 19 01.97       7.29      9.64   0.145   4.1   1.8 131.3 06510+31143B
06 51 10.781 +31 11 28.81      72.37    100.78   0.141   4.2   2.5   9.8 06510+31143B

• 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) = sqrt(5.4**2 + Size**2)  (1/SNR + 1/20)  arcsec
  

  where 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 0.2 arcsec by our ability to fit source positions in maps with 1.8 arcsec pixels. Systematic errors in the positions are smaller than 0.05 arcsec.

• `W' is a warning flag indicating that the source may be a sidelobe of a nearby bright source. In the catalog, 4,813 sources (3.5%) are flagged as possible sidelobes. The flagging algorithm is an artificial intelligence approach based on oblique decision trees. Our analysis indicates that <10% of the objects flagged as sidelobes are real sources and that considerably less than 1% of the unflagged sources in the catalog are sidelobes. Complete details of the sidelobe identification procedure are included in our catalog paper.

• 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.

  A FITS image giving the rms noise as a function of position on the sky is available. This is a 2200 x 299 pixel image (2.6 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 is less than 5 times the coverage map rms value at that position.

• 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 5.4 arcsec FWHM point-spread function.

  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 < 0
  

  and similarly for Min(fitted). For most purposes the best estimates of the source size are max(Maj,0) and max(Min,0).

  NOTE: For sources with minor axes smaller than zero, the position angle in the catalog is always zero. This is the result of a mistake in the computation of the deconvolved position angle and is repaired in later releases.

  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.

• The Field Name is the name of the coadded image (available through several archives) 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.

  Field names that end with A were constructed entirely from 1993 observations, while fields names than end with B include at least some data from 1994 observations. Thus the A or B gives a rough indication of the epoch of the observation.

The preliminary catalog released in January 1995 is still available for historical purposes. We recommend that the new catalog be used where possible for all projects. In addition to having 5 times as many sources as the preliminary catalog, the new catalog has these improvements compared with the preliminary version:

• The rms noise level is more accurate in fields with bright sources.
• Sources that are probably sidelobes of bright objects are flagged.
• The CLEAN bias correction has been added to flux densities.

In the overlapping region the new catalog is very similar to the preliminary catalog, so it should not be difficult to migrate on-going projects to the new version.

FIRST Home Page