From: Richard L. White

Optical Counterparts for 70,000 Radio Sources: APM Identifications for the FIRST Radio Survey

Authors: Richard G. McMahon, Richard L. White, David J. Helfand, Robert H. Becker

Comments: Accepted for publication in ApJ; 28 pages, 23 figures

We describe a program to identify optical counterparts to radio sources from the VLA FIRST survey using the Cambridge APM scans of the POSS-I plates. We use radio observations covering 4150 square degrees of the north Galactic cap to a 20 cm flux density threshold of 1.0 mJy; the 382,892 sources detected all have positional uncertainties of <1" (radius of 90% confidence). Our description of the APM catalog, derived from the 148 POSS-I O and E plates covering this region, includes an assessment of its astrometric and photometric accuracy, a photometric recalibration using the Minnesota APS catalog, a discussion of the classification algorithm, and quantitative tests of the catalog's reliability and completeness. We go on to show how the use of FIRST sources as astrometric standards allows us to improve the absolute astrometry of the POSS plates by nearly an order of magnitude to ~0.15" rms. Matching the radio and optical catalogs yields counterparts for over 70,000 radio sources; we include detailed discussions of the reliability and completeness of these identifications as a function of optical and radio morphology, optical magnitude and color, and radio flux density. An analysis of the problem of radio sources with complex morphologies (e.g., double-lobed radio galaxies) is included. We conclude with a brief discussion of the source classes represented among the radio sources with identified counterparts.

Format for the Table of FIRST-APM Optical Counterparts

From the paper:

Column  Name        Description
------  ----        -----------
1-2     RA, Dec     radio source coordinates (epoch J2000.0)
3-5     dRA, dDec   offsets (arcsec) to the optical object in RA and Dec
5       Sep         radius (arcsec) to the optical object
6       Ecl         APM classification on E plate
7       Epsf        APM psf parameter on E plate
8       Emag        E (red) magnitude
9       Ocl         APM classification on O plate
10      Opsf        APM psf parameter on O plate
11      Omag        O (blue) magnitude 
12      O-E         Color
13      Plate       Number of POSS-I plate
14      Fpeak       FIRST peak radio flux density (mJy)
15      Fint        FIRST integrated radio flux density (mJy)
16      Maj         FIRST source deconvolved major axis (FWHM, arcsec)
17      Min         FIRST source deconvolved minor axis (FWHM, arcsec)
18      PA          FIRST position angle for major axis (degrees)

Notes on parameters:

- APM positions and magnitudes have been calibrated as described in the
  paper.
- The classification codes (Ecl, Ocl) are -1=stellar, 1=non-stellar,
  2=blended, 0=noise.
- The Epsf and Opsf parameters measure by how many sigma the object
  deviates from being a point-source.
- Negative magnitudes indicate that source is undetected on the plate.
  In that case, the absolute value of the magnitude is an upper limit.
- The O-E color is -9.99 if the object is detected on only one plate.
- The radio parameters are described in more detail in White, Becker,
  Helfand, & Gregg (1997, ApJ, 475, 479) and on the FIRST web pages
  (http://sundog.stsci.edu).
- Negative values for the FIRST major and minor axes indicate that the
  source size before deconvolution was smaller than the beam, so the
  source is unresolved along that axis.