APM Optical Counterparts to FIRST Radio Sources

McMahon, R. G., White, R. L., Helfand, D. J., & Becker, R. H.
"Optical Counterparts for 70,000 Radio Sources: APM Identifications for the FIRST Radio Survey"
2001, Astrophysical Journal Supplement, in press

We have identified optical counterparts to radio sources from the VLA FIRST survey using the Cambridge APM scans of the POSS-I plates. In the process of identifying approximately 73,000 optical counterparts to FIRST sources, we also have improved the astrometric and photometric calibration of the APM catalog. The paper includes 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 also included.

This paper is available both in HTML and in a gzip-compressed postscript version. The paper can also be obtained from the astro-ph e-print archive at http://arXiv.org/abs/astro-ph/0110437.

A number of files are available with data from this paper:

List of closest APM-FIRST matches. This table lists the radio and optical parameters (positions, fluxes, magnitudes, etc.) for APM-FIRST identifications separated by less than 5 arcsec. Each APM and FIRST source appears only once -- if a source has more than one match, only the closest is listed in the table. Most of these matches represent true associations between the radio and optical sources; within 4.2 arcsec, only 10% of the associations are the result of chance.

This is a 3.5 Mbyte gzip-compressed text file (11 Mbytes uncompressed) with 85,611 entries. The format is described in the paper and in the readme file.

List of all APM-FIRST matches. This table lists the radio and optical parameters for APM-FIRST identifications separated by up to than 20 arcsec. Both APM and FIRST sources may appear more than once in the table -- if a source has more than one match, all matches are listed. Optical objects can also appear as several different entries in the catalog if they lie in the narrow regions where the edges of POSS-I plates overlap. Most of the wider separation matches are chance coincidences, although even some pairs with separations as large as 15 arcsec can be confidently associated.

This is a 16 Mbyte gzip-compressed text file (64 Mbytes uncompressed) with 496,791 entries. It includes 261,434 unique radio sources and 454,754 unique optical sources (including plate-overlap duplicates.) The format is the same as the close match list.

apm2aps_corr.dat Photometric calibration of APM plates. Coefficients for polynomial corrections are given for the 148 POSS-I plates included in the survey area.
intraplate_distortions.dat Astrometric calibration for APM plates. Distortion correction for APM positions is given as a function of plate position. It is applied to each source based on the source's APM (X,Y) pixel coordinates.
plate_offsets.dat Astrometric calibration for APM plates. RA & Dec offsets are given for each APM plate. They are applied as a zero-point shift to all the sources on a plate.

Further descriptions of all tables can be found in the paper.

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Richard L. White, rlw@stsci.edu
2001 October 18