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

We describe a program to identify optical counterparts to radio sources fromthe VLA FIRST survey using the Cambridge APM scans of the POSS-I plates. We useradio observations covering 4150 square degrees of the north Galactic cap to a20 cm flux density threshold of 1.0 mJy; the 382,892 sources detected all havepositional uncertainties of <1" (radius of 90% confidence). Our description ofthe APM catalog, derived from the 148 POSS-I O and E plates covering thisregion, includes an assessment of its astrometric and photometric accuracy, aphotometric recalibration using the Minnesota APS catalog, a discussion of theclassification algorithm, and quantitative tests of the catalog's reliabilityand completeness. We go on to show how the use of FIRST sources as astrometricstandards allows us to improve the absolute astrometry of the POSS plates bynearly an order of magnitude to ~0.15" rms. Matching the radio and opticalcatalogs yields counterparts for over 70,000 radio sources; we include detaileddiscussions of the reliability and completeness of these identifications as afunction of optical and radio morphology, optical magnitude and color, andradio flux density. An analysis of the problem of radio sources with complexmorphologies (e.g., double-lobed radio galaxies) is included. We conclude witha brief discussion of the source classes represented among the radio sourceswith identified counterparts.Comment: Accepted for publication in ApJ; 28 pages, 23 figure