7 The Catalog of FIRST Optical Counterparts

Table 4 displays a sample page of the catalog of APM-FIRST matches which is available from the FIRST home page (http://sundog.stsci.edu). Columns 1 and 2 contain the radio source coordinates (epoch J2000.0) and are followed by the offsets in Right Ascension, Declination, and radius (all in arcsec) of the nearest optical object falling within $5\arcsec$ of the radio position. The global APM/FIRST shift, the plate-to-plate shifts, and the intraplate shifts as determined by FIRST (§ 4) have all been applied to the raw APM catalog positions before calculating these offsets. Columns 6-8 contain the optical classification, the psf parameter (see § 3), and the magnitude of the counterpart on the red ($E$) plate. The optical classification codes are: stellar (consistent with the magnitude- and position-dependent point spread function, cl$=-1$), non-stellar (a measurably extended source, cl=1), merged objects (sources with two local maxima within a single set of connected above-threshold pixels, cl=2), and noise (objects with nonphysical morphologies, cl=0). For further details of the principles involved, see § 3 and Maddox et al. (1991a,b). A negative magnitude indicates that the object was not detected in this waveband and the absolute value is a 2$\sigma$ upper limit. Columns 9-11 list the same information for the blue ($O$) plate. Column 12 is the ($O-E$) color derived from columns 8 and 11 unless the object is only detected on one plate, whereupon the color is designated -9.99. All raw APM magnitudes have been corrected for plate to plate systematic calibration errors as described in § 3.4; the uncertainties (rms) in the magnitudes are $\sim0.2$. The POSS plate number in column 13 is followed by selected radio parameters for the FIRST source, including the peak and integrated flux densities, and the deconvolved source major axis, minor axis, and position angle. The major and minor axes (FWHM) have been deconvolved using a 5.4'' synthesized beam. Negative values indicate that the source size (before deconvolution) was smaller than the beam. These radio parameters are described in further detail in WBHG.

As demonstrated above, 95% of the counterparts within $2\arcsec$ are actual FIRST identifications. There are, however, a number of reasons for studying the contents of the optical sky even farther from the FIRST positions: double and multiple-component sources are likely to have parent galaxies with substantial offsets from the radio positions, and the clustering of optical objects around FIRST sources appears to be a fruitful area for investigation (Helfand et al. 1998). In addition, catalog users may find it helpful to be able to calculate false rates for particular ranges of optical magnitude, radio flux density, or source morphology. Thus, we have constructed a second catalog (also available on-line) which includes all optical objects appearing on either POSS-I plate which lie within $20\arcsec$ of a FIRST position; 68% of the FIRST sources have at least one such counterpart. The format of this database is identical to that of the primary catalog; it contains nearly half a million entries representing 454,754 unique optical objects in the vicinity of 261,434 radio catalog entries. Also as the FIRST survey grows in size we shall periodically update these on-line optical catalogues.