2 The FIRST Survey

In its initial five years, the FIRST project surveyed $\sim$4670 deg$^{2}$ of the high Galactic latitude sky to a point-source flux density limit of 1.0 mJy at a wavelength of 20 cm. Using the B-configuration of the Very Large Array (VLA) in bandwidth-synthesis mode, we have produced nearly uniform-sensitivity images of the sky with a median rms of 0.14 mJy and an angular resolution of 5"; the positions for each of the half million sources in our current catalog are accurate to better than 1" (90% confidence). Details of the observing strategy and the pipeline processing system that creates the final images from the raw uv data are presented in Becker, White, and Helfand (1995; hereafter, BWH).

For the program described here, we have utilized the 98Feb04 version of the FIRST catalog, which is available from our Web homepage (http://sundog.stsci.edu). The catalog contains positions, peak and integrated flux densities, source morphological parameters, and information on the field from which each entry was derived. A detailed description of the catalog's construction can be found in White et al. (1997; hereafter WBHG); an exhaustive discussion of the astrometric, photometric, and morphological uncertainties is included in that paper and in BWH, and is summarized on our Web homepage.

The catalog has a mean surface density of $\sim$90 ``sources'' per square degree. However, it is important to note that a significant fraction of radio sources at these flux densities are extended. In particular, classical double-lobed radio sources in which the components are separated by more than $\sim$3" are typically represented by two catalog entries; for complex sources extended on scales of $\sim$1', several components can be required to fit adequately the source surface brightness distribution (see WBHG). We have not yet completed a definitive study of multi-component sources; even when this is done, it will not be possible to decide unambiguously whether or not two nearby catalog entries are parts of the same source hosted by a single optical counterpart, or whether they are chance alignments of unrelated objects. We briefly investigate these issues in § 5.4; prior to that discussion, we use the term ``sources'' synonymously with ``radio catalog entries''.

For clarity, we present in this paper only the results of the APM identification program for the $\sim$ 4150 deg$^2$ in the north Galactic cap. This region has approximate boundaries of $+22^{\circ}<\delta<+58^{\circ}$ and $07^{\rm h}30^{\rm m}<RA<17^{\rm h}30^{\rm m}$ and includes $\sim$ 382,892 radio sources. Subsequent extensions to this work for the two southern cap strips ( $+1.6^{\circ}>\delta<-2.5^{\circ}$ and $-7^{\circ}>\delta>-11^{\circ}$ in the range $21^{\rm h}30^{\rm m}<RA<03^{\rm h}00^{\rm m}$ for a total of 610 deg$^2$) as well as the remainder of the north cap region will be added to the Web site as they become available.