A Catalog of 1.4 GHz Radio Sources from the FIRST Survey

We present a catalog of 138,665 radio sources derived from the initial 1550 deg² of the FIRST survey. The survey parameters are reviewed, and a map depicting the coverage for the first two observing sessions is presented. We then describe in detail our algorithm for radio source detection and parameterization, as well as our procedures for constructing the final catalog. The results of extensive tests for astrometric and photometric accuracy, as well as for uncertainties in source extent and morphological characterization, are presented. Source positions are all good to better than 1", and the flux density scale is accurate to 5%; our elliptical Gaussian fitting procedure is shown both to provide a high-fidelity representation of source morphology for most objects and to characterize correctly bright sources extended on scales down to ~1/3 the synthesized beam size of 5.4". We use the catalog to construct a log N-log S relation for 20 cm radio sources over four decades of flux density, finding excellent agreement with previous determinations from smaller surveys. As one example of the catalog's utility, we present comparisons with the Guide Star Catalog, the IRAS Faint Source catalog, and the ROSAT WGA catalog, identifying radio counterparts to thousands of the objects. Complete instructions for access to this FIRST catalog and subsequent updates thereto are provided.