The Arecibo Legacy Fast ALFA Survey. III. H I Source Catalog of the Northern Virgo Cluster Region

We present the first installment of H I sources extracted from the Arecibo Legacy Fast ALFA (ALFALFA) extragalactic survey, initiated in 2005. Sources have been extracted from three-dimensional spectral data cubes exploiting a matched filtering technique and then examined interactively to yield global H I parameters. A total of 730 H I detections are cataloged within the solid angle 11^h44^m<R.A. (J2000.0)<14^h00^m and +12^deg<decl. (J2000.0)<+16^deg and redshift range -1600 km s^-1<cz<18,000 km s^-1. In comparison, the H I Parkes All-Sky Survey detected 40 H I signals in the same region. Optical counterparts are assigned via examination of digital optical imaging databases. ALFALFA H I detections are reported for three distinct classes of signals: (1) detections, typically with S/N >6.5, (2) high-velocity clouds in the Milky Way or its periphery, and (3) signals of lower S/N (to ~4.5) which coincide spatially with an optical object of known similar redshift. Although this region of the sky has been heavily surveyed by previous targeted observations based on optical flux- or size-limited samples, 69% of the extracted sources are newly reported H I detections. The resulting positional accuracy of H I sources is dependent on S/N: it averages 24" (20" median) for all sources with S/N>6.5 and is of order ~17" (14" median) for signals with S/N>12. The median redshift of the sample is ~7000 km s^-1, and its distribution reflects the known local large-scale structure including the Virgo Cluster and the void behind it, the A1367-Coma supercluster at cz~7000 km s^-1, and a third, more distant overdensity at cz~13,000 km s^-1. Distance uncertainties in and around the Virgo Cluster perturb the derived H I mass distribution. Specifically, an apparent deficiency of the lowest H I mass objects can be attributed, at least in part, to the incorrect assignment of some foreground objects to the cluster distance. Several extended H I features are found in the vicinity of the Virgo Cluster. A small percentage (6%) of H I detections have no identifiable optical counterpart, more than half of which are high-velocity clouds in the Milky Way vicinity; the remaining 17 objects do not appear connected to or associated with any known galaxy. Based on these initial results, ALFALFA is expected to fulfill, and even exceed, its predicted performance objectives in terms of the number and quality of H I detections.