The Arecibo legacy fast alfa survey. III. HI 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 h 44 m < R.A. (J2000.0) < 14 h 00 m and +12° < decl. (J2000.0) < +16° 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 ...