2008AJ....136..713K

We present the third installment of HI sources extracted from the Arecibo Legacy Fast ALFA extragalactic survey. This data set continues the work of the Virgo ALFALFA catalog. The catalogs and spectra published here consist of data obtained during the 2005 and 2006 observing sessions of the survey. The catalog consists of 578 HI detections within the range 11^h36^m < right ascension (R.A.)(J2000) <13^h52^m and +08° < declination (decl.)(J2000) <+12°, and cz_☉< 18, 000 km/s. The catalog entries are matched with optical counterparts where possible through the examination of digitized optical images. The catalog detections can be classified into three categories: (a) detections of high reliability with a signal-to-noise ratio (S/N) > 6.5; (b) high-velocity clouds in the Milky Way or its periphery; and (c) signals of lower S/N which coincide spatially with an optical object and known redshift. 75% of the sources are newly published HI detections. Of particular note is a complex of HI clouds projected between M87 and M49 that do not coincide with any optical counterparts. Candidate objects without optical counterparts are few. The median redshift for this sample is 6500 km/s and the cz distribution exhibits the local large-scale structure consisting of Virgo and the background void and the A1367-Coma supercluster regime at cz_☉∼ 7000 km/s. Position corrections for telescope pointing errors are applied to the data set by comparing the ALFALFA continuum centroid with those cataloged in the NRAO VLA⁷ Sky Survey (NVSS). The uncorrected positional accuracy averages 27'(21' median) for all sources with S/N > 6.5 and is of order ∼21'(16' median) for signals with S/N > 12. Uncertainties in distances toward the Virgo cluster can affect the calculated HI mass distribution.