Optically unseen HI detections toward the Virgo cluster detected in the Arecibo Legacy Fast ALFA survey

We report the discovery by the Arecibo Legacy Fast ALFA ( ALFALFA) survey of eight H I features not coincident with stellar counterparts in the Virgo Cluster region. All of the H i clouds have km s(-1) 1 cz ! 3000 and, if at the Virgo distance, H i masses between and. Four of the eight objects were 7 9 1.9 * 10 1.1 * 10 M, reported or hinted at by previous studies and rediscovered by ALFALFA. While some clouds appear to be associated with optical galaxies in their vicinity, others show no clear association with a stellar counterpart. Two of them are embedded in relatively dense regions of the cluster and are associated with M49 and M86; they were previously known. The others are mostly located in peripheral regions of the cluster. Especially notable are a concentration of objects toward the so- called M cloud, 3 degrees - 5 degrees to the northwest of M87, and a complex of several clouds projected roughly halfway between M87 and M49. The object referred to as VIRGOHI 21 and proposed to be a dark galaxy is also detected and shown to be a tidal feature associated with NGC 4254.