ALFALFA Discovery of an HI Cloud Complex in the Virgo Cluster

Giovanelli et al. (Cornell University)

One of the goals of ALFALFA (the Arecibo Legacy Fast ALFA Survey) is that of verifying whether a significant population of optically faint, baryon rich, low mass systems exists. The preliminary ALFALFA results indicate that such systems are relatively abundant. While this statement is still based on very preliminary statistical grounds (the lowest S/N detections need still to be corroborated), it may be useful to note that about 10 nearby, extragalactic HI sources have been detected at encouragingly high S/N, which appear not to have optical counterparts, nor do they appear to be the result of recent tidal events. ALFALFA has easily detected the cloud VIRGOHI 21, reported by Davies et al. (2004). The more comprehensive and higher quality ALFALFA dataset shows a clear tidal stream associated with the galaxy NGC 4254 (Phookun, Vogel & Mundy 1993) and extending well beyond the boundary of the HI indicated by either the single pixel Arecibo (Minchin et al. 2005a) and more recent WSRT measurements (Minchin et al. 2005b). Additionally, a newly discovered, very widespread ALFALFA source (AHI 123026.0+092800; see figure below), also in the periphery of the Virgo cluster, has been confirmed by follow-up observations at Arecibo. Thanks to the NRAO rapid response proposal mode, a synthesis map of the central region of this feature has very recently been obtained with the VLA. As evident in the figure, the source consists of a complex of clouds stretching for some 250 kpc, if located at the Virgo distance. The total mass of the system approaches 109 M_sun, including a main component containing about half the mass in the system. No optical counterpart has been identified, and no ordered velocity field is apparent in the complex. The synthesis data of the largest cloud reveal a high degree of clumpiness. A very low surface brightness dwarf is offset 1.6' (7.5 kpc) from one of the smaller clumps of gas. The clouds in the complex appear not to be gravitationally bound to one other, and may represent the remnant of an ancient episode of harassment. A paper on this discovery is currently in preparation.

HI distribution in the vicinity of the newly discovered cloud AHI 123026.0+092800. The ALFALFA moment map 50 km/s wide is shown in the center (lower part of the map affected by poor sky coverage), while the VLA-BnC moment 0 map is displayed to the right. SDSS images of (lower left) the HI detection field over a 6' x 6' box and (upper left) of one centered on a low surface brightness dwarf nearest to the HI cloud complex are inset.

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