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.
References Cited:
Davies,
J., Minchin, R., Sabatini, S., van Driel, W., Baes, M. et al. 2004,
MNRAS, 349, 922
Minchin,
R.F., Davies, J., Disney, M.J., Boyce, P. Garcia, D. et al. 2005a,
ApJ, 622, L21
Minchin,
R.F., Davies, J., Disney, M.J., Marble, A.R., Impey, C.D. et al.
2005b, astro-ph/0508153
Phookun,
B., Vogel, S.N. & Mundy, L.G. 1993, ApJ, 418, 113