2008A&A...487..161G

Neutral hydrogen clouds are found in the Milky Way and Andromeda halo both as large complexes and smaller isolated clouds. Here we present a search for HI clouds in the halo of M 33, the third spiral galaxy of the Local Group. We have used two complementary data sets: a 3°x3° map of the area provided by the Arecibo Legacy Fast ALFA (ALFALFA) survey and deeper pointed observations carried out with the Arecibo telescope in two fields that permit sampling of the northeastern and southwestern edges of the HI disc. The total amount of HI around M 33 detected by our survey is about 10⁷M_☉. At least 50% of this mass is made of HI clouds that are related both in space and velocity to the galaxy. We discuss several scenarios for the origin of these clouds focusing on the two most interesting ones: (a) dark-matter dominated gaseous satellites, (b) debris from filaments flowing into M 33 from the intergalactic medium or generated by a previous interaction with M 31. Both scenarios seem to fit with the observed cloud properties. Some structures are found at anomalous velocities, particularly an extended HI complex previously detected by Thilker et al. (2002, in Seeing Through the Du st: the Detection of HI an d the Exploration of the ISM in Galaxies, ed. A.R. Taylor, T.L. Landecker, & A.G. Willis (San Francisco: ASP), 370). Even though the ALFALFA observations seem to indicate that this cloud is possibly connected to M 33 by a faint gas bridge, we cannot firmly establish its extragalactic nature or its relation to M 33. Taking into account that the clouds associated with M 33 are likely to be highly ionised by the extragalactic ultraviolet (UV) radiation, we predict that the total gas mass associated with them is ≥5x10⁷M_☉. If the gas is steadily falling towards the M 33 disc it can provide the fuel needed to sustain a current star formation rate of 0.5M_☉/yr.