Dark galaxies or tidal debris? Kinematical clues to the origin of massive isolated HI clouds

An extended HI cloud (VIRGOHI21) with an HI mass of similar to 10(8)M(circle dot) and no apparent optical counterpart was recently discovered in the Virgo cluster. In order to understand the origin of physical properties of apparently isolated HI clouds like VIRGOHI21, we numerically investigate physical properties of tidal HI debris that were formed by galaxy-galaxy interactions in clusters of galaxies. Our hydrodynamical simulations demonstrate that tidal debris with total HI masses of 10(8)-10(9)M(circle dot) can have (1) a wide spread of HI velocities (> 200 km s(-1)), (2) a small mass fraction of stars (similar to 10 per cent), and (3) a mean B-band surface brightness of the stellar components fainter than 30 mag arcsec(-2). These results suggest that VIRGOHI21, which lies at a projected distance of similar to 150 kpc from the one-armed, HI-rich spiral galaxy M99 (NGC 4254), is tidal debris. We propose that the comparison between the simulated and the observed velocity fields of HI clouds allows us to better understand their nature and origin (e. g. whether they are just tidal debris or are 'dark galaxies' that have HI gas only and are embedded within dark matter haloes).