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

An extended H i cloud (VIRGOH i 21) with an H i mass of ∼108 M⊙ and no apparent optical counterpart was recently discovered in the Virgo cluster. In order to understand the origin of physical properties of apparently isolated H i clouds like VIRGOH i 21, we numerically investigate physical properties of tidal H i debris that were formed by galaxy—galaxy interactions in clusters of galaxies. Our hydrodynamical simulations demonstrate that tidal debris with total H i masses of 108–109 M⊙ can have (1) a wide spread of H i velocities (>200 km s−1), (2) a small mass fraction of stars (∼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 VIRGOH i 21, which lies at a projected distance of ∼150 kpc from the one-armed, H i-rich spiral galaxy M99 (NGC 4254), is tidal debris. We propose that the comparison between the simulated and the observed velocity fields of H i clouds allows us to better understand their nature and origin (e.g. whether they are just tidal debris or are ‘dark galaxies’ that have H i gas only and are embedded within dark matter haloes).