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

An extended HI cloud (VIRGOHI 21) with an HI mass of $\sim10^8$ M$_{\odot}$ 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 the VIRGOHI21, we numerically investigate physical properties of tidal HI debris that were formed by galaxy-galaxy interaction in clusters of galaxies. Our hydrodynamical simulations demonstrate that tidal debris with total HI masses of $10^8-10^9$ M$_{\odot}$ can have (1) wide HI velocity widths ($>200$ km s$^{-1}$), (2) a small mass fraction of stars ($\sim$10%), and (3) a mean $B$-band surface brightness of the stellar components fainter than 30 mag arcsec$^{-2}$. These results suggest that the VIRGOHI21, which lies at a projected distance of $\sim$ 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 ``dark galaxies'' that have HI gas only and are embedded by dark matter halos). We also discuss the timescales for isolated HI gas to be evaporated by thermal conduction of the hot intracluster gas.