Kinematic clues to the origins of starless HI clouds : dark galaxies or tidal debris?

Abstract

Isolated H i clouds with no optical counterparts are often taken as evidence for galaxy–galaxy interactions, though an alternative hypothesis is that these are primordial ‘dark galaxies’ that have not formed stars. Similarly, certain kinematic features in H i streams are also controversial, sometimes taken as evidence of dark galaxies but also perhaps explicable as the result of harassment. We numerically model the passage of a galaxy through the gravitational field of cluster. The galaxy consists of smoothed particle hydrodynamics particles for the gas and N-bodies for the stars and dark matter, while the cluster includes the gravitational effects of substructure using 400 subhaloes (the effects of the intracluster medium are ignored). We find that harassment can indeed produce long H i streams and these streams can include kinematic features resembling dark galaxy candidates such as VIRGOHI21. We also show that apparent clouds with diameter <20 kpc and velocity widths <50 km s−1 are almost invariably produced in these simulations, making tidal debris a highly probable explanation. In contrast, we show that the frequency of isolated clouds of the same size but velocity width >100 km s−1 is negligible – making this a very unlikely explanation for the observed clouds in the Virgo cluster with these properties.