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

## TL;DR

This study uses numerical simulations to investigate whether isolated HI clouds are primordial dark galaxies or tidal debris, finding that tidal interactions are a more probable explanation for most observed clouds.

## Contribution

The paper demonstrates through simulations that tidal debris from galaxy harassment can produce HI clouds with properties similar to dark galaxy candidates, challenging the dark galaxy hypothesis.

## Key findings

- Tidal harassment can create long HI streams with features resembling dark galaxy candidates.
- Most small HI clouds are likely tidal debris, not primordial dark galaxies.
- High-velocity width clouds are rare in tidal debris simulations, suggesting different origins.

## Abstract

Isolated HI 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' which have not formed stars. Similarly, certain kinematic features in HI 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 SPH particles for the gas and n-bodies for the stars and dark matter, while the cluster includes the gravitational effects of substructure using 400 subhalos (the effects of the intracluster medium are ignored). We find that harassment can indeed produce long HI 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 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 is negligible - making this a very unlikely explanation for the observed clouds in the Virgo cluster with these properties.