Massive HI clouds with no optical counterparts as high-density regions of intragroup HI rings and arcs

We present a new scenario in which massive intragroup H i clouds are the high-density parts of large H i rings/arcs formed by dynamical interaction between galaxy groups and gas-rich, low surface brightness (LSB) galaxies with extended gas discs. Our hydrodynamical simulations demonstrate that the group tidal field is very efficient at stripping the outer H i gas of the disc if the gaseous disc of the LSB galaxy extends 2–5 times further than the stellar disc. We find that a massive, extended 'leading stream' orbiting the centre of the group can form out of the stripped outer H i envelope, while the severely shrunken LSB galaxy, whose stellar disc remains unaffected, continues on its path. The result is a relatively isolated, massive H i cloud with a ring- or arc-like shape, a very inhomogeneous density distribution (NH i~1.0 x 10^17-1.1 x 10^20 atom cm^-2) , and, initially, no stellar content. Only the high-density peaks of the simulated intragroup H i ring/arc can be detected in many current H i observations. These will appear as relatively isolated 'H i islands' near the group centre. We also find that star formation can occur within the ring/arc, if the total gas mass within the intragroup ring/arc is very large (~4 x 10^9 M⊙) . We discuss these results in terms of existing observations of intragroup gas (e.g. the Leo Ring and HIPASS J0731-69) and intergalactic H ii regions.