Massive H I clouds with no optical counterparts as high-density regions of intragroup H I 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 (N-HI approximate to 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 (approximate to 4 x 10(9) M circle dot). 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.