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 HI clouds are the high-density parts of large HI rings/arcs formed by dynamical interaction between galaxy groups and gas-rich, low surface brightness (LSB) galaxies with extended gas disks. Our hydrodynamical simulations demonstrate that the group tidal field is very efficient at stripping the outer HI gas of the disk if the gaseous disk of the LSB galaxy extends $2 - 5$ times further than the stellar disk. We find that a massive, extended `leading stream' orbiting the group's center can form out of the stripped outer HI envelope, while the severely shrunk LSB galaxy, whose stellar disk remains unaffected, continues on its path. The result is a relatively isolated, massive HI cloud with a ring- or arc-like shape, a very inhomogeneous density distribution ($N_{\rm HI} \sim 1.0 \times 10^{17} - 1.1 \times 10^{20}$ atoms cm$^{-2}$), and, initially, no stellar content. Only the high density peaks of the simulated intragroup HI ring/arc can be detected in many current HI observations. These will appear as relatively isolated `HI islands' near the group center. 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 ($\sim$ 4 $\times$ $10^9$ ${\rm M}_{\odot}$). We discuss these results in terms of existing observations of intragroup gas (e.g., the Leo Ring and HIPASS J0731--69) and intergalactic HII regions. 
K. Bekki, B. S. Koribalski, S. D. Ryder, W. J. Couch