Neutral hydrogen distributions and kinematics of giant low surface brightness disk galaxies

We present high sensitivity VLA H I observations of four giant low surface brightness (LSB) disk galaxies. As in the optical, the H I surface brightnesses of these galaxies are low, though their total H I masses are quite high (≃ 10¹⁰ M_⊙h^-2₇₅). The H I extents are similar to the optical disk sizes, in some cases as large as 130 kpc h^-1₇₅. The rotation curves rise slowly and are consistent with negligible disk contribution, similar to many previously observed dwarf galaxies. However, the peak rotation velocities of these galaxies are high (> 200 km s^-1) and infer high dynamical masses. These galaxies provide the first examples of galaxies that are both massive and dark matter dominated. Therefore, their rotation curves provide an important constraint on the structure of dark matter halos. Due to the small contribution of the baryonic component, interpreting the shapes of these curves is not strongly dependent on assumptions of stellar M/L. Also, the large masses of these LSB halos makes it difficult for large-scale baryonic blow-outs to affect the structure of the halos as can occur in dwarf galaxies. At large radii even fairly low surface densities of gas can rise above the critical density threshold described by Kennicutt (1989, ApJ, 344, 685). This is observed in two of these galaxies, however the regions with gas surface density above the threshold do not have attendant star formation. This implies that σ_gas may be greater than 10 km s^-1 in these cases. In one of these systems, F568-6, we find gas moving at high speed with respect to the regularly rotating disk and coincident with a region of active star formation. Most likely these are the remains of a dwarf fallen through the disk and having set off star formation in the process. This provides a possible clue as to how stars ever formed in these low volume density systems.