NEUTRAL HYDROGEN DISTRIBUTIONS AND KINEMATICS OF GIANT LOW SURFACE BRIGHTNESS DISK GALAXIES

We present high sensitivity VLA H I observations of four giant low sur face brightness (LSB) disk galaxies. As in the optical, the H I surfac e brightnesses of these galaxies are low, though their total H I masse s are quite high (similar or equal to 10(10) M.h(75)(-2) ). The H I ex tents are similar to the optical disk sizes, in some cases as large as 130 kpc h(75)(-1). The rotation curves rise slowly and are consistent with negligible disk contribution, similar to many previously observe d dwarf galaxies. However, the peak rotation velocities of these galax ies are high (> 200 km s(-1)) and infer high dynamical masses. These g alaxies provide the first examples of galaxies that are both massive a nd 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 IL. Also, the large masses of these LSB halos makes it difficult for l arge-scale baryonic blow-outs to affect the structure of the halos as can occur in dwarf galaxies. At large radii even fairly low surface de nsities 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 thres hold do not have attendant star formation. This implies that sigma(gas ) may be greater than 10 km s(-1) in these cases. In one of these syst ems, F568-6, we find gas moving at high speed with respect to the regu larly rotating disk and coincident with a region of active star format ion. Most likely these are the remains of a dwarf fallen through the d isk 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. (C) 1997 American Astronomical Society.