HI OBSERVATIONS OF LOW SURFACE BRIGHTNESS GALAXIES - PROBING LOW-DENSITY GALAXIES

We present Very Large Array (VLA) and Westerbork Synthesis Radio Teles cope (WSRT) 21-cm HI observations of 19 late-type low surface brightne ss (LSB) galaxies. Our main findings are that these galaxies, as well as having low surface brightnesses, have low HI surface densities, abo ut a factor of similar to 3 lower than in normal late-type galaxies. W e show that LSB galaxies in some respects resemble the outer parts of late-type normal galaxies, but may be less evolved. LSB galaxies are m ore gas-rich than their high surface brightness counterparts, The rota tion curves of LSB galaxies rise more slowly than those of HSB galaxie s of the same luminosity, with amplitudes between 50 and 120 km s(-1), and are often still increasing at the outermost measured point. The s hape of the rotation curves suggests that LSB galaxies have low matter surface densities, We use the average total mass surface density of a galaxy as a measure for the evolutionary state, and show that LSB gal axies are among the least compact, least evolved galaxies. We show tha t both M(HI)/L(B) and M(dyn)/L(B) depend strongly on central surface b rightness, consistent with the surface brightness-mass-to-light ratio relation required by the Tully-Fisher relation. LSB galaxies are there fore slowly evolving galaxies, and may well be low surface density sys tems in all respects.