HI Observations of Low Surface Brightness Galaxies: Probing Low Density Galaxies

Abstract

We present Very Large Array (vla) and Westerbork Synthesis Radio Telescope (wsrt)21-cm Hi observations of 19 late-type low surface brightness (LSB) galaxies. Our mainfindings are that these galaxies, as well as having low surface brightnesses, have lowHi surface densities, about a factor of 3 lower than in normal late-type galaxies. Weshow that LSB galaxies in some respects resemble the outer parts of late-type normalgalaxies, but may be less evolved. LSB galaxies are more gas-rich than their high surfacebrightness counterparts. The rotation curves of LSB galaxies rise more slowly than thoseof HSB galaxies of the same luminosity, with amplitudes between 50 and 120 km s−1,and are often still increasing at the outermost measured point. The shape of the rotationcurves suggests that LSB galaxies have low matter surface densities. We use the averagetotal mass surface density of a galaxy as a measure for the evolutionary state, and showthat LSB galaxies are among the least compact, least evolved galaxies. We show thatboth MHI/LB and Mdyn/LB depend strongly on central surface brightness, consistentwith the surface brightness–mass-to-light ratio relation required by the Tully-Fisherrelation. LSB galaxies are therefore slowly evolving galaxies, and may well be low surfacedensity systems in all respects.