Kinematics and star formation properties of low surface brightness galaxies

Abstract

We present detailed studies of the kinematics and star formation properties of low surface brightness (LSB) galaxies. A total of five giant LSB galaxies including the prototype, Malin 1, were imaged in the 21-cm line of H sc I to provide the first glimpse into the kinematics of these systems. We find that these are some of the first examples to be uncovered of galaxies that are both massive and dark matter dominated. We also find that most of the galaxies have gas surface densities that lie below the critical density for star formation at all radii, consistent with their lack of star formation. In a couple of cases, though, the gas exceeds the critical density where there is no star formation implying a higher gas velocity dispersion or strong flaring of the gas disk. Long-slit optical spectroscopy and broadband CCD imaging are presented for a total of 71 LSB galaxies. We find that these galaxies follow a Fisher-Tully relation with a slope that is in good agreement with the slopes found for other samples of LSB and HSB galaxies. We interpret both the optical and H sc I rotation curves in terms of mass models consisting of a halo only or a stellar component plus a halo of either an isothermal form or an NFW halo of the form described by Navarro et al. (1996) and find that the rotation curves are generally best modeled by low mass-to-light ratio stellar components. By modeling the rotation curves with only an NFW halo and comparing the results with the predictions of cosmological simulations we find that these data are marginally consistent at best with Standard Cold Dark Matter and generally favor lower density models. However, we also find that when including the stellar component, the NFW halo gives a significantly worse fit than an isothermal halo in many cases. This suggests that the inner parts of galactic halos may not be well described by the NFW halo profile.