The star formation law in disk galaxies --- a Bayesian view

J.Köppen, H.-E.Fröhlich

ABSTRACT The dependence of the star formation rate SFR propto g^x/r^y on gas surface density g and galactocentric radius r are studied in disk galaxies. The sample includes the Milky Way galaxy and 11 nearby Sb and Sc galaxies with known radial gas surface densities and Halpha surface brightnesses, used as a measure of the SFR. The Bayesian approach permits to compute genuine probabilities for the validity of several different functional forms. We do not find any significant explicit dependence of the SFR on radius. A linear dependence on gas density SFR propto g is found to be an order of magnitude more probable than any universal non-linear law. If each galaxy has its own exponent x, the non-linear law SFR propto g^x would also be highly probable. These findings are fairly independent of the CO-H2 conversion recipe used.