The Relationship Between Gas, Stars, and Star Formation in Irregular Galaxies: A Test of Simple Models

Abstract

We explore various instability models for the onset of star formation in irregular galaxies. Critical gas densities are calculated for gravitational instabilities with a thin, pure-gas disk, and with a thick disk composed of gas and a star-like fluid. We also calculated the stability properties of three dimensional systems including dark matter, considered the thermal state of the gas, and used a modified threshold column density written in terms of the local rate of shear instead of the epicyclic frequency. The model predictions were compared to the azimuthally-averaged present day star formation activity traced by the Halpha surface brightness, and to the 1 Gyr-integrated star formation activity represented by the stellar surface brightness. The ratio of the observed gas density to the critical gas density is lower by a factor of 2 in most of the Im galaxies than it is in spiral galaxies. Star formation ends before this ratio drops significantly in the outer regions, and it remains high in the inner regions where the ratio is often low. These results suggest that the critical gas density does not trace star formation with the same detail in irregular galaxies as it appears to trace it in giant spiral galaxies. The only azimuthally-averaged quantity that correlates with the current star formation activity in irregulars is the stellar surface density. Theoretical implications are discussed.