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

Irregular galaxies are a unique test of models for the physical laws regulating star formation because of their lack of spiral density waves and rotational shear. Here we explore various instability models for the onset of star formation in irregular galaxies. If the gas is unstable, clouds and eventually stars can form, and so these models should predict where star formation occurs. Critical gas densities were calculated for gravitational instabilities in two models, one with a thin, pure-gas disk (Σc), and another with a thick disk composed of gas and a star-like fluid (Σc,2f ). 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 Hα surface brightness, and to the 1 Gyr-integrated star formation activity represented by the stellar surface brightness. We find that the ratio of the observed gas density to the critical gas density, Σg/Σc, is lower by a factor of ∼ 2 in most of the Im galaxies than it is in spiral galaxies, at both the intermediate radii where Σg/Σc is highest, and at the outer radii where star formation ends. We also find that, although star formation in irregulars usually occurs at intermediate radii where Σg/Σc is highest, this activity often ends before Σg/Σc drops significantly in the outer regions, and it remains high in the inner regions where