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 starlike 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, both at 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 Σg/Σc is often low. There are also no correlations between the peak, average, or edge values of Σg/Σc and the integrated star formation rates in irregulars. These results suggest that Σg/Σc does not trace star formation with the same detail in irregular galaxies as it appears to trace it in giant spiral galaxies. The low value of α also implies that either the gas in irregulars is more stable than it is in spirals, or Σc is not a good threshold for star-forming instabilities. Dark matter in the disks of irregulars makes the gas more unstable, but stars do the same for the disks of spirals, which leaves the ratio of the two α-values about the same. Moreover, the instability parameter with dark matter still does not follow the star formation activity in irregulars. The thermal model suggests that irregulars have difficulty in sustaining a cool, dense gas phase, and it also fails to predict where star formation occurs. An alternative model in which cloud formation involves a competition between self-gravity and shear, rather than an instability in the usual sense, is more successful in defining the threshold for star formation, but it does not predict where star formation ends either. The failure of these models suggests that processes other than spontaneous instabilities are important for star formation in irregular galaxies. The role of Σg/Σc in spiral galaxies is also questioned. The observed sensitivity of the star formation rate to Σg/Σc may be strongly dependent on instabilities specific to spiral arms and not on general instabilities of the type for which Σg/Σc was originally derived. In that case, large-scale star formation may end in the outer disks of spirals because the stellar density waves end there, at the outer Lindblad resonance. The only azimuthally averaged quantity that correlates with the current star formation activity in irregulars is the stellar surface density. A causal connection is possible if stellar energy input to the interstellar medium acts as a feedback process to star formation. If this process played a key role in initiating star formation in irregulars from the beginning, then it could explain why irregular galaxies began their evolution slowly compared to larger disk systems with spiral arms.