Sextans A: A case study of star formation and gas densities in irregular galaxies

As part of an effort to understand how stars form in irregular galaxies, we have used UBV and H alpha imaging to trace the past and current star formation in the tiny irregular galaxy Sextans A. We have also used a published rotation curve to calculate the critical gas density necessary for instabilities that lead to cloud formation and then to star formation according to star formation models. The current star formation is not uniformly distributed throughout the galaxy but appears to move around on large spatial scales. A comparison with the observed gas surface densities shows that the current star formation is located in the region of the galaxy with higher radially averaged observed gas densities relative to the critical density for stability of the gas. However, this ratio is lower than the low end of what is observed for actively star-forming spiral galaxies and suggests that the gas in Sextans A is marginally stable against collapse. This means that clouds in Sextans A have a harder time forming, which is consistent with the low absolute rate of star formation in Sextans A integrated over its lifetime compared to spirals. Because Sextans A is such a small galaxy, the coming and going of normal giant H II regions or gas clouds can dominate the star formation and interstellar medium of the galaxy: Star formation is a very grainy process in small galaxies. It is also likely that other processes, such as feedback from newly formed massive stars, play important roles in facilitating future cloud formation.