Testing the star formation law in bulgeless disk galaxies

We study the relation between the surface density of gas and star formation rate in twenty bulgeless disk galaxies using data from the IRAM 30m, MDM, Spitzer IRAC, and the VLA. Recent work has provided constraints on the physics that sets the star formation efficiency in varying environments of the interstellar medium; however, a single theory for star formation has yet to stand out among its peers. The general motivation for our study is to test the predictive power of current theories with substantially improved observations of late-type and low-mass disks, which are underrepresented in most surveys. We specifically study star formation and the properties of the cold and warm interstellar medium above and below the circular velocity threshold of 120 km s-1 (stellar mass ~ 1010 M&sun;), where Dalcanton et al. (2004) found that edge-on galaxies show an abrupt transition in their dust scale heights. This transition also corresponds to a disk stability transition according to the Toomre Q parameter. In our sample, we find no difference between low-mass (Vcirc < 120 km s-1) and high-mass (Vcirc > 120 km s-1) disks on the star formation law, which indicates that the physical processes responsible for star formation act on smaller scales than the tens of parsecs probed by dust lanes. We also find that the correlation between the star formation rate surface density and the total (atomic plus molecular) gas surface density is stronger than either the atomic or molecular correlation