Star formation thresholds in two atypical star-forming dwarfs

Star formation is a major driver of galaxy evolution.  A galaxy's luminosity, colours, metallicity and gas content are all controlled by its star formation activity.  A quantitative assessment of galactic-scale star formation and its links to the global properties of the ISM therefore serves as an important step towards understanding the life-cycles of galaxies.  Nearby galaxies serve as ideal laboratories in which to study star formation.  In this talk I present new, high-quality ultraviolet, infrared and HI imaging of two nearby dwarfs, NGC 2915 and NGC 1705.  These are atypical star-forming systems, each having a small stellar disk embedded in a large, extended HI disk.  It is not clear why star formation should only occur near the very centre of each galaxy's HI disk.  A suite of star formation models is examined for the galaxies in order to relate their star-forming properties to those of their inter-stellar media, with the aim of better understanding the interplay between the key properties of the ISM that regulate the star formation activity.  The star formation threshold of each galaxy is shown not to be a local phenomenon, depending on global kinematics as well as the stellar and dark matter potentials.  Treating ~10^9 solar masses of dark matter as being distributed within the HI disk of each galaxy leads to gravitational instabilities that are consistent with observations.  Alternatively, adopting the rotational shear as a characterization of the global ISM kinematics allows for accurate descriptions of the star-forming cores of the galaxies. These results therefore suggest that dark matter could play a potentially important role in regulating a galaxy's star formation activity, and that rotational shear can largely prevent star formation in the outer, differentially-rotating disks of late-type systems.