Star Formation Models for the Dwarf Galaxies NGC 2915 and NGC 1705
  Crucial to a quantitative understanding of galaxy evolution are the
properties of the inter-stellar medium that regulate galactic-scale star
formation activity. We present here the results of a suite of star formation
models applied to the nearby blue compact dwarf galaxies NGC 2915 and NGC 1705.
Each of these galaxies has a stellar disk embedded in a much larger,
essentially star-less HI disk. These atypical stellar morphologies allow for
rigorous tests of star formation models that examine the effects on star
formation of the HI, stellar and dark matter mass components, as well as the
kinematics of the gaseous and stellar disks. We use far ultra-violet and 24
micron imaging from the Galaxy Evolution Explorer and the Spitzer Infrared
Nearby Galaxies Survey respectively to map the spatial distribution of the
total star formation rate surface density within each galaxy. New
high-resolution HI line observations obtained with the Australia Telescope
Compact Array are used to study the distribution and dynamics of each galaxy's
neutral inter-stellar medium. The standard Toomre Q parameter is unable to
distinguish between active and non-active star forming regions, predicting the
HI disks of the dwarfs to be sub-critical. Two-fluid instability models
incorporating the stellar and dark matter components of each galaxy, in
addition to the gaseous component, yield portions of the inner disk unstable.
Finally, a formalisation in which the HI kinematics are characterised by the
rotational shear of the gas produces models that very accurately match the
observations. This suggests the time available for perturbations to collapse in
the presence of rotational shear to be an important factor governing
galactic-scale star formation.
