Timescales on which Star Formation Affects the Neutral ISM
  Turbulent neutral hydrogen (HI) line widths are often thought to be driven
primarily by star formation (SF), but the timescale for converting SF energy to
HI kinetic energy is unclear. As a complication, studies on the connection
between Hi line widths and SF in external galaxies often use broadband tracers
for the SF rate, which must implicitly assume that SF histories (SFHs) have
been constant over the timescale of the tracer. In this paper, we compare
measures of HI energy to time-resolved SFHs in a number of nearby dwarf
galaxies. We find that HI energy surface density is strongly correlated only
with SF that occurred 30-40 Myr ago. This timescale corresponds to the
approximate lifetime of the lowest mass supernova progenitors (~ 8 Msun). This
analysis suggests that the coupling between SF and the neutral ISM is strongest
on this timescale, due either to an intrinsic delay between the release of the
peak energy from SF or to the coherent effects of many SNe during this
interval. At \Sigma_SFR > 10^-3 Msun yr^-1 kpc^-2, we find a mean coupling
efficiency between SF energy and HI energy of \epsilon = 0.11 +/- 0.04 using
the 30-40 Myr timescale. However, unphysical efficiencies are required in lower
\Sigma_SFR systems, implying that SF is not the primary driver of HI kinematics
at \Sigma_SFR < 10^-3 Msun yr^-1 kpc^-2.
