Timescales on which Star Formation Affects the Neutral ISM

Abstract: 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.