Drivers of HI Turbulence in Dwarf Galaxies
  Neutral hydrogen (HI) velocity dispersions are believed to be set by
turbulence in the interstellar medium (ISM). Although turbulence is widely
believed to be driven by star formation (SF), recent studies have shown that
this driving mechanism may not be dominant in regions of low SF rate surface
density (SFRSD), such as found in dwarf galaxies or the outer regions of
spirals. We have generated average HI line profiles in a number of nearby
dwarfs and low-mass spirals by co-adding HI spectra in regions with either a
common radius or SFRSD. We find that the spatially-resolved superprofiles are
composed of a central narrow peak (5-15 km/s) with higher velocity wings to
either side. With the assumption that the central peak reflects the turbulent
velocity dispersion, we compare HI kinematics to local ISM properties,
including surface mass densities and measures of SF. The HI velocity dispersion
is correlated most strongly with surface mass density, which points at a
gravitational origin for turbulence, but it is unclear which instabilities can
operate efficiently in these systems. SF energy is produced at a level
sufficient to drive HI turbulent motions where SFRSD > 10^-4 Msun yr^-1 kpc^-2.
At low SF intensities, SF does not supply enough energy for turbulence, nor
does it uniquely determine the velocity dispersion. Nevertheless, SF appears to
provide a lower threshold for HI velocity dispersions. We find that coupling
efficiency decreases with increasing SFRSD, consistent with a picture where SF
couples to the ISM with constant efficiency, but that less of that energy is
found in HI at higher SFRSD. We examine a number of potential drivers of HI
turbulence, including SF, gravitational instabilities, the magnetorotational
instability, and accretion, and find that no single mechanism can drive the
observed levels of turbulence at low SFRSD. We discuss possible solutions to
this conundrum.
