What is Driving the HI Velocity Dispersion?

Abstract

We explore what dominant physical mechanism sets the kinetic energy contained in neutral, atomic (HI) gas. We compare the HI line widths predicted from turbulence driven by supernova (SN) explosions and magneto-rotational instability (MRI) to direct observations in 11 disk galaxies. We use high-quality maps of the HI mass surface density and line width, obtained by the THINGS survey. We show that all sample galaxies exhibit a systematic radial decline in the HI line width, which appears to be a generic property of HI disks and also implies a radial decline in kinetic energy density of HI. At a galactocentric radius of r25 there is a characteristic value of the HI velocity dispersion of $10\pm2$ \kms. Inside this radius, galaxies show HI line widths above the thermal value expected from a warm HI component, implying that turbulence drivers must be responsible for maintaining this line width. Therefore, we compare maps of HI kinetic energy to maps of the star formation rate (SFR) and to predictions for energy generated by MRI. We find a positive correlation between kinetic energy of HI and SFR. For a given turbulence dissipation timescale we can estimate the energy input required to maintain the observed kinetic energy. The SN rate implied by the observed recent SFR is sufficient to maintain the observed velocity dispersion, if the SN feedback efficiency is at least ε_SN\simeq0.1. Beyond r25, this efficiency would have to increase to unrealistic values, $ε>1$, suggesting that mechanical energy from young stars does not supply most energy in outer disks. On the other hand, both thermal broadening and turbulence driven by MRI can produce the velocity dispersions and kinetic energies that we observe in this regime.