The SAMI Galaxy Survey: the difference between ionized gas and stellar velocity dispersions

We investigate the mean locally measured velocity dispersions of ionized gas (sigma(gas)) and stars (sigma(*)) for 1090 galaxies with stellar masses log (M*/M-circle dot) = 9.5 from the SAMI Galaxy Survey. For star-forming galaxies, sigma(*) tends to be larger than sigma(gas), suggesting that stars are in general dynamically hotter than the ionized gas (asymmetric drift). The difference between sgas and s*(Delta sigma) correlates with various galaxy properties. We establish that the strongest correlation of Delta(sigma) is with beam smearing, which inflates sgas more than sigma(*), introducing a dependence of Delta sigma on both the effective radius relative to the point spread function and velocity gradients. The second strongest correlation is with the contribution of active galactic nuclei (AGN) (or evolved stars) to the ionized gas emission, implying that the gas velocity dispersion is strongly affected by the power source. In contrast, using the velocity dispersion measured from integrated spectra (sigma(ap)) results in less correlation between the aperture-based Delta sigma (Delta sigma(ap)) and the power source. This suggests that the AGN (or old stars) dynamically heat the gas without causing significant deviations from dynamical equilibrium. Although the variation of Delta sigma(ap) is much smaller than that of Delta sigma, a correlation between Delta sigma(ap) and gas velocity gradient is still detected, implying that there is a small bias in dynamical masses derived from stellar and ionized gas velocity dispersions.