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We investigate the mean locally-measured velocity dispersions of ionised gas($\sigma_{\rm gas}$) and stars ($\sigma_*$) for 1090 galaxies with stellarmasses $\log\,(M_*/M_{\odot}) \geq 9.5$ from the SAMI Galaxy Survey. Forstar-forming galaxies, $\sigma_*$ tends to be larger than $\sigma_{\rm gas}$,suggesting that stars are in general dynamically hotter than the ionised gas(asymmetric drift). The difference between $\sigma_{\rm gas}$ and $\sigma_*$($\Delta\sigma$) correlates with various galaxy properties. We establish thatthe strongest correlation of $\Delta\sigma$ is with beam smearing, whichinflates $\sigma_{\rm gas}$ more than $\sigma_*$, introducing a dependence of$\Delta\sigma$ on both the effective radius relative to the point spreadfunction and velocity gradients. The second-strongest correlation is with thecontribution of active galactic nuclei (AGN) (or evolved stars) to the ionisedgas emission, implying the gas velocity dispersion is strongly affected by thepower source. In contrast, using the velocity dispersion measured fromintegrated spectra ($\sigma_{\rm aper}$) results in less correlation betweenthe aperture-based $\Delta\sigma$ ($\Delta\sigma_{\rm aper}$) and the powersource. This suggests that the AGN (or old stars) dynamically heat the gaswithout causing significant deviations from dynamical equilibrium. Although thevariation of $\Delta\sigma_{\rm aper}$ is much smaller than that of$\Delta\sigma$, a correlation between $\Delta\sigma_{\rm aper}$ and gasvelocity gradient is still detected, implying there is a small bias indynamical masses derived from stellar and ionised gas velocity dispersions.

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