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

We investigate the mean locally-measured velocity dispersions of ionised gas (σgas) and stars (σ∗) for 1090 galaxies with stellar masses log (M∗/M⊙) ≥ 9.5 from the SAMI Galaxy Survey. For star-forming galaxies, σ∗ tends to be larger than σgas, suggesting that stars are in general dynamically hotter than the ionised gas (asymmetric drift). The difference between σgas and σ∗ (∆σ) correlates with various galaxy properties. We establish that the strongest correlation of ∆σ is with beam smearing, which inflates σgas more than σ∗, introducing a dependence of ∆σ 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 ionised gas emission, implying the gas velocity dispersion is strongly affected by the power source. In contrast, using the velocity dispersion measured from integrated spectra (σap) results in less correlation between the aperture-based ∆σ (∆σap) 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 ∆σap is much smaller than that of ∆σ, a correlation between ∆σap and gas velocity gradient is still detected, implying there is a small bias in dynamical masses derived from stellar and ionised gas velocity dispersions.

Publication Date: 
September 2021