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

Abstract

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* (Δσ) correlates with various galaxy properties. We establish that the strongest correlation of Δσ is with beam smearing, which inflates σgas more than σ*, introducing a dependence of Δσ 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 Δσ (Δσap) 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 Δσap is much smaller than that of Δσ, a correlation between Δσap and gas velocity gradient is still detected, implying there is a small bias in dynamical masses derived from stellar and ionised gas velocity dispersions.

The SAMI Survey: Evidence for dynamical coupling of ionised gas and young stellar populations

“…Following Varidel et al (2016) and Oh et al (2022), we compute the projected mean velocity gradient within an elliptical 1𝑅 𝑒 aperture (bottom). RHS: same as LHS, but using LOESS smoothing to highlight colour gradients.…”

“…Barat et al ( 2019) compared rotational and central dispersion of gas and stars in the Sydney-AAO Multi Integral field (SAMI) Galaxy Survey and find a steeper slope for the 𝑆0.5 − log 10 𝑀 * relationship in stars compared to gas kinematics. More recently, Oh et al (2022) showed that the velocity dispersion of the ionised gas tends to be lower than that of the stars in SAMI galaxies, though caution that beam smearing may be playing a role in artificially enhancing differences. As the precursor to stars, cold gas kinematics can also be compared to stellar kinematics.…”

Spatially resolved gas and stellar kinematics in compact starburst galaxies

“…In J1400, which looks like a spiral galaxies viewed almost face on, we find that average the stars have a larger velocity dispersion than the gas. This could be caused by asymmetric drift, where the velocity dispersion of the stars is increased due to random motions, resulting in a larger velocity dispersion than that of the gas (Oh et al 2022).…”

“…A common feature observed is that the stellar rotation speed is slower than the rotation speed of the gas. This asymmetric drift is caused by the random motions of the stars (Martinsson et al 2013;Oh et al 2022). Galaxy bulges stand out in the stellar kinematics as they typically have a larger velocity dispersion than the stellar disk (e.g.…”

“…Over the last 20 years, large galaxy surveys with integral field spectrographs at low (e.g. SAURON: Bacon et al (2001), GHASP: Epinat et al (2010), CALIFA: Sánchez et al (2012), DYNAMO: Green et al (2014), MaNGA: Bundy et al (2015), SAMI: Scott et al (2018); Oh et al (2022)) and high redshift Based on observations collected at the European Southern Observatory at Paranal, Chile (ESO programme 097.B-1061).…”