The SAMI Galaxy Survey: Toward a Unified Dynamical Scaling Relation for Galaxies of All Types

We take advantage of the first data from the Sydney-AAO Multi-object Integral field Galaxy Survey to investigate the relation between the kinematics of gas and stars, and stellar mass in a comprehensive sample of nearby galaxies. We find that all 235 objects in our sample, regardless of their morphology, lie on a tight relation linking stellar mass (M _*) to internal velocity quantified by the S _0.5 parameter, which combines the contribution of both dispersion (σ) and rotational velocity (V _rot) to the dynamical support of a galaxy (S_0.5=\sqrt{0.5 V_rot²+σ ²}). Our results are independent of the baryonic component from which σ and V _rot are estimated, as the S _0.5 of stars and gas agree remarkably well. This represents a significant improvement compared to the canonical M _* versus V _rot and M _* versus σ relations. Not only is no sample pruning necessary, but also stellar and gas kinematics can be used simultaneously, as the effect of asymmetric drift is taken into account once V _rot and σ are combined. Our findings illustrate how the combination of dispersion and rotational velocities for both gas and stars can provide us with a single dynamical scaling relation valid for galaxies of all morphologies across at least the stellar mass range 8.5 <log (M _*/M _⊙) < 11. Such relation appears to be more general and at least as tight as any other dynamical scaling relation, representing a unique tool for investigating the link between galaxy kinematics and baryonic content, and a less biased comparison with theoretical models.