The SAMI Galaxy Survey: Towards a unified dynamical scaling relation for galaxies of all types

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We take advantage of the first data from the Sydney-AAO Multi-object Integral field (SAMI) 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 S₀.₅ parameter, which combines the contribution of both dispersion (σ) and rotational velocity (Vrot) to the dynamical support of a galaxy (S₀.₅=√0.5Vrot²+σ²). Our results are independent of the baryonic component from which σ and Vrot are estimated, as the S₀.₅ of stars and gas agree remarkably well. This represents a significant improvement compared to the canonical M* vs. Vrot and M* vs. 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 Vrot 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.