Fingerprints of the hierarchical building up of the structure on the gas kinematics of galaxies

TL;DR

This study uses cosmological simulations to explore how galaxy gas kinematics, influenced by hierarchical structure formation, affect the Tully-Fisher relation, highlighting the importance of combined velocity indicators and the impact of mergers.

Contribution

It demonstrates that combining rotation and dispersion velocities reduces scatter in Tully-Fisher relations and identifies the maximum rotation curve as the best velocity proxy across morphologies.

Findings

Dispersion-dominated systems show higher $\sigma / V_{rot}$ ratios.

Mergers increase scatter in the Tully-Fisher relation.

Maximum rotation velocity at the rotation curve peak minimizes scatter.

Abstract

Recent observational and theoretical works have suggested that the Tully-Fisher Relation might be generalised to include dispersion-dominated systems by combining the rotation and dispersion velocity in the definition of the kinematical indicator. Mergers and interactions have been pointed out as responsible of driving turbulent and disordered gas kinematics, which could generate Tully-Fisher Relation outliers. We intend to investigate the gas kinematics of galaxies by using a simulated sample which includes both, gas disc-dominated and spheroid-dominated systems. Cosmological hydrodynamical simulations which include a multiphase model and physically-motivated Supernova feedback were performed in order to follow the evolution of galaxies as they are assembled. Both the baryonic and stellar Tully-Fisher relations for gas disc-dominated systems are tight while, as more dispersion-dominated systems are included, the scatter increases. We found a clear correlation between $\sigma / V_{\rm rot}$ and morphology, with dispersion-dominated systems exhibiting the larger values ($> 0.7$). Mergers and interactions can affect the rotation curves directly or indirectly inducing a scatter in the Tully-Fisher Relation larger than the simulated evolution since $z \sim 3$. Kinematical indicators which combine rotation velocity and dispersion velocity can reduce the scatter in the baryonic and the stellar mass-velocity relations. Our findings also show that the lowest scatter in both relations is obtained if the velocity indicators are measured at the maximum of the rotation curve. Moreover, the rotation velocity estimated at the maximum of the gas rotation curve is found to be the best proxy for the potential well regardless of morphology.