$V/\sigma$ Trends with Mass for Dwarf Galaxies from the Marvelous Massive Dwarfs Suite

Abstract

Galaxy formation scenarios can be interpreted through galaxy morphology and the level of rotational versus pressure support, quantified through the ratio of a galaxy's rotation speed to its velocity dispersion: $V/\sigma$. Observational studies of dwarf galaxies find that $V/\sigma$ does not strongly depend on environment, and may weakly depend on galaxy mass, which could shift our understanding of how dwarf galaxies form. We utilize the Marvelous Massive Dwarfs suite to examine whether $V/\sigma$ depends on mass in simulations, and understand how this varies for different baryonic components of the galaxy: HI gas, young stars ($<$ 1 Gyr) and old stars ($>$ 1 Gyr). We use a simulation sample of 67 isolated dwarf galaxies with M$_\star=10^6-10^9$ M$_\odot$ and produce line-of-sight maps for rotation speed and dispersion for different viewing angles of each galaxy. We find that $V/\sigma$ increases with mass, and that HI gas and young stars are more rotation-supported ($V/\sigma\approx 1-13$) while old stars are more dispersion-supported ($V/\sigma\approx 0.2-5$). This result is consistent with the scenario where young stars are born from dynamically cold gas in the interstellar medium and undergo dynamical heating over time. We quantify the effects of spatial resolution in observational determinations of $V/\sigma$ and find that existing observations using old stars may underestimate the intrinsic $V/\sigma$. We find a correlation between $V/\sigma_\mathrm{HI,global}$ and HI line profile shape that is qualitatively similar to previous simulation results, but we find higher $V/\sigma_\mathrm{HI,global}$ compared to prior work which found values $\lesssim 2$ for most galaxies in this mass range. Our results motivate future work to examine $V/\sigma$ and dwarf galaxy formation with different kinematic tracers of the galaxy.