Measuring dynamical masses from gas kinematics in simulated high-redshift galaxies

Wellons et al., available on arXiv

Abstract: Advances in instrumentation have recently extended detailed measurements of gas kinematics to large samples of high-redshift galaxies. Relative to most nearby, thin disk galaxies, in which gas rotation accurately traces the gravitational potential, the interstellar medium (ISM) of z>1 galaxies is typically more dynamic and exhibits elevated turbulence. If not properly modeled, these effects can strongly bias dynamical mass measurements. We use high-resolution FIRE-2 cosmological zoom-in simulations to analyze the physical effects that must be considered to correctly infer dynamical masses from gas kinematics. Our analysis covers a wide range of galaxy properties, from low-redshift Milky-Way-mass galaxies to massive high-redshift galaxies (M_* > 10^11 M_sun at z=1). Selecting only snapshots where a well-ordered disk is present, we calculate the rotational profile (r) of the cool (10^3.5 K < T < 10^4.5 K) gas and compare it to the circular velocity v_c=sqrt(GM_enc/r) assuming spherical symmetry. In the simulated massive high-redshift galaxies, the gas rotation traces the circular velocity reasonably well at intermediate radii r~1-3 kpc, but the two quantities diverge significantly outside that range. At larger radii, gradients in the turbulent pressure can bias dynamical mass measurements low by ~10-40%. In the interior, the assumption of a spherically-symmetric gravitational potential becomes increasingly poor owing to a massive disk component, reducing the gas rotational velocities by >~10%. Finally, in the interior and exterior, the gas’ motion can be significantly non-circular due to e.g. bars, satellites, and inflows/outflows. We discuss the accuracy of commonly-used analytic models for pressure gradients (or “asymmetric drift”) in the ISM of high-redshift galaxies.