Predictions for the angular dependence of gas mass flow rate and metallicity in the circumgalactic medium

Abstract

ABSTRACT We use cosmological hydrodynamical simulations to examine the physical properties of the gas in the circumgalactic media (CGM) of star-forming galaxies as a function of angular orientation. We utilize TNG50 of the IllustrisTNG project, as well as the EAGLE simulation to show that observable properties of CGM gas correlate with azimuthal angle, defined as the galiocentric angle with respect to the central galaxy. Both simulations are in remarkable agreement in predicting a strong modulation of flow rate direction with azimuthal angle: inflow is more substantial along the galaxy major axis, while outflow is strongest along the minor axis. The absolute rates are noticeably larger for higher ($\log {(M_\star / \rm {M}_\odot)} \sim 10.5$) stellar mass galaxies, up to an order of magnitude compared to $\dot{M} \lesssim 1$ M⊙ yr−1 sr−1 for $\log {(M_\star / \rm {M}_\odot)}\sim 9.5$ objects. Notwithstanding the different numerical and physical models, both TNG50 and EAGLE predict that the average metallicity of the CGM is higher along the minor versus major axes of galaxies. The angular signal is robust across a wide range of galaxy stellar mass $8.5 \lt \log {(M_\star / \rm {M}_\odot)} \lt 10.5$ at z < 1. This azimuthal dependence is particularly clear at larger impact parameters b ≥ 100 kpc. Our results present a global picture, whereby despite the numerous mixing processes, there is a clear angular dependence of the CGM metallicity. We make forecasts for future large survey programmes that will be able to compare against these expectations. Indeed, characterizing the kinematics, spatial distribution and metal content of CGM gas is key to a full understanding of the exchange of mass, metals, and energy between galaxies and their surrounding environments.