Long way to go: how outflows from large galaxies propagate through the hot halo gas

TL;DR

This study uses hydrodynamic simulations to analyze how supernova-driven outflows from Milky Way-like galaxies propagate through hot halo gas, revealing scaling relations, temperature distributions, and outflow dynamics.

Contribution

It provides new insights into the mass loss scaling, temperature bimodality, and outflow velocities in galaxy halos, incorporating the effects of hot halo gas and multiple starburst events.

Findings

Mass loss scales linearly with star formation

Hot halo gas increases mass loading at the virial radius

Outflow velocities are close to the hot halo sound speed

Abstract

Using hydrodynamic simulations, we study the mass loss due to supernova-driven outflows from Milky Way type disk galaxies, paying particular attention to the effect of the extended hot halo gas. We find that the total mass loss at inner radii scales roughly linearly with total mass of stars formed, and that the mass loading factor at the virial radius can be several times its value at inner radii because of the swept up hot halo gas. The temperature distribution of the outflowing material in the inner region ($\sim $10 kpc) is bimodal in nature, peaking at $10^5$ K and $10^{6.5}$ K, responsible for optical and X-ray emission, respectively. The contribution of cold/warm gas with temperature $\le 10^{5.5}$ K to the outflow rate within 10 kpc is $\approx 0.3\hbox{--}0.5$. The warm mass loading factor, $\eta_{3e5}$ ($T\le 3 \times 10^5$ K) is related to the mass loading factor at the virial radius ($\eta_{v}$) as $\eta_{v} \approx 25\, \eta_{3e5}\, \left(\mbox{SFR}/{\rm M}_\odot{\rm yr}^{-1} \right)^{-0.15}$ for a baryon fraction of 0.1 and a starburst period of 50 Myr. We also discuss the effect of multiple bursts that are separated by both short and long periods. The outflow speed at the virial radius is close to the sound speed in the hot halo, $\lesssim 200$ km s$^{-1}$. We identify two `sequences' of outflowing cold gas at small scales: a fast ($\approx 500$ km~s$^{-1}$) sequence, driven by the unshocked free-wind; and a slow sequence ($\approx \pm 100$ km s$^{-1}$) at the conical interface of the superwind and the hot halo.