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

Abstract

Using hydrodynamic simulations, we study the mass-loss due to supernova-driven outflows from Milky Way type disc 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 (~10 kpc) is bimodal in nature, peaking at 105 K and 106.5 K, responsible for optical and X-ray emission, respectively. The contribution of cold/warm gas with temperature ≤105.5 K to the outflow rate within 10 kpc is ≈0.3-0.5. The warm mass loading factor, η3e5 (T ≤ 3 × 105 K) is related to the mass loading factor at the virial radius (ηv) as ηv ≈ 25 η3e5 (SFR/M⊙ yr-1)-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, ≲ 200 km s-1. We identify two 'sequences' of outflowing cold gas at small scales: a fast (≈500 km s-1) sequence, driven by the unshocked free-wind; and a slow sequence (≈± 100 km s-1) at the conical interface of the superwind and the hot halo.