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 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 (∼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 10 5.5 K to the outflow rate within 10 kpc is ≈ 0.3-0.5. The warm mass loading factor, η 3e5 (T 3 × 10 5 K) is related to the mass loading factor at the virial radius (η v ) as η v ≈ 25 η 3e5 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.

Multiwavelength features of Fermi bubbles as signatures of a Galactic wind

“…The low latitude cold/warm gas can have a wide angle and a large (∼ 100 km s −1 ) line-of-sight velocity because the clouds are following non-radial trajectories (e.g., see the S2 sequence of clouds in Fig. 12 of Sarkar et al 2015). Figure 9 shows the position-velocity diagram as seen from the Galactic centre.…”

“…The details of the initial set up for milky way (MW) type galaxies are given in a previous paper Sarkar et al (2015). However, for the sake of completeness, we briefly discuss the set up below.…”

“…Since we are interested in the Galactic wind and not the disc ISM, we constrain the cooling of the disc material (but not the injected material) to be zero for a height less than 1.2 kpc above the disc plane. A more detailed description about the code implementation can be found in Sarkar et al (2015).…”

An origin for multiphase gas in galactic winds and haloes

“…35), but high resolution multi-dimensional simulations are needed in order to resolve the question of whether or not the entire medium cools monolithically or whether something akin to discrete cool clouds of a characteristic size precipitate out of the surrounding hot background. Our speculations on the fate of a cool wind-driven circumgalactic bubble also need to be confronted with both parameterized semianalytic calculations and multi-dimensional numerical simulations with radiative cooling (e.g., Oñorbe et al 2015;Sarkar et al 2015).…”

Diffuse X-Ray Emission From Star-Forming Galaxies

“…It has been observed (Strickland et al 2002;Strickland & Heckman 2007) and noticed in numerical simulations (Hopkins et al 2012;Sarkar et al 2015, hereafter S15) that a significant fraction (∼0.3-0.5) of the input mechanical energy is stored in a hot (  T 10 6 K), X-ray emitting gas. Therefore, it is necessary to decipher the origin of diffuse X-ray emission from star-forming galaxies to understand the feedback mechanisms.…”