The SILCC (SImulating the LifeCycle of molecular Clouds) project – II. Dynamical evolution of the supernova-driven ISM and the launching of outflows

Abstract

The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a more self-consistent understanding of the interstellar medium (ISM) on small scales and its link to galaxy evolution. We present three-dimensional (magneto)hydrodynamic simulations of the ISM in a vertically stratified box including self-gravity, an external potential due to the stellar component of the galactic disc, and stellar feedback in the form of an interstellar radiation field and supernovae (SNe). The cooling of the gas is based on a chemical network that follows the abundances of H + , H, H 2 , C + , and CO and takes shielding into account consistently. We vary the SN feedback by comparing different SN rates, clustering and different positioning, in particular SNe in density peaks and at random positions, which has a major impact on the dynamics. Only for random SN positions the energy is injected in sufficiently low-density environments to reduce energy losses and enhance the effective kinetic coupling of the SNe with the gas. This leads to more realistic velocity dispersions (σ HI ≈ 0.8σ 300−8000 K ∼ 10−20 km s −1 , σ Hα ≈ 0.6σ 8000−3×10 5 K ∼ 20 − 30 km s −1 ), and strong outflows with mass loading factors (ratio of outflow to star formation rate) of up to 10 even for solar neighbourhood conditions. Clustered SNe abet the onset of outflows compared to individual SNe but do not influence the net outflow rate. The outflows do not contain any molecular gas and are mainly composed of atomic hydrogen. The bulk of the outflowing mass is dense (ρ ∼ 10 −25 − 10 −24 g cm −3 ) and slow (v ∼ 20 − 40 km s −1 ) but there is a high-velocity tail of up to v ∼ 500 km s −1 with ρ ∼ 10 −28 − 10 −27 g cm −3 .

Quantifying Supernovae-driven Multiphase Galactic Outflows

“…Therefore, the scaling relations as shown in Fig 16 should hold for other Σ gas cases as well. Girichidis et al (2016b) find that clustering of some SNe does not affect the mass outflow rate. We find a very mild increase in mass flux, although with large fluctuations.…”

“…SNe scale height Where SNe explode is critical for feedback efficiency. A SN exploding in a dense medium quickly radiates away its energy, and has little impact on the large-scale ISM, let alone contributing to driving winds (Girichidis et al 2016b). On the other hand, if a SN explodes in an environment dominated by tenuous gas, then the cooling is much less efficient, and a significant fraction of energy can be preserved (e.g.…”

“…Overall, once the ISM is hot-dominated, clustering of SNe does not help with the loading factors, and may even be negative for loading mass. Girichidis et al (2016b) have found that for the solar neighbourhood, SNe can blow away most of the gas in the midplane, and drive outflows with a mass loading up to 10. This is higher than our value, which is around 2-3.…”

“…Including selfgravity would make the cold phase smaller in volume, thus may facilitate feedback for those SNe that explode in the inter-cloud space (e.g. Girichidis et al 2016b;Kim & Ostriker 2016). But we note that (i) the external gravitational field in our simulations does include that from the gas, so the effect of self-gravity is not fully missed, at least in the vertical direction.…”

CO-dark gas and molecular filaments in Milky Way-type galaxies – II. The temperature distribution of the gas

“…The lack of stellar feedback means that we will tend to over-produce molecular clouds, although this is offset to some extent by the absence of self-gravity, which will decrease the number of large clouds formed (c.f. Girichidis et al 2016). Note that absence of self-gravity is unlikely to be a major obstacle in forming H2, as the transition from H to H2 typically occurs at number densities n < 100 cm −3 , where the contribution from self-gravity remains small in comparison to the effects of the global potential.…”