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Metallicity effects on the multiphase ISM

Figure 1: Time-averaged values for the volume filling fraction (VFF) and mass fraction (MF) of the different gas phases for every run (see text for details). The time interval in which we average goes from the time of the first episode of star formation, different for every simulation, to the end of the runs at 100 Myr. We define the gas phases depending on their temperature, T : cold gas, T ≤ 300 K, warm gas, 300 < T ≤ 3 × 105 K (here split into neutral and ionised gas), and hot gas, T ≥ 3 × 105 K. We note that the VFF of the warm gas is dominant, and that the VFF of hot gas diminishes from MW to SMC, and increases for Z10 and Z10 noB. Moreover, it decreases at fixed metallicity (Z10, Z10_noB, Z30, Z50 runs) and increasing gas surface density. Regarding the MF of the cold gas, it decreases with metallicity and fixed surface density of the gas, whereas it increases for fixed metallicity and denser gas.

Vittoria Brugaletta

Even though metals constitute only a small fraction of the mass of the interstellar medium, their presence impacts the structure and the evolution of the ISM in many ways. To understand these effects we run several SILCC simulations using the same setup as in Rathjen et al. (2021), with different initial environmental metallicity values: the MW run (Z = Z), the LMC (Z = ½ Z), the SMC (Z = 1/5 Z)and the runs Z10, Z10_noB, Z30, Z50, which have all Z = 0.02 Z but different gas surface densities (given by the number in their name in units of M pc-2) and the possibility of having magnetic fields (Z10_noB is the only run without).

In Figure 1 we compute the time average of the volume filling fraction (VFF) and mass fraction (MF) for the different gas phases, excluding the time evolution from the beginning of every simulation until their first star formation episode. We observe that for almost every simulation, Z10_noB excluded, the VFF of warm gas is dominant (more than 50%), whereas the hot gas contributes for around 30% – 40%, and the cold gas for less than 1%. The predominance of the warm gas in the VFF could be explained by the presence, in addition to supernovae, of feedback elements such as radiation, stellar winds and cosmic rays, which increase the VFF of warm gas at the cost of the VFF of hot gas (Rathjen et al., 2021).

Regarding the behavior of VFFs as a function of metallicity and gas surface density, we observe that the hot gas VFF diminishes with decreasing metallicity, going from MW to SMC. For the Z10 and Z10_noB runs it increases, reaching ~60% in the latter. Moreover, considering all simulations with a Z = 0.02 Z , higher gas surface density decreases the hot VFF. There are two primary causes of this. The first is that for fixed gas surface density (MW, LMC, SMC, Z10, Z10_noB runs), the number of formed massive stars (and consequently SNe) in our simulations decreases going from MW to SMC, whereas their number increases for the Z10_noB run. Since supernovae form the hot VFF, their number influences how large the hot VFFs can be. Second, the observed trend with surface density of gas at Z = 0.02 Z can be explained taking into account that in denser environments cooling processes are more efficient, therefore the hot gas is cooled down to warm gas temperatures more easily. For this reason the MF of cold gas increases at a Z = 0.02 Z going from 10 to 50 M pc-2 . We also note a decrease going from MW to Z10_noB, at fixed surface density. In fact, if more metals are present then the cooling processes become more efficient.