SILCC -- IX. The multi-phase interstellar medium at low metallicity

TL;DR

This work investigates how gas-phase metallicity alters the multiphase ISM using high-resolution MHD simulations with non-equilibrium chemistry, variable UV fields, and cosmic-ray transport across seven metallicities from $0.02Z_\odot$ to $1Z_\odot$. The simulations reveal that as metallicity decreases, the star formation rate plummets by more than a factor of ten, the cold gas mass fraction drops from roughly 60% to 2.3%, and the warm phase becomes increasingly dominant in both volume and mass. Molecular hydrogen becomes progressively scarce with metallicity, with H$_2$ fractions scaling approximately linearly with $Z$ for both total and dense gas, while H$_2$ in diffuse gas remains substantial at high $Z$ but declines at low $Z$. Fragmentation weakens at low metallicity, and star formation shifts toward atomic gas, driven by weaker photoelectric and cosmic-ray heating and by lower cooling efficiency, leading to outflow properties that are only weakly tied to metallicity at fixed gas surface density. The study underscores the strong influence of gas-phase metallicity on ISM structure, star formation, and feedback processes in metal-poor environments.

Abstract

The gas-phase metallicity affects heating and cooling processes in the star-forming galactic interstellar medium (ISM) as well as ionising luminosities, wind strengths, and lifetimes of massive stars. To investigate its impact, we conduct magnetohydrodynamic simulations of the ISM using the FLASH code as part of the SILCC project. The simulations assume a gas surface density of 10 M$_\odot$ pc$^{-2}$ and span metallicities from 1/50 Z$_\odot$ to 1 Z$_\odot$. We include non-equilibrium thermo-chemistry, a space- and time-variable far-UV background and cosmic ray ionisation rate, metal-dependent stellar tracks, the formation of HII regions, stellar winds, type II supernovae, and cosmic ray injection and transport. With the metallicity decreasing over the investigated range, the star formation rate decreases by more than a factor of ten, the mass fraction of cold gas decreases from 60% to 2.3%, while the volume filling fraction of the warm gas increases from 20% to 80%. Furthermore, the fraction of H$_\mathrm{2}$ in the densest regions drops by a factor of four, and the dense ISM fragments into approximately five times fewer structures at the lowest metallicity. Outflow mass loading factors remain largely unchanged, with values close to unity, except for a significant decline at the lowest metallicity. Including the major processes that regulate ISM properties, this study highlights the strong impact of gas phase metallicity on the star-forming ISM.

Figures (22)

• Figure 1: Equilibrium temperature (upper panel) and pressure (bottom panel) as a function of the gas density, for every metallicity analysed in this work. These equilibrium curves are computed using our chemical network without considering the self-shielding of H$_2$ and CO, assuming a constant external hydrogen column density $N_\text{H, tot}$ = 10$^{20}$ cm$^{-2}$, a constant $G_0$ = 1.7 and a constant $\zeta$ = 3 $\times$ 10$^{-17}$ s$^{-1}$.
• Figure 2: Snapshot of the $\Sigma$010-Z0.2 run at $t$ = 104 Myr. The top row represents an edge–on view of the simulation box, the bottom squares show the corresponding face–on views. From left to right we depict the total gas column density $\Sigma_{\text{gas}}$ (projection), the temperature $T$ (slice), the column densities of H$^{+}$, H, and H$_2$ (projections), the magnetic field strength $B$ (slice), and the energy density of CRs $e_{\text{CR}}$ (slice). The top elongated panels show only a part of the box (1 kpc around the midplane instead of 4 kpc). All slices are taken at $y$ = 0 (upper panels), and $z$ = 0 (bottom panels). The projections are computed along the y-axis (upper panels) or along the z-axis (bottom panels), respectively. The white circles in the first and third panel represent the active star clusters, whereas the transparent circles represent star clusters that are no longer active. The stellar feedback due to the presence of star clusters shapes the structure and governs the evolution of the multiphase ISM.
• Figure 3: Volume-weighted average filling fraction (top) and average mass fraction (bottom) for the different gas phases defined in the text. The averages are computed in the 200 Myr period after the beginning of star formation, in the region where $|z|<$ 250 pc. We note that the warm gas occupies around 50% or more of the volume, and constitutes a large fraction of the gas mass, especially at low metallicity. The hot gas volume filling fraction shows a correlation with metallicity, whereas the cold gas volume and mass fractions decrease for lower metallicity. We add the values measured by Tielens2005 in the Solar neighbourhood as a comparison.
• Figure 4: Temperature-density (left column) and pressure-density (right column) phase diagrams of all our simulations, at a time in which the H II region branch is most visible in every run. From top to bottom, the metallicity goes from 1 to 0.02 Z$_\odot$, and the selected snapshots are at simulation times $t$ = 45.3, 68.4, 43.1, 104.5, 72.4, 52.2, 45.3 Myr, respectively. We take into consideration the region $|z|<$ 250 pc. We overplot the unshielded equilibrium curves (red dashed lines) from Fig. \ref{['fig:eq_curves']}, computed assuming $G_0$ = 1.7 and $\zeta$ = 3 $\times$ 10$^{-17}$ s$^{-1}$, and a (orange dashed) line corresponding to T = 10$^4$ K in the left column. In the pressure panel for $\Sigma$010 we add black isothermal lines corresponding to 10$^4$ K (solid), 100 K (dotted), 30 K (dashed). The presence of the two-phase medium is less evident at low Z due to the lack of cold gas.
• Figure 5: Top panel: average ratio of the molecular hydrogen surface density over the sum of molecular and atomic surface densities, as a function of the metallicity. The average has been computed considering a time interval of 200 Myr. In the region $|z|$$<$ 250 pc we consider either the total amount of H$_2$ and H (round markers) or the amount of H$_2$ and H found for a gas denser than 10$^{-22}$ g cm$^{-3}$ (triangles). The solid and dotted lines represent the best fit for the total amount and that for dense gas, respectively. Bottom panel: average mass fraction of diffuse H$_2$, as a function of metallicity. We find this fraction to be around 50% for the highest Z, and around 30% for the lowest.