FIRE-3: Updated Stellar Evolution Models, Yields, & Microphysics and Fitting Functions for Applications in Galaxy Simulations

Abstract

Increasingly, uncertainties in predictions from galaxy formation simulations (at sub-Milky Way masses) are dominated by uncertainties in stellar evolution inputs. In this paper, we present the full set of updates from the FIRE-2 version of the Feedback In Realistic Environments (FIRE) project code, to the next version, FIRE-3. While the transition from FIRE-1 to FIRE-2 focused on improving numerical methods, here we update the stellar evolution tracks used to determine stellar feedback inputs, e.g. stellar mass-loss (O/B and AGB), spectra (luminosities and ionization rates), and supernova rates (core-collapse and Ia), as well as detailed mass-dependent yields. We also update the low-temperature cooling and chemistry, to enable improved accuracy at T 10 4 K and densities n 1 cm −3 , and the meta-galactic ionizing background. All of these synthesize newer empirical constraints on these quantities and updated stellar evolution and yield models from a number of groups, addressing different aspects of stellar evolution. To make the updated models as accessible as possible, we provide fitting functions for all of the relevant updated tracks, yields, etc, in a form specifically designed so they can be directly "plugged in" to existing galaxy formation simulations. We also summarize the default FIRE-3 implementations of "optional" physics, including spectrally-resolved cosmic rays and supermassive black hole growth and feedback.

Formation of proto-globular cluster candidates in cosmological simulations of dwarf galaxies at $z>4$

“…We note that our treatment of stellar mass-loss processes is different than the standard FIRE-2 physics(Hopkins et al 2018b), where 100% of stellar mass-loss energy is converted into macroscopic momentum, but is consistent with the approach adopted in FIRE-3(Hopkins et al 2022).MNRAS 000,1-14 (2022) …”

“…These results strongly indicate that the treatment of unresolved 𝑃 𝑑𝑉 work from stellar mass-loss or other forms of unresolved early feedback is crucial in resolving the formation of star clusters in progenitors of 𝑧 = 0 dwarf galaxies while the choice of density threshold (if any) is not. Preliminary work using FIRE-3 galaxy formation physics (Hopkins et al 2022), which follows the treatment of unresolved 𝑃 𝑑𝑉 work adopted here and does not adopt a density threshold for star formation -it requires star-forming gas to be self-gravitating, Jeans unstable, and within a converging flow -supports this conclusion (Sameie et al 2022, in preparation).…”

“…Finally, the galaxy formation models employed here, which are part of the FIRE-2 suite, are subject to further refinement. Details of the treatment of various aspects of star formation will likely be important for accurately capturing the formation of bound, long-lived star clusters; updates incorporated into FIRE-3 (Hopkins et al 2022) are a starting point in this direction. In the near future, however, it will be possible to run the kinds of simulations presented here with numerical parameters that allow us to form pc-scale clusters and to follow the evolution of the clusters and their host galaxies from birth in the high-redshift Universe to present day.…”

“…Relative to earlier FIRE simulations, including the Fitts et al (2017) suite, our fiducial FIRE-2 model includes metal-diffusion physics, a slightly updated ultraviolet background radiation model, and a correction to the cosmic ray heating source term that avoids spurious heating at very early times in the simulations. We also adopt an updated stellar mass-loss algorithm (as we discuss further in section 4.2) that differs from the default FIRE-2 implementation outlined in Hopkins et al 2018b but follows the default treatment in FIRE-3 (Hopkins et al 2022). In a second set of simulations, we use the default FIRE-2 algorithm for the stellar mass-loss processes (as described in Hopkins et al 2018b and in section 4.2 below) or vary the density threshold from our standard choice of 𝑛 thresh = 1000 cm −3 , keeping all other fiducial FIRE-2 physics unchanged, to study the impact on star formation activity and any potential difference in efficiency of galaxies in forming GCCs.…”

Effects of the environment and feedback physics on the initial mass function of stars in the STARFORGE simulations

“…"Non-isothermal" or "cooling" STARFORGE runs utilize the radiative cooling and thermochemistry module from Hopkins et al (2022) that contains detailed metallicity-dependent cooling and heating physics from 𝑇 = 10 − 10 10 K, including recombination, thermal bremsstrahlung, metal lines (following Wiersma et al 2009), molecular lines, fine structure (following Glover & Abel 2008) and dust collisional processes. The cooling module selfconsistently solves for the internal energy and ionization state of the gas (see Hopkins et al (2022) and Appendix B of Hopkins et al 2018b). The gas adiabatic index is calculated as a function of density based on the results of Vaidya et al (2015).…”