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

Philip F. Hopkins, Andrew Wetzel+19 more|arXiv (Cornell University)|Feb 28, 2022

Astronomy and Astrophysical Research4 citations

TL;DR

This paper presents FIRE-3, a major update to the Feedback In Realistic Environments (FIRE) galaxy simulation framework, incorporating state-of-the-art stellar evolution models, updated nucleosynthetic yields, and improved microphysics for dense, cold interstellar medium. The key contribution is a comprehensive set of fitting functions that enable direct integration into galaxy simulations, significantly enhancing accuracy in modeling stellar feedback, cooling, and chemical evolution across diverse galactic environments.

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 $T \lesssim 10^{4}\,$K and densities $n\gg 1\,{ m cm^{-3}}$, 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.

Motivation & Objective

• Address growing uncertainties in galaxy formation simulations at sub-Milky Way masses by improving stellar feedback inputs.
• Integrate the latest empirical and theoretical constraints on stellar evolution, mass-loss, luminosities, ionization rates, and supernova rates.
• Enhance low-temperature cooling and chemistry models for dense, cold ISM gas (T ≲ 10⁴ K, n ≫ 1 cm⁻³) to improve accuracy in metal-poor and early-universe conditions.
• Provide accessible, ready-to-use fitting functions for stellar tracks, yields, and microphysics to enable adoption in non-FIRE simulations.
• Anchor galaxy simulations in the current state of stellar astrophysics while enabling improved predictive power for observational diagnostics like CO and CII emission.

Proposed method

• Updated stellar evolution tracks using recent models from multiple groups, incorporating mass-dependent yields and improved treatment of O/B and AGB mass-loss.
• Revised supernova rates (core-collapse and Type Ia) based on updated stellar population synthesis and delay-time distribution functions.
• Improved low-temperature cooling and chemistry routines to accurately model dense, cold ISM gas at T ≲ 10⁴ K and high densities.
• Updated the meta-galactic ionizing background using current observational constraints and theoretical models.
• Developed a suite of new, robust fitting functions for all key stellar and ISM physics inputs, designed for direct integration into galaxy simulation codes.
• Provided default implementations for optional physics, including spectrally-resolved cosmic rays and supermassive black hole growth and feedback, for future extension.

Experimental results

Research questions

• RQ1 How do updated stellar evolution models and yields improve the accuracy of galaxy simulation predictions for sub-Milky Way mass systems?
• RQ2 To what extent do improved low-temperature cooling and chemistry models affect the dynamics and thermodynamics of dense, cold ISM in metal-poor or early galaxies?
• RQ3 How do updated supernova rates and ionization rates from evolved stars impact the energy and momentum injection into the ISM?
• RQ4 Can fitting functions derived from multi-source synthesis provide reliable, plug-and-play inputs for galaxy simulations across diverse codes and redshifts?
• RQ5 How do the updated microphysics and yields affect predictions for observable tracers such as CO and CII emission in galaxies?

Key findings

• The FIRE-3 models incorporate updated stellar evolution and nucleosynthetic yields from multiple recent studies, significantly improving the physical fidelity of feedback inputs.
• Cooling and chemistry routines are now accurate down to T ≲ 10⁴ K and densities n ≫ 1 cm⁻³, enabling better modeling of cold, dense phases in low-metallicity or early-universe environments.
• Fitting functions for stellar evolution, yields, and microphysics are provided in a form directly usable in galaxy simulations, reducing implementation barriers for external codes.
• The updated models reduce uncertainties in stellar feedback inputs, which were previously the dominant source of uncertainty in sub-Milky Way galaxy simulations.
• The new fitting functions are new to the literature and were carefully vetted to avoid erroneous extrapolations, ensuring robustness across the full dynamic range of interest.
• The updated models enable improved predictive power for observational diagnostics such as CO and CII emission, as well as detailed stellar abundance patterns in galaxies.