Gas Accretion and Galactic Chemical Evolution: Theory and Observations

Abstract

This chapter reviews how galactic inflows influence galaxy metallicity. The goal is to discuss predictions from theoretical models, but particular emphasis is placed on the insights that result from using models to interpret observations. Even as the classical "G-dwarf problem" endures in the latest round of observational confirmation, a rich and tantalizing new phenomenology of relationships between M * , Z, SFR, and gas fraction is emerging both in observations and in theoretical models. A consensus interpretation is emerging in which star-forming galaxies do most of their growing in a quiescent way that balances gas inflows and gas processing, and metal dilution with enrichment. Models that explicitly invoke this idea via equilibrium conditions can be used to infer inflow rates from observations, while models that do not assume equilibrium growth tend to recover it self-consistently. Mergers are an overall subdominant mechanism for delivering fresh gas to galaxies, but they trigger radial flows of previously-accreted gas that flatten radial gas-phase metallicity gradients and temporarily suppress central metallicities. Radial gradients are generically expected to be steep at early times and then flattened by mergers and enriched inflows of recycled gas at late times. However, further theoretical work is required in order to understand how to interpret observations. Likewise, more observational work is needed in order to understand how metallicity gradients evolve to high redshifts.

Weak evolution of the mass–metallicity relation at cosmic dawn in the FirstLight simulations

“…This model reduces the mass loading factor (the gas outflow rate in units of SFR) in comparison to what would be permitted in the case without a velocity floor (Torrey et al 2019). A reduced loading factor naturally increases the galaxy metallicity at a fixed stellar mass because more metals are retained in the ISM rather than expelled (Lilly et al 2013;Finlator 2017).…”

Galactic Chemical Evolution of Radioactive Isotopes

“…OMEGA includes galactic inflows and outflows in order to consider, in a simplified way, the interactions between galaxies and their surrounding environment (see e.g., Somerville & Davé 2015;Anglés-Alcázar et al 2017;Naab & Ostriker 2017). Inflows introduce gas into the galaxy, fuel star formation, and usually dilute the gas metallicity inside the galaxy (e.g., Finlator 2017). Outflows on the other hand expel gas from the galaxy (e.g., Veilleux et al 2005;Bustard et al 2016;Pillepich et al 2018).…”

Validating Semi-analytic Models of High-redshift Galaxy Formation Using Radiation Hydrodynamical Simulations

“…where the four terms on the right-hand side are the inflow rate from the CGM into the galaxy (Ṁ g,in ), the combined mass-loss rate of all stars (Ṁ ej ), the star formation rate (Ṁ ), and the outflow rate from the galaxy into the CGM (Ṁ g,out ). While the magnitude of the star formation rate drives how much metal mass is ejected by stars, the galactic inflows typically dilute the metallicity of the galactic gas (Finlator 2017). We refer to Davé et al (2012) for an analytical model where all the physical processes stated above are in equilibrium.…”