LATIS: The Stellar Mass-Metallicity Relation of Star-forming Galaxies at $z\sim 2.5$

TL;DR

This study investigates the stellar mass-metallicity relation of star-forming galaxies at redshift ~2.5, revealing a correlation between stellar mass and metallicity, high alpha-enhancement, and insights into galactic wind properties and black hole activity.

Contribution

It provides the first detailed stellar metallicity measurements at this redshift using FUV spectra and models, and links these to galactic wind parameters and black hole feedback.

Findings

Stellar metallicity increases with stellar mass and flattens at high masses.

Galaxies at this redshift show high alpha-element enhancement (~0.6).

Mass loading of galactic winds decreases with stellar mass, indicating possible black hole-driven outflows.

Abstract

We present the stellar mass - stellar metallicity relation for 3491 star-forming galaxies at $2 \lesssim z \lesssim 3$ using rest-frame far-ultraviolet (FUV) spectra from the Ly$\alpha$ Tomography IMACS Survey (LATIS). We fit stellar population synthesis models from the Binary Population And Spectral Synthesis code (BPASS v$2.2.1$) to medium resolution (R $\sim 1000$) and high signal-to-noise ($>30$ per 100 km/s over a wavelength range of 1221 - 1800 \r{A}) composite spectra of galaxies in bins of stellar mass to determine their stellar metallicity, primarily tracing $\rm Fe/H$. We find a strong correlation between stellar mass and stellar metallicity, with stellar metallicity monotonically increasing with stellar mass at low masses and flattening at high masses ($M_* \gtrsim 10^{10.3} M_\odot$). Additionally, we compare our stellar metallicity measurements with the gas-phase oxygen abundance of galaxies at similar redshift and estimate the average $\rm [\alpha/Fe] \sim 0.6$. Such high $\alpha$-enhancement indicates that high-redshift galaxies have not yet undergone significant iron enrichment through Type Ia supernovae. Moreover, we utilize an analytic chemical evolution model to constrain the mass loading parameter of galactic winds as a function of stellar mass. We find that as the stellar mass increases, the mass loading parameter decreases. The parameter then flattens or reaches a turning point at around $M_* \sim 10^{10.5} M_\odot$. Our findings may signal the onset of black hole-driven outflows at $z \sim 2.5$ for galaxies with $M_* \gtrsim 10^{10.5} M_\odot$.