The Chemical Evolution of Star-Forming Galaxies Over the Last 11 Billion Years

Abstract: We calculate the stellar mass-metallicity relation at five epochs ranging to z~2.3. We quantify evolution in the shape of the mass-metallicity relation as a function of redshift; the mass-metallicity relation flattens at late times. There is an empirical upper limit to the gas-phase oxygen abundance in star-forming galaxies that is independent of redshift. From examination of the mass-metallicity relation and its observed scatter we show that the flattening at late times is a consequence of evolution in the stellar mass where galaxies enrich to this empirical upper metallicity limit; there is also evolution in the fraction of galaxies at a fixed stellar mass that enrich to this limit. The stellar mass where metallicities begin to saturate is ~0.7 dex smaller in the local universe than it is at z~0.8.

The Chemical Evolution of Star-Forming Galaxies Over the Last 11 Billion Years

The research paper "The Chemical Evolution of Star-Forming Galaxies Over the Last 11 Billion Years" by Zahid et al. provides a methodical examination of the stellar mass-metallicity relation (MZ relation) across various epochs, extending up to redshift ( z \sim 2.3 ). The study focuses on analyzing the evolution of the gas-phase oxygen abundance as a function of stellar mass in star-forming galaxies. This relationship is pivotal in the context of galaxy evolution as it offers insights regarding the complex interplay between star formation, gas inflows, and gas outflows over cosmic time.

Key Findings

1. Mass-Metallicity Relation Evolution:

   • The study presents the MZ relation at five distinct epochs, illustrating that metallicity—as quantified by gas-phase oxygen abundance—correlates with stellar mass. This correlation holds across different redshifts, with notable mass-dependent evolution.
   • A significant finding is the empirical upper limit of the gas-phase oxygen abundance that remains largely unaffected by redshift variations.

2. Metallicity Saturation and Flattening:

   • The analysis reveals that metallicities in more massive galaxies tend to saturate, producing a flattening effect observed in the MZ relation at lower redshifts. The stellar mass threshold at which this saturation occurs diminishes over cosmic time.
   • The saturation is attributed to galaxies achieving a balance in which the generation of metals by stellar processes is offset by metal dispersal into the interstellar medium through various mechanisms.

3. Scatter and Evolution Dynamics:

   • The study analyzes scatter within the MZ relation, showing that at higher redshifts, there is a relatively uniform scatter across different stellar masses. As time progresses, massive galaxies exhibit a tightened scatter due to metallicity saturation.

Methodology and Data

The investigation relies on data compiled from major astronomical surveys such as the Sloan Digital Sky Survey (SDSS), the Smithsonian Hectospec Lensing Survey (SHELS), and the Deep Extragalactic Evolutionary Probe 2 Survey (DEEP2). These sources provide a robust dataset spanning from the local universe to high redshift regions. The authors employ a combination of spectroscopy and stellar population synthesis models to derive reliable metallicity and stellar mass estimates.

Implications and Future Directions

The implications of this study are multifaceted. On a practical level, understanding the evolution of the MZ relation aids in refining current models of galaxy formation and evolution. The persistent upper limit on metallicity suggests a self-regulating mechanism potentially governed by star formation efficiency, outflows, and the availability of pristine gas inflows.

The findings promote a nuanced perspective on "chemical downsizing," where massive galaxies' rapid evolution to saturation levels precedes similar processes in less massive systems. This observation aligns with broader notions of galaxy downsizing but from a chemical enrichment perspective.

In future work, advancements in near-infrared spectroscopy could enable more comprehensive studies of high-redshift galaxies, enhancing our understanding of metal enrichment processes during key eras of galaxy evolution. Observational constraints obtained from such studies will be vital for elaborating theoretical models, potentially unveiling new aspects of metal distribution mechanics in galaxies over cosmic time.