Merger-driven infall of metal-poor gas in luminous infrared galaxies: a deep dive beneath the mass-metallicity relation

TL;DR

This study investigates how metal-poor gas infall during galaxy mergers affects the chemical composition of luminous infrared galaxies, revealing deviations from typical mass-metallicity relations and highlighting the role of gas accretion in galaxy evolution.

Contribution

It introduces new infrared diagnostics for chemical abundances and provides evidence for merger-driven metal-poor gas infall impacting galaxy metallicity.

Findings

Most luminous IR galaxies follow the mass-metallicity relation.

Four late-stage merger galaxies show heavily depressed metallicities.

Metal-poor gas infall occurs during late merger stages, affecting chemical evolution.

Abstract

The build up of heavy elements and the stellar mass assembly are fundamental processes in the formation and evolution of galaxies. Although they have been extensively studied through observations and simulations, the key elements that govern these processes, such as gas accretion and outflows, are not fully understood. This is especially true for luminous and massive galaxies, which usually suffer strong feedback in the form of massive outflows, and large-scale gas accretion triggered by galaxy interactions. For a sample of 77 luminous infrared (IR) galaxies, we derive chemical abundances using new diagnostics based on nebular IR lines, which peer through the dusty medium of these objects and allow us to include the obscured metals in our abundance determinations. In contrast to optical-based studies, our analysis reveals that most luminous IR galaxies remain close to the mass-metallicity relation. Nevertheless, four galaxies with extreme star-formation rates ($> 60$M$_{\odot }$yr$^{-1}$) in their late merger stages show heavily depressed metallicities of 12+log(O/H) $\sim 7.7$--$8.1$ along with solar-like N/O ratios, indicative of gas mixing processes affecting their chemical composition. This evidence suggests the action of a massive infall of metal-poor gas in a short phase during the late merger stages, eventually followed by a rapid enrichment. These results challenge the classical gas equilibrium scenario usually applied to main-sequence galaxies, suggesting that the chemical enrichment and stellar-mass growth in luminous IR galaxies are regulated by different processes.