Evidence for the Accretion of Gas in Star-Forming Galaxies: High N/O Abundances in Regions of Anomalously-Low Metallicity

TL;DR

This study provides evidence that accretion of metal-poor gas occurs in star-forming galaxies, affecting their chemical composition, and likely sustains ongoing star formation in late-type galaxies.

Contribution

It demonstrates the use of N/O and O/H abundance ratios to identify gas accretion in galaxy regions with low metallicity, providing observational evidence for gas inflow.

Findings

Regions with anomalously low O/H show high N/O ratios.

Gas mixing indicates accretion of metal-poor gas.

Accretion likely sustains star formation in late-type galaxies.

Abstract

While all models for the evolution of galaxies require the accretion of gas to sustain their growth via on-going star formation, it has proven difficult to directly detect this inflowing material. In this paper we use data of nearby star-forming galaxies in the SDSS IV Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to search for evidence of accretion imprinted in the chemical composition of the interstellar medium. We measure both the O/H and N/O abundance ratios in regions previously identified as having anomalously low values of O/H. We show that the unusual locations of these regions in the N/O vs. O/H plane indicate that they have been created through the mixing of disk gas having higher metallicity with accreted gas having lower metallicity. Taken together with previous analysis on these anomalously low-metallicity regions, these results imply that accretion of metal-poor gas can probably sustain star formation in present-day late-type galaxies.