On the oxygen and nitrogen chemical abundances and the evolution of the "green pea" galaxies

TL;DR

This study investigates the chemical abundances in green pea galaxies, revealing their low metallicity, high N/O ratios, and potential evolutionary processes involving gas inflow and outflow, providing insights into galaxy evolution.

Contribution

It provides the first detailed analysis of oxygen and nitrogen abundances in green pea galaxies, highlighting their unique chemical properties and evolutionary implications.

Findings

Green pea galaxies are genuinely metal-poor with 20% solar oxygen abundance.

They exhibit systematically higher N/O ratios at given metallicity compared to normal galaxies.

Their mass-metallicity relation is offset towards lower metallicities by over 0.3 dex.

Abstract

We have investigated the oxygen and nitrogen chemical abundances in extremely compact star-forming galaxies with redshifts between $\sim$0.11-0.35, popularly referred to as "green peas". Direct and strong-line methods sensitive to the N/O ratio applied to their SDSS spectra reveals that these systems are genuine metal-poor galaxies, with mean oxygen abundances 20% solar. At a given metallicity these galaxies display systematically large N/O ratios compared to normal galaxies, which can explain the strong difference between our metallicities measurements and previous ones. While their N/O ratios follow the relation with stellar mass of local star-forming galaxies in the SDSS, we find that the mass--metallicity relation of the "green peas" is offset $\ga$0.3 dex to lower metallicities. We argue that recent interaction-induced inflow of gas, possibly coupled with a selective metal-rich gas loss, driven by supernova winds, may explain our findings and the known galaxy properties, namely high specific star formation rates, extreme compactness, and disturbed optical morphologies. The "green pea" galaxy properties seem to be not common in the nearby Universe, suggesting a short and extreme stage of their evolution. Therefore, these galaxies may allow us to study in great detail many processes, such as starburst activity and chemical enrichment, under physical conditions approaching those in galaxies at higher redshifts.