Influence of the Void Environment on Chemical Abundances in Dwarf Galaxies and Implications for Connecting Star Formation and Halo Mass

Abstract

We study how the void environment affects the chemical evolution of galaxies in the universe by comparing the oxygen and nitrogen abundances of dwarf galaxies in voids with dwarf galaxies in denser regions. Using spectroscopic observations from the Sloan Digital Sky Survey Data Release 7, we estimate the oxygen, nitrogen, and neon abundances of 889 void dwarf galaxies and 672 dwarf galaxies in denser regions. We use the Direct T e method for calculating the gas-phase chemical abundances in the dwarf galaxies because it is best suited for low-metallicity, low-mass (dwarf) galaxies. A substitute for the [O ii ] λ 3727 doublet is developed, permitting oxygen abundance estimates of SDSS dwarf galaxies at all redshifts with the Direct T e method. We find that void dwarf galaxies have about the same oxygen abundance and Ne/O ratio as dwarf galaxies in denser environments. However, we find that void dwarf galaxies have slightly higher neon (∼10%) abundances than dwarf galaxies in denser environments. The opposite trend is seen in both the nitrogen abundance and N/O ratio: void dwarf galaxies have slightly lower nitrogen abundances (∼5%) and lower N/O ratios (∼7%) than dwarf galaxies in denser regions. Therefore, we conclude that the void environment has a slight influence on dwarf galaxy chemical evolution. Our mass–N/O relationship shows that the secondary production of nitrogen commences at a lower stellar mass in void dwarf star-forming galaxies than in dwarf star-forming galaxies in denser environments. We also find that star-forming void dwarf galaxies have higher H i masses than the star-forming dwarf galaxies in denser regions. Our star-forming dwarf galaxy sample demonstrates a strong anti-correlation between the sSFR and N/O ratio, providing evidence that oxygen is produced in higher-mass stars than those which synthesize nitrogen. The lower N/O ratios and smaller stellar mass for secondary nitrogen production seen in void dwarf galaxies may indicate both delayed star formation as predicted by ΛCDM cosmology and a dependence of cosmic downsizing on the large-scale environment. A shift toward slightly higher oxygen abundances and higher H i masses in void dwarf galaxies could be evidence of larger ratios of dark matter halo mass to stellar mass in voids compared with denser regions.