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] λ3727 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 starforming 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.

The Influence of the Void Environment on the Ratio of Dark Matter Halo Mass to Stellar Mass in SDSS MaNGA Galaxies

“…If instead, the dark matter halo is more shallow in void galaxies (more pancake-shaped) than in wall galaxies, we would expect to see lower ratios of dark matter halo mass to stellar mass in the void dwarf galaxies with this data based on the conclusions of Douglass & Vogeley (2017b) and Douglass et al (2018). Our results eliminate this potential geometry of the dark matter halo.…”

“…The studies by Douglass & Vogeley (2017b) and Douglass et al (2018) suggest that a shift towards higher M DM /M * in void galaxies could explain why void dwarf galaxies have gas-phase metallicities similar to dwarf galaxies in denser regions. However, as seen in Figures 5 and 6 and in Table 1, there is no statistically significant difference in the mass ratios for galaxies as a function of their large-scale environment.…”

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

“…As described in Douglass et al (2018), both similar distributions or a shift towards higher metallicities in void dwarf galaxies could be due to a larger ratio of dark matter halo mass to stellar mass in voids, as seen in simulations by Jung et al (2014) and Tonnesen & Cen (2015). If the ratio of dark matter halo mass to stellar mass is larger in void galaxies, then their gravitational potential wells are slightly deeper, allowing them to retain more of their heavy metals.…”

“…There is very little difference between the metallicity distributions of the void and wall dwarf galaxy sample populations. (Image courtesy of Douglass et al 2018. ) line (when available) or the relationship shown in Fig.…”

“…) line (when available) or the relationship shown in Fig. 1 (described in Douglass et al 2018) is used to calculate the abundance of singly-ionized oxygen.…”