Large-scale Environmental Dependence of the Abundance Ratio of Nitrogen to Oxygen in Blue, Star-forming Galaxies Fainter than L*

Abstract

Abstract We examine how the cosmic environment affects the chemical evolution of galaxies in the universe by comparing the N/O ratio of dwarf galaxies in voids with that of dwarf galaxies in denser regions. Ratios of the forbidden [O iii] and [S ii] transitions provide estimates of a region’s electron temperature and number density. We estimate the abundances of oxygen and nitrogen using these temperature and density estimates and the emission-line fluxes [O ii] , [O iii] , and [N ii] with the direct T e method. Using spectroscopic observations from the Sloan Digital Sky Survey Data Release 7, we are able to estimate the N/O ratio in 42 void dwarf galaxies and 89 dwarf galaxies in denser regions. The N/O ratio for void dwarfs ( ) is slightly lower ( ) than for dwarf galaxies in denser regions. We also estimate the nitrogen and oxygen abundances of 2050 void galaxies and 3883 galaxies in denser regions with . These somewhat brighter galaxies (but still fainter than L *) also display similar minor shifts in the N/O ratio. The shifts in the average and median element abundance values in all absolute magnitude bins studied are in the same direction, suggesting that the large-scale environment may influence the chemical evolution of galaxies. We discuss possible causes of such a large-scale environmental dependence of the chemical evolution of galaxies, including retarded star formation and a higher ratio of dark matter halo mass to stellar mass in void galaxies.