Large-scale environmental dependence of the abundance ratio of nitrogen   to oxygen in blue, star-forming galaxies fainter than L*
  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 dwarf galaxies in more dense 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] 3727, [O III] 4959, 5007, and [N II] 6548, 6584 with the direct Te 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 more dense regions. The N/O ratio for void dwarfs (Mr > -17) is slightly lower (12%) than for dwarf galaxies in denser regions. We also estimate the nitrogen and oxygen abundances of 2050 void galaxies and 3883 galaxies in more dense regions with Mr > -20. 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 dark matter halo mass to stellar mass ratio in void galaxies. 
Kelly A. Douglass and Michael S. Vogeley
astro-ph.GA