The MOSDEF Survey: Sulfur Emission-line Ratios Provide New Insights into Evolving Interstellar Medium Conditions at High Redshift

Abstract

We present results on the emission-line properties of 1.3 <= z <= 2.7 galaxies drawn from the complete the MOSFIRE Deep Evolution Field (MOSDEF) survey. Specifically, we use observations of the emission-line diagnostic diagram of [O III]lambda 5007/H beta versus [S II] lambda lambda 6717,6731/H alpha, i.e., the "[S II] BPT diagram," to gain insight into the physical properties of high-redshift star-forming regions. High-redshift MOSDEF galaxies are offset toward lower [S II] lambda lambda 6717,6731/H alpha at fixed [O III]lambda 5007/H beta, relative to local galaxies from the Sloan Digital Sky Survey (SDSS). Furthermore, at fixed [O III]lambda 5007/H beta, local SDSS galaxies follow a trend of decreasing [S II] lambda lambda 6717,6731/H alpha as the surface density of star formation (Sigma(S)(FR)) increases. We explain this trend in terms of the decreasing fractional contribution from diffuse ionized gas (f(DIG)) as Sigma(S)(FR) increases in galaxies, which causes galaxy-integrated line ratios to shift toward the locus of pure H II-region emission. The z similar to 0 relationship between f(DIG) and Sigma(S)(FR) implies that high-redshift galaxies have lower f(DIG) values than typical local systems, given their significantly higher typical Sigma(S)(FR). When an appropriate low-redshift benchmark with zero or minimal f(DIG) is used, high-redshift MOSDEF galaxies appear offset toward higher [S II]lambda lambda 6717,6731/H alpha and/or [O III]lambda 5007/H beta. The joint shifts of high-redshift galaxies in the [S II] and [N II] BPT diagrams are best explained in terms of the harder spectra ionizing their star-forming regions at fixed nebular oxygen abundance (expected for chemically young galaxies), as opposed to large variations in N/O ratios or higher ionization parameters. The evolving mixture of H II regions and diffuse ionized gas is an essential ingredient of our description of the interstellar medium over cosmic time.