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 ] λ 5007/H β versus [S ii ] λλ 6717,6731/H α , 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 ] λλ 6717,6731/H α at fixed [O iii ] λ 5007/H β , relative to local galaxies from the Sloan Digital Sky Survey (SDSS). Furthermore, at fixed [O iii ] λ 5007/H β , local SDSS galaxies follow a trend of decreasing [S ii ] λλ 6717,6731/H α as the surface density of star formation (Σ SFR ) increases. We explain this trend in terms of the decreasing fractional contribution from diffuse ionized gas ( f DIG ) as Σ SFR increases in galaxies, which causes galaxy-integrated line ratios to shift toward the locus of pure H ii -region emission. The z  ∼ 0 relationship between f DIG and Σ SFR implies that high-redshift galaxies have lower f DIG values than typical local systems, given their significantly higher typical Σ SFR . When an appropriate low-redshift benchmark with zero or minimal f DIG is used, high-redshift MOSDEF galaxies appear offset toward higher [S ii ] λλ 6717,6731/H α and/or [O iii ] λ 5007/H β . 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.