2019ApJ...881L..35S

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/Hβ versus [S II]λλ6717,6731/Hα, 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]λλ6717,6731/Hα at fixed [O III]λ5007/Hβ, relative to local galaxies from the Sloan Digital Sky Survey (SDSS). Furthermore, at fixed [O III]λ5007/Hβ, local SDSS galaxies follow a trend of decreasing [S II]λλ6717,6731/Hα 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]λλ6717,6731/Hα and/or [O III]λ 5007/Hβ. 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.