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

Alice E. Shapley, Ryan L. Sanders, Peng Shao, Naveen A. Reddy, Mariska Kriek, Alison L. Coil, Bahram Mobasher, Brian Siana, Irene Shivaei, William R. Freeman, Mojegan Azadi, Sedona H. Price, Gene C.K. Leung, Tara Fetherolf, Laura De Groot, Tom Zick, Francesca M. Fornasini, Guillermo Barro

Research output: Contribution to journalArticlepeer-review

42 Scopus citations


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 (ΣSFR) increases. We explain this trend in terms of the decreasing fractional contribution from diffuse ionized gas (f DIG) as ΣSFR 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 ΣSFR implies that high-redshift galaxies have lower f DIG values than typical local systems, given their significantly higher typical ΣSFR. 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.

Original languageEnglish
Article numberL35
JournalAstrophysical Journal Letters
Issue number2
StatePublished - Aug 20 2019

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ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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