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

  • Author(s): Shapley, Alice E
  • Sanders, Ryan L
  • Shao, Peng
  • Reddy, Naveen A
  • Kriek, Mariska
  • Coil, Alison L
  • Mobasher, Bahram
  • Siana, Brian
  • Shivaei, Irene
  • Freeman, William R
  • Azadi, Mojegan
  • Price, Sedona H
  • Leung, Gene CK
  • Fetherolf, Tara
  • Groot, Laura de
  • Zick, Tom
  • Fornasini, Francesca M
  • Barro, Guillermo
  • et al.

We present results on the emission-line properties of 1.3<=z<=2.7 galaxies drawn from the complete MOSFIRE Deep Evolution Field (MOSDEF) survey. Specifically, we use observations of the emission-line diagnostic diagram of [OIII]5007/Hb vs. [SII]6717,6731/Ha, i.e., the [SII] BPT diagram, to gain insight into the physical properties of high-redshift star-forming regions. High-redshift MOSDEF galaxies are offset towards lower [SII]6717,6731/Ha at fixed [OIII]5007/Hb, relative to local galaxies from the Sloan Digital Sky Survey (SDSS). Furthermore, at fixed [OIII]5007/Hb, local SDSS galaxies follow a trend of decreasing [SII]6717,6731/Ha as the surface density of star formation (Sigma_SFR) increases. We explain this trend in terms of the decreasing fractional contribution from diffuse ionized gas (f_DIG) as Sigma_SFR increases in galaxies, which causes galaxy-integrated line ratios to shift towards the locus of pure HII-region emission. The z~0 relationship between f_DIG and Sigma_SFR implies that high-redshift galaxies have lower f_DIG values than typical local systems, given their significantly higher typical Sigma_SFR. When an appropriate low-redshift benchmark with zero or minimal f_DIG is used, high-redshift MOSDEF galaxies appear offset towards higher [SII]6717,6731/Ha and/or [OIII]5007/Hb. The joint shifts of high-redshift galaxies in the [SII] and [NII] 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 HII regions and DIG is an essential ingredient to our description of the ISM over cosmic time.

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