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29 March 2024
 
  » arxiv » 1907.7189

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The MOSDEF Survey: Sulfur Emission-line Ratios Provide New Insights into Evolving ISM 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 ;
Date 16 Jul 2019
AbstractWe 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.
Source arXiv, 1907.7189
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