The MOSDEF Survey: Sulfur Emission-line Ratios Provide New Insights into Evolving ISM Conditions at High Redshift. (arXiv:1907.07189v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Shapley_A/0/1/0/all/0/1">Alice E. Shapley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sanders_R/0/1/0/all/0/1">Ryan L. Sanders</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shao_P/0/1/0/all/0/1">Peng Shao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Reddy_N/0/1/0/all/0/1">Naveen A. Reddy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kriek_M/0/1/0/all/0/1">Mariska Kriek</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Coil_A/0/1/0/all/0/1">Alison L. Coil</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mobasher_B/0/1/0/all/0/1">Bahram Mobasher</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Siana_B/0/1/0/all/0/1">Brian Siana</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shivaei_I/0/1/0/all/0/1">Irene Shivaei</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Freeman_W/0/1/0/all/0/1">William R. Freeman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Azadi_M/0/1/0/all/0/1">Mojegan Azadi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Price_S/0/1/0/all/0/1">Sedona H. Price</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leung_G/0/1/0/all/0/1">Gene C. K. Leung</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fetherolf_T/0/1/0/all/0/1">Tara Fetherolf</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Groot_L/0/1/0/all/0/1">Laura de Groot</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zick_T/0/1/0/all/0/1">Tom Zick</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fornasini_F/0/1/0/all/0/1">Francesca M. Fornasini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Barro_G/0/1/0/all/0/1">Guillermo Barro</a>

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.

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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