The MOSDEF Survey: [SIII] as a New Probe of Evolving ISM Conditions. (arXiv:1910.13594v1 [astro-ph.GA])

The MOSDEF Survey: [SIII] as a New Probe of Evolving ISM Conditions. (arXiv:1910.13594v1 [astro-ph.GA])
<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:+Jones_T/0/1/0/all/0/1">Tucker Jones</a>, <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:+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:+Siana_B/0/1/0/all/0/1">Brian Siana</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:+Shivaei_I/0/1/0/all/0/1">Irene Shivaei</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:+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:+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:+Zick_T/0/1/0/all/0/1">Tom O. Zick</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:+Barro_G/0/1/0/all/0/1">Guillermo Barro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fornasini_F/0/1/0/all/0/1">Francesca M. Fornasini</a>

We present measurements of [SIII]$lambdalambda$9069,9531 for a sample of
$zsim1.5$ star-forming galaxies, the first sample with measurements of these
lines outside of the low-redshift universe. We employ the line ratio
S$_{32}$$equiv$[SIII]$lambdalambda$9069,9531/[SII]$lambdalambda$6716,6731
as a novel probe of evolving ISM conditions. Since this ratio includes the
low-ionization line [SII], it is crucial that the effects of diffuse ionized
gas (DIG) on emission-line ratios be accounted for in $zsim0$ integrated
galaxy spectra, or else that comparisons be made to samples of local HII
regions in which DIG emission is not present. We find that S$_{32}$ decreases
with increasing stellar mass at both $zsim1.5$ and $zsim0$, but that the
dependence is weak suggesting S$_{32}$ has a very shallow anticorrelation with
metallicity, in contrast with O$_{32}$ that displays a strong metallicity
dependence. As a result, S$_{32}$ only mildly evolves with redshift at fixed
stellar mass. The $zsim1.5$ sample is systematicallty offset towards lower
S$_{32}$ and higher [SII]/H$alpha$ at fixed [OIII]/H$beta$ relative to $z=0$
HII regions. By comparing to photoionization model grids, we find that such
trends can be explained by a scenario in which the ionizing spectrum is harder
at fixed O/H with increasing redshift, but are inconsistent with an increase in
ionization parameter at fixed O/H. This analysis demonstrates the advantages of
expanding beyond the strongest rest-optical lines for evolutionary studies, and
the particular utility of [SIII] for characterizing evolving ISM conditions and
stellar compositions.

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