The MOSDEF-LRIS Survey: The Connection Between Massive Stars and Ionized Gas in Individual Galaxies at $zsim2$. (arXiv:2008.02282v1 [astro-ph.GA])

The MOSDEF-LRIS Survey: The Connection Between Massive Stars and Ionized Gas in Individual Galaxies at $zsim2$. (arXiv:2008.02282v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Topping_M/0/1/0/all/0/1">Michael W. Topping</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:+Sanders_R/0/1/0/all/0/1">Ryan L. Sanders</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:+Kriek_M/0/1/0/all/0/1">Mariska Kriek</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>

We present constraints on the massive star and ionized gas properties for a
sample of 62 star-forming galaxies at $zsim2.3$. Using BPASS stellar
population models, we fit the rest-UV spectra of galaxies in our sample to
estimate age and stellar metallicity which, in turn, determine the ionizing
spectrum. In addition to the median properties of well-defined subsets of our
sample, we derive the ages and stellar metallicities for 30 high-SNR individual
galaxies — the largest sample of individual galaxies at high redshift with
such measurements. Most galaxies in this high-SNR subsample have stellar
metallicities of $0.001<Z_*<0.004$. We then use Cloudy+BPASS photoionization
models to match observed rest-optical line ratios and infer nebular properties.
Our high-SNR subsample is characterized by a median ionization parameter and
oxygen abundance, respectively, of $log(U)_{textrm{med}}=-2.98pm0.25$ and
$12+log(textrm{O/H})_{textrm{med}}=8.48pm0.11$. Accordingly, we find that
all galaxies in our sample show evidence for $alpha$-enhancement. In addition,
based on inferred $log(U)$ and $12+log(textrm{O/H})$ values, we find that
the local relationship between ionization parameter and metallicity applies at
$zsim2$. Finally, we find that the high-redshift galaxies most offset from the
local excitation sequence in the BPT diagram are the most $alpha$-enhanced.
This trend suggests that $alpha$-enhancement resulting in a harder ionizing
spectrum at fixed oxygen abundance is a significant driver of the high-redshift
galaxy offset on the BPT diagram relative to local systems. The ubiquity of
$alpha$-enhancement among $zsim2.3$ star-forming galaxies indicates important
differences between high-redshift and local galaxies that must be accounted for
in order to derive physical properties at high redshift.

We present constraints on the massive star and ionized gas properties for a
sample of 62 star-forming galaxies at $zsim2.3$. Using BPASS stellar
population models, we fit the rest-UV spectra of galaxies in our sample to
estimate age and stellar metallicity which, in turn, determine the ionizing
spectrum. In addition to the median properties of well-defined subsets of our
sample, we derive the ages and stellar metallicities for 30 high-SNR individual
galaxies — the largest sample of individual galaxies at high redshift with
such measurements. Most galaxies in this high-SNR subsample have stellar
metallicities of $0.001<Z_*<0.004$. We then use Cloudy+BPASS photoionization
models to match observed rest-optical line ratios and infer nebular properties.
Our high-SNR subsample is characterized by a median ionization parameter and
oxygen abundance, respectively, of $log(U)_{textrm{med}}=-2.98pm0.25$ and
$12+log(textrm{O/H})_{textrm{med}}=8.48pm0.11$. Accordingly, we find that
all galaxies in our sample show evidence for $alpha$-enhancement. In addition,
based on inferred $log(U)$ and $12+log(textrm{O/H})$ values, we find that
the local relationship between ionization parameter and metallicity applies at
$zsim2$. Finally, we find that the high-redshift galaxies most offset from the
local excitation sequence in the BPT diagram are the most $alpha$-enhanced.
This trend suggests that $alpha$-enhancement resulting in a harder ionizing
spectrum at fixed oxygen abundance is a significant driver of the high-redshift
galaxy offset on the BPT diagram relative to local systems. The ubiquity of
$alpha$-enhancement among $zsim2.3$ star-forming galaxies indicates important
differences between high-redshift and local galaxies that must be accounted for
in order to derive physical properties at high redshift.

http://arxiv.org/icons/sfx.gif