Constraints on the production and escape of ionizing radiation from the emission-line spectra of metal-poor star-forming galaxies. (arXiv:1909.07386v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Plat_A/0/1/0/all/0/1">Ad&#xe8;le Plat</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Charlot_S/0/1/0/all/0/1">Stephane Charlot</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bruzual_G/0/1/0/all/0/1">Gustavo Bruzual</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Feltre_A/0/1/0/all/0/1">Anna Feltre</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vidal_Garcia_A/0/1/0/all/0/1">Alba Vidal-Garc&#xed;a</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Morisset_C/0/1/0/all/0/1">Christophe Morisset</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chevallard_J/0/1/0/all/0/1">Jacopo Chevallard</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Todt_H/0/1/0/all/0/1">Helge Todt</a>

We explore the production and escape of ionizing photons in young galaxies by
investigating the ultraviolet and optical emission-line properties of models of
ionization-bounded and density-bounded HII regions, active-galactic-nucleus
(AGN) narrow-line regions and radiative shocks computed all using the same
physically-consistent description of element abundances and depletion on to
dust grains down to very low metallicities. We compare these models with a
reference sample of metal-poor star-forming galaxies and Lyman-continuum (LyC)
leakers at various redshifts, which allows the simultaneous exploration of more
spectral diagnostics than typically available at once for individual
subsamples. We confirm that current single- and binary-star population
synthesis models do not produce hard-enough radiation to account for the
high-ionization emission of the most metal-poor galaxies. Introducing either an
AGN or radiative-shock component brings models into agreement with
observations. A published model including X-ray binaries is an attractive
alternative to reproduce the observed rise in HeII4686/Hbeta ratio with
decreasing oxygen abundance in metal-poor star-forming galaxies, but not the
high observed HeII4686/Hbeta ratios of galaxies with large EW(Hbeta). A source
of harder ionizing radiation appears to be required in these extreme objects,
such as an AGN or radiative-shock component, perhaps linked to an
initial-mass-function bias toward massive stars at low metallicity. This would
also account for the surprisingly high [OI]/[OIII] ratios of confirmed LyC
leakers relative to ionization-bounded models. We find no simple by-eye
diagnostic of the nature of ionizing sources and the escape of LyC photon,
which require proper simultaneous fits of several lines to be discriminated
against.

We explore the production and escape of ionizing photons in young galaxies by
investigating the ultraviolet and optical emission-line properties of models of
ionization-bounded and density-bounded HII regions, active-galactic-nucleus
(AGN) narrow-line regions and radiative shocks computed all using the same
physically-consistent description of element abundances and depletion on to
dust grains down to very low metallicities. We compare these models with a
reference sample of metal-poor star-forming galaxies and Lyman-continuum (LyC)
leakers at various redshifts, which allows the simultaneous exploration of more
spectral diagnostics than typically available at once for individual
subsamples. We confirm that current single- and binary-star population
synthesis models do not produce hard-enough radiation to account for the
high-ionization emission of the most metal-poor galaxies. Introducing either an
AGN or radiative-shock component brings models into agreement with
observations. A published model including X-ray binaries is an attractive
alternative to reproduce the observed rise in HeII4686/Hbeta ratio with
decreasing oxygen abundance in metal-poor star-forming galaxies, but not the
high observed HeII4686/Hbeta ratios of galaxies with large EW(Hbeta). A source
of harder ionizing radiation appears to be required in these extreme objects,
such as an AGN or radiative-shock component, perhaps linked to an
initial-mass-function bias toward massive stars at low metallicity. This would
also account for the surprisingly high [OI]/[OIII] ratios of confirmed LyC
leakers relative to ionization-bounded models. We find no simple by-eye
diagnostic of the nature of ionizing sources and the escape of LyC photon,
which require proper simultaneous fits of several lines to be discriminated
against.

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