Nearby dwarf galaxies provide a unique laboratory in which to test stellar
population models below $Z_ødot/2$. Such tests are particularly important for
interpreting the surprising high-ionization UV line emission detected at $z>6$
in recent years. We present HST/COS ultraviolet spectra of ten nearby
metal-poor star-forming galaxies selected to show He II emission in SDSS
optical spectra. The targets span nearly a dex in gas-phase oxygen abundance
($7.8<12+łogO/H<8.5$) and present uniformly large specific star
formation rates (sSFR $10^2$ $Gyr^-1$). The UV spectra confirm
that metal-poor stellar populations can power extreme nebular emission in
high-ionization UV lines, reaching C III equivalent widths comparable to those
seen in systems at $z6-7$. Our data reveal a marked transition in UV
spectral properties with decreasing metallicity, with systems below
$12+łogO/H8.0$ ($Z/Z_1/5$) presenting minimal
stellar wind features and prominent nebular emission in He II and C IV. This is
consistent with nearly an order of magnitude increase in ionizing photon
production beyond the $He^+$-ionizing edge relative to H-ionizing flux
as metallicity decreases below a fifth solar, well in excess of standard
stellar population synthesis predictions. Our results suggest that often
neglected sources of energetic radiation such as stripped binary products and
very massive O-stars produce a sharper change in the ionizing spectrum with
decreasing metallicity than expected. Consequently, nebular emission in C IV
and He II powered by these stars may provide useful metallicity constraints in
the reionization era.
Description
[1706.00881] Ultraviolet spectra of extreme nearby star-forming regions --- approaching a local reference sample for JWST
%0 Generic
%1 senchyna2017ultraviolet
%A Senchyna, Peter
%A Stark, Daniel P.
%A Vidal-García, Alba
%A Chevallard, Jacopo
%A Charlot, Stéphane
%A Mainali, Ramesh
%A Jones, Tucker
%A Wofford, Aida
%A Feltre, Anna
%A Gutkin, Julia
%D 2017
%K UV ionization low metallicity population stellar
%T Ultraviolet spectra of extreme nearby star-forming regions ---
approaching a local reference sample for JWST
%U http://arxiv.org/abs/1706.00881
%X Nearby dwarf galaxies provide a unique laboratory in which to test stellar
population models below $Z_ødot/2$. Such tests are particularly important for
interpreting the surprising high-ionization UV line emission detected at $z>6$
in recent years. We present HST/COS ultraviolet spectra of ten nearby
metal-poor star-forming galaxies selected to show He II emission in SDSS
optical spectra. The targets span nearly a dex in gas-phase oxygen abundance
($7.8<12+łogO/H<8.5$) and present uniformly large specific star
formation rates (sSFR $10^2$ $Gyr^-1$). The UV spectra confirm
that metal-poor stellar populations can power extreme nebular emission in
high-ionization UV lines, reaching C III equivalent widths comparable to those
seen in systems at $z6-7$. Our data reveal a marked transition in UV
spectral properties with decreasing metallicity, with systems below
$12+łogO/H8.0$ ($Z/Z_1/5$) presenting minimal
stellar wind features and prominent nebular emission in He II and C IV. This is
consistent with nearly an order of magnitude increase in ionizing photon
production beyond the $He^+$-ionizing edge relative to H-ionizing flux
as metallicity decreases below a fifth solar, well in excess of standard
stellar population synthesis predictions. Our results suggest that often
neglected sources of energetic radiation such as stripped binary products and
very massive O-stars produce a sharper change in the ionizing spectrum with
decreasing metallicity than expected. Consequently, nebular emission in C IV
and He II powered by these stars may provide useful metallicity constraints in
the reionization era.
@misc{senchyna2017ultraviolet,
abstract = {Nearby dwarf galaxies provide a unique laboratory in which to test stellar
population models below $Z_\odot/2$. Such tests are particularly important for
interpreting the surprising high-ionization UV line emission detected at $z>6$
in recent years. We present HST/COS ultraviolet spectra of ten nearby
metal-poor star-forming galaxies selected to show He II emission in SDSS
optical spectra. The targets span nearly a dex in gas-phase oxygen abundance
($7.8<12+\log\mathrm{O/H}<8.5$) and present uniformly large specific star
formation rates (sSFR $\sim 10^2$ $\mathrm{Gyr}^{-1}$). The UV spectra confirm
that metal-poor stellar populations can power extreme nebular emission in
high-ionization UV lines, reaching C III] equivalent widths comparable to those
seen in systems at $z\sim 6-7$. Our data reveal a marked transition in UV
spectral properties with decreasing metallicity, with systems below
$12+\log\mathrm{O/H}\lesssim 8.0$ ($Z/Z_\odot \lesssim 1/5$) presenting minimal
stellar wind features and prominent nebular emission in He II and C IV. This is
consistent with nearly an order of magnitude increase in ionizing photon
production beyond the $\mathrm{He^+}$-ionizing edge relative to H-ionizing flux
as metallicity decreases below a fifth solar, well in excess of standard
stellar population synthesis predictions. Our results suggest that often
neglected sources of energetic radiation such as stripped binary products and
very massive O-stars produce a sharper change in the ionizing spectrum with
decreasing metallicity than expected. Consequently, nebular emission in C IV
and He II powered by these stars may provide useful metallicity constraints in
the reionization era.},
added-at = {2017-06-06T10:06:12.000+0200},
author = {Senchyna, Peter and Stark, Daniel P. and Vidal-García, Alba and Chevallard, Jacopo and Charlot, Stéphane and Mainali, Ramesh and Jones, Tucker and Wofford, Aida and Feltre, Anna and Gutkin, Julia},
biburl = {https://www.bibsonomy.org/bibtex/21b201acdff1f3094c27d478ab531185d/miki},
description = {[1706.00881] Ultraviolet spectra of extreme nearby star-forming regions --- approaching a local reference sample for JWST},
interhash = {307d727ac2a613deca08c5f63536e851},
intrahash = {1b201acdff1f3094c27d478ab531185d},
keywords = {UV ionization low metallicity population stellar},
note = {cite arxiv:1706.00881Comment: 26 pages, 13 figures, 11 tables, submitted to MNRAS},
timestamp = {2017-06-06T10:06:12.000+0200},
title = {Ultraviolet spectra of extreme nearby star-forming regions ---
approaching a local reference sample for JWST},
url = {http://arxiv.org/abs/1706.00881},
year = 2017
}