@article{4c71dbcaf0024d1b971ae4d5876aa8ae,
title = "Ultraviolet spectra of extreme nearby star-forming regions - approaching a local reference sample for JWST",
abstract = "Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z⊙/2. Such tests are particularly important for interpreting the surprising highionization ultraviolet (UV) line emission detected at z > 6 in recent years. We present HST/COS UV spectra of 10 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 + logO/H < 8.5) and present uniformly large specific star formation rates (sSFR ~102 Gyr-1). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching CIII] equivalent widths comparable to those seen in systems at z ~ 6-7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + logO/H ≲ 8.0 (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.",
keywords = "Galaxies: evolution, Galaxies: stellar content, Stars: massive, Ultraviolet:galaxies",
author = "Peter Senchyna and Stark, {Daniel P.} and Alba Vidal-Garc{\'i}a and Jacopo Chevallard and St{\'e}phane Charlot and Ramesh Mainali and Tucker Jones and Aida Wofford and Anna Feltre and Julia Gutkin",
note = "Funding Information: 1 IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. Funding Information: We thank the referee for their timely and helpful report. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. Support for program #14168 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. The scientific results reported in this article are based in part on observations made by the Chandra X-ray Observatory and published previously in cited articles. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Funding Information: DPS acknowledges support from the National Science Foundation through the grant AST-1410155. AV, JC and SC acknowledge support from the European Research Council via an Advanced Grant under grant agreement no. 321323-NEOGAL. TJ acknowledges support from NASA through Hubble Fellowship grant HST-HF2-51359.001-A awarded by the Space Telescope Science Institute. AF acknowledges support from the ERC via an Advanced Grant under grant agreement no. 339659-MUSICOS. Publisher Copyright: {\textcopyright} 2017 The Author.",
year = "2017",
month = dec,
doi = "10.1093/mnras/stx2059",
language = "English (US)",
volume = "472",
pages = "2608--2632",
journal = "Monthly Notices of the Royal Astronomical Society",
issn = "0035-8711",
publisher = "Oxford University Press",
number = "3",
}