%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 }