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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 high-ionization 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 + log O/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 C III] 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 + log O/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.
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https://hal-insu.archives-ouvertes.fr/insu-03710600
Contributor : Nathalie POTHIER Connect in order to contact the contributor
Submitted on : Friday, July 1, 2022 - 8:40:52 AM
Last modification on : Tuesday, August 2, 2022 - 4:29:41 AM

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Peter Senchyna, Daniel P. Stark, Alba Vidal-García, Jacopo Chevallard, Stéphane Charlot, et al.. Ultraviolet spectra of extreme nearby star-forming regions - approaching a local reference sample for JWST. Monthly Notices of the Royal Astronomical Society, 2017, 472, pp.2608-2632. ⟨10.1093/mnras/stx2059⟩. ⟨insu-03710600⟩

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