Mon. Not. R. Astron. Soc., 472, 2608-2632 (2017/December-2)
Ultraviolet spectra of extreme nearby star-forming regions - approaching a local reference sample for JWST.
SENCHYNA P., STARK D.P., VIDAL-GARCIA A., CHEVALLARD J., CHARLOT S., MAINALI R., JONES T., WOFFORD A., FELTRE A. and GUTKIN J.
Abstract (from CDS):
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.
© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
stars: massive - galaxies: evolution - galaxies: stellar content - ultraviolet: galaxies - ultraviolet: galaxies
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