Ultraviolet spectra of extreme nearby star-forming regions – approaching a local reference sample for JWST
Stark, Daniel P.
AffiliationUniv Arizona, Steward Observ
MetadataShow full item record
PublisherOXFORD UNIV PRESS
CitationUltraviolet spectra of extreme nearby star-forming regions – approaching a local reference sample for JWST 2017, 472 (3):2608 Monthly Notices of the Royal Astronomical Society
Rights© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
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AbstractNearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z(circle dot)/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 + logO/H < 8.5) and present uniformly large specific star formation rates (sSFR similar to 10(2) 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 similar to 6-7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + logO/H less than or similar to 8.0 (Z/Z(circle dot) less than or similar to 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.
VersionFinal published version
SponsorsNASA through Space Telescope Science Institute ; NASA [NAS 5-26555]; W.M. Keck Foundation; National Science Foundation [AST-1410155]; European Research Council [321323-NEOGAL]; NASA through Hubble Fellowship - Space Telescope Science Institute [HST-HF2-51359.001-A]; ERC [339659-MUSICOS]