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 $Z_{\odot }/2$. 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 ($7.8<12+\log O/H<8.5$) 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 $12+\log O/H\lesssim 8.0$ ($Z/Z_{\odot }\lesssim 1/5$) 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 $\mathrm{H}{\mathrm{e}}^{+}$-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.

27 pages, 13 figures, 11 tables, accepted for publication in MNRAS

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