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\npopulation models below $Z_\\odot/2$. Such tests are particularly important for\ninterpreting the surprising high-ionization UV line emission detected at $z>6$\nin recent years. We present HST/COS ultraviolet spectra of ten nearby\nmetal-poor star-forming galaxies selected to show He II emission in SDSS\noptical spectra. The targets span nearly a dex in gas-phase oxygen abundance\n($7.8<12+\\log\\mathrm{O/H}<8.5$) and present uniformly large specific star\nformation rates (sSFR $\\sim 10^2$ $\\mathrm{Gyr}^{-1}$). The UV spectra confirm\nthat metal-poor stellar populations can power extreme nebular emission in\nhigh-ionization UV lines, reaching C III] equivalent widths comparable to those\nseen in systems at $z\\sim 6-7$. Our data reveal a marked transition in UV\nspectral properties with decreasing metallicity, with systems below\n$12+\\log\\mathrm{O/H}\\lesssim 8.0$ ($Z/Z_\\odot \\lesssim 1/5$) presenting minimal\nstellar wind features and prominent nebular emission in He II and C IV. This is\nconsistent with nearly an order of magnitude increase in ionizing photon\nproduction beyond the $\\mathrm{He^+}$-ionizing edge relative to H-ionizing flux\nas metallicity decreases below a fifth solar, well in excess of standard\nstellar population synthesis predictions. Our results suggest that often\nneglected sources of energetic radiation such as stripped binary products and\nvery massive O-stars produce a sharper change in the ionizing spectrum with\ndecreasing metallicity than expected. Consequently, nebular emission in C IV\nand He II powered by these stars may provide useful metallicity constraints in\nthe reionization era.\n