The Ionizing Spectra of Extremely Metal-poor O Stars: Constraints from the Only H ii Region in Leo P

Abstract

Metal-poor, star-forming dwarf galaxies produce extreme nebular emission and likely played a major role in cosmic reionization. Yet, determining their contribution to the high-redshift ionizing photon budget is hampered by the lack of observations constraining the ionizing spectra of individual massive stars more metal-poor than the Magellanic Clouds (20%–50% Z ⊙ ). We present new Keck Cosmic Web Imager (KCWI) optical integral field unit spectroscopy of the only H ii region in Leo P (3% Z ⊙ ), which is powered by a single O star. We calculate the required production rate of photons capable of ionizing hydrogen and helium from the observed H β and He i λ 4471 emission-line fluxes. Remarkably, we find that the ionizing photon production rate and spectral hardness predicted by a tlusty model fit to the stellar spectral energy distribution agrees with our observational measurements within the uncertainties. We then fit C loudy photoionization models to the full suite of optical emission lines in the KCWI data and show that the shape of the same tlusty ionizing continuum simultaneously matches lines across a wide range of ionization energies. Finally, we detect O iii ] and N iii ] nebular emission in the Hubble Space Telescope far-ultraviolet spectrum of the Leo P H ii region, and highlight that the rarely observed N iii ] emission cannot be explained by our C loudy models. These results provide the first observational evidence that widely used, yet purely theoretical, model spectra accurately predict the ionizing photon production rate from late-O stars at very low metallicity, validating their use to model metal-poor galaxies both locally and at high redshift.