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%
present new Keck Cosmic Web Imager (KCWI) optical integral field unit
spectroscopy of the only HII region in Leo P (3% ), which is powered
by a single O star. We calculate the required production rate of photons
capable of ionizing H and He from the observed H and HeI 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 SED
agrees with our observational measurements within the uncertainties. We then
fit Cloudy 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 OIII] and NIII] nebular emission in the Hubble Space
Telescope far-ultraviolet spectrum of the Leo P HII region, and highlight that
the rarely observed NIII] emission cannot be explained by our Cloudy 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.
The Ionizing Spectra of Extremely Metal-Poor O Stars: Constraints from the Only HII Region in Leo P
O. Grace Telford, Kristen B. W. McQuinn, John Chisholm, Danielle A. Berg
Submitted on 31 October 2022
Comment: Submitted to AAS Journals; comments welcome! 9 figures, 19 pages
Subjects: Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Astrophysics of Galaxies