Low-metallicity massive single stars with rotation. III. Source of ionization and C IV emission in I Zw 18

Chemically homogeneously evolving stars have been proposed to account for several exotic phenomena, including gravitational-wave emissions, gamma-ray bursts and certain types of supernovae. Here we study whether these stars can explain the observations of the metal-poor star-forming dwarf galaxy, I Zwicky 18. We apply our synthetic spectral models from Paper II to (i) establish a classification sequence for these hot stars, (ii) predict the photonionizing flux and the strength of observable emission lines from a I Zw 18-like stellar population, and (iii) compare our predictions to all available observations of this galaxy. Adding two new models computed with we report that (i) these stars follow a unique sequence of classes: O ̊ightarrow WN ̊ightarrow WO (i.e. without ever being WC). From our population synthesis with standard assumptions, we predict that (ii) the source of the UV C IV and other emission bumps is a couple of dozen WO-type Wolf--Rayet stars (not WC as previously assumed) which are the result of chemically homogeneous evolution, while these, combined with the rest of the O-star population, account for the high He II ionizing flux and the spectral hardness. Contrasting our results against published optical and UV data from the literature and accounting for different aperture sizes and spatial regions probed by the observations, we find that (iii) our models are highly consistent with existing measurements. Since our “massive Pop II stars” might just as well exist in early star-forming regions, our findings have implications for upcoming James Webb Space Telescope (JWST) surveys: the first galaxies in the high-redshift Universe may also experience the extra contribution of UV photons and the kinds of exotic explosions that chemically homogeneous stellar evolution predicts. Given that our results apply for binary populations too as long as the same fraction (10%) of the systems evolves chemically homogeneously, we conclude that the stellar progenitors of gravitational waves may very well exist today in I Zw 18.