Low-metallicity massive single stars with rotation

III. Source of ionization and C IV emission in I Zw 18

¹ Institute of Astronomy – Faculty of Physics, Astronomy and Informatics – Nicolaus Copernicus University, Grudziądzka 5, 87-100 Toruń, Poland
² Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir km 4, E-28850 Torrejón de Ardoz, Madrid, Spain
³ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
⁴ Astronomický ústav, Akademie věd České republiky, Fričova 298, 251 65 Ondřejov, Czech Republic
⁵ Instituto de Astrofísica de Andalucía (IAA/CSIC), Glorieta de la Astronomía s/n Aptdo. 3004, E-18080 Granada, Spain
⁶ Observatório Nacional/MCTIC, R. Gen. José Cristino, 77, 20921-400 Rio de Janeiro, Brazil
⁷ Ústav teoretické fyziky a astrofyziky, Masarykova univerzita, Kotlářská 267/2, 611 37 Brno, Czech Republic
⁸ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
⁹ Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany

^⋆ Corresponding authors: dorottya.szecsi@gmail.com; brankica.kubatova@asu.cas.cz

Received: 4 October 2024
Accepted: 1 July 2025

Abstract

Context. 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.

Aims. Here we study whether these stars can explain the observations of the metal-poor star-forming dwarf galaxy, I Zwicky 18.

Methods. 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.

Results. Adding two new models computed with PoWR, we report that (i) these stars follow a unique sequence of classes: O → WN → WO (i.e. without ever being WC). From our population synthesis with standard assumptions, we predict that (ii) the source of the UV C IVλ1550 Å 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.

Conclusions. 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.