Three-Dimensional Modelling of Ionized Gas. I. Did very massive stars of different metallicities drive the second cosmic reionization?

TL;DR

This paper models the impact of very massive stars with varying metallicities on cosmic reionization, especially focusing on helium ionization and the potential link to supermassive black hole formation.

Contribution

It introduces a comprehensive 3D radiative transfer model using realistic stellar spectra to study reionization driven by VMSs across different metallicities.

Findings

Metallicity significantly affects the spectrum hardness and reionization timeline.

VMSs could be key in reionizing HeII by z~2.5.

Number of VMSs needed aligns with SMBH formation scenarios.

Abstract

The first generation of stars that formed directly from the primordial gas played a crucial role in the early phase of the reionization of the universe. Because of the short lifetimes of these stars the metals produced in their cores were quickly returned to the environment, from which early PopII stars with a different initial mass function and different SEDs were formed, already much earlier than the time at which the universe became completely reionized at a redshift of z~6. Using a state-of-the-art model atmosphere code we calculate realistic SEDs of very massive stars (VMSs) of different metallicities to serve as input for the 3-dimensional multi-frequency radiative transfer code we have developed to simulate the temporal evolution of the ionization of the inhomogeneous interstellar and intergalactic medium, using multiple stellar clusters as sources of ionizing radiation. Our tool handles distributions of numerous radiative sources characterized by high resolution synthetic SEDs, and yields occupation numbers of the required energy levels of the most important elements which are treated in NLTE consistently with the 3d radiative transfer. We further demonstrate that the increasing metallicity of the radiative sources in the transition from PopIII stars to PopII stars has a strong impact on the hardness of the emitted spectrum, and hence on the reionization history of helium. A top-heavy stellar mass distribution characterized by VMSs forming in chemically evolved clusters of high core mass density may not only provide the progenitors of intermediate-mass and supermassive black holes (SMBHs), but also play an important role for the reionization of HeII. The number of VMSs required to reionize HeII by a redshift of z~2.5 is astonishingly close to the number of VMSs required to explain galactic SMBHs if one assumes that these have been formed by mergers of smaller black holes.