Observable and ionizing properties of star-forming galaxies with very massive stars and different initial mass functions

¹ Observatoire de Genève, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
³ LUPM, Université de Montpellier, CNRS, Place Eugène Bataillon, F-34095 Montpellier, France

^⋆ Corresponding author; daniel.schaerer@unige.ch

Received: 11 July 2024
Accepted: 30 September 2024

Abstract

Context. The presence of very massive stars (VMS) with masses > 100 M_⊙ is now firmly established in the Local Group, nearby galaxies, and out to cosmological distances. If present, these stars could boost the UV luminosity and ionizing photon production of galaxies, helping to alleviate the overabundance of UV-bright galaxies found with JWST at high redshift.

Aims. To examine these questions, we quantify the impact of VMS on properties of integrated stellar populations, exploring different stellar initial mass functions (IMFs) extending up to 400 M_⊙, and with slopes between standard (Salpeter-like) and flatter, more top-heavy IMFs.

Methods. Combing consistent stellar evolution and atmosphere models tailored to VMS at 1/2.5 Z_⊙ metallicity with BPASS evolutionary synthesis models and including nebular emission, we computed integrated spectral energy distributions (SEDs) and derived quantities for a large set of models.

Results. We find that VMS contribute significantly to the UV luminosity and Lyman continuum emission of young stellar populations, and they are characterized by strong stellar He II emission, with EW(He IIλ1640) up to 4–8 Å at young ages or ∼2.5 − 4 Å for a constant star formation rate (SFR) (for the IMFs considered here). For IMFs with a Salpeter slope, the boost of the UV luminosity is relatively modest (up to a factor of ∼1.6). However, small changes in the IMF slope (e.g., from α₂ = −2.35 to −2) lead to large increases in L_UV and the ionizing photon production, Q. The ionizing photon efficiency, ξ_ion, is also increased with VMS, by typically 0.14–0.2 dex for a Salpeter slope, and by up to ∼0.4 dex when the IMF is extended up to 400 M_⊙. Stronger H recombination lines are also predicted in the presence of VMS. Interestingly, SEDs including VMS show smaller Lyman breaks, and the shape of the ionizing spectra remain essentially unaltered up to ∼35 eV, but become softer at higher energies. We derive and discuss the maximum values that quantities such as L_UV per stellar mass or unit SFR, ξ_ion, Q, and others can reach when VMS are included, and we show that these values become essentially independent of the IMF. We propose observational methods to test for the presence of VMS and constrain the IMF in star-forming galaxies. Finally, using published JWST observations, we examine if high redshift (z ≳ 5 − 6) galaxies show some evidence of the presence of VMS and/or signs of non-standard IMFs. Very top-heavy IMFs can be excluded on average, but we find that the IMF could well extend into the regime of VMS and be flatter than Salpeter in the bulk of high-z galaxies.

Conclusions. The predictions should improve our understanding of the stellar content and IMF of the most distant galaxies and allow us to establish if “extreme” or “unusual” populations extending well beyond 100 M_⊙ existed in the early Universe.