The Binary Population and Spectral Synthesis suite of binary stellar evolution models and synthetic stellar populations provides a framework for the physically motivated analysis of both the integrated light from distant stellar populations and the detailed properties of those nearby. We present a new version 2.1 data release of these models, detailing the methodology by which Binary Population and Spectral Synthesis incorporates binary mass transfer and its effect on stellar evolution pathways, as well as the construction of simple stellar populations. We demonstrate key tests of the latest Binary Population and Spectral Synthesis model suite demonstrating its ability to reproduce the colours and derived properties of resolved stellar populations, including well-constrained eclipsing binaries. We consider observational constraints on the ratio of massive star types and the distribution of stellar remnant masses. We describe the identification of supernova progenitors in our models, and demonstrate a good agreement to the properties of observed progenitors. We also test our models against photometric and spectroscopic observations of unresolved stellar populations, both in the local and distant Universe, finding that binary models provide a self-consistent explanation for observed galaxy properties across a broad redshift range. Finally, we carefully describe the limitations of our models, and areas where we expect to see significant improvement in future versions.

Determining the properties of old stellar populations (those with age >1 Gyr) has long involved the comparison of their integrated light, either in the form of photometry or spectroscopic indexes, with empirical or synthetic templates. Here we re-evaluate the properties of old stellar populations using a new set of stellar population synthesis models, designed to incorporate the effects of binary stellar evolution pathways as a function of stellar mass and age. We find that single-aged stellar population models incorporating binary stars, as well as new stellar evolution and atmosphere models, can reproduce the colours and spectral indices observed in both globular clusters and quiescent galaxies. The best-fitting model populations are often younger than those derived from older spectral synthesis models and may also lie at slightly higher metallicities.

We report the discovery of a candidate galaxy with a photo-z of z ∼ 12 in the first epoch of the James Webb Space Telescope (JWST) Cosmic Evolution Early Release Science Survey. Following conservative selection criteria, we identify a source with a robust z _phot = 11.8 − 0.2 + 0.3 (1σ uncertainty) with m _F200W = 27.3 and ≳7σ detections in five filters. The source is not detected at λ < 1.4 μm in deep imaging from both Hubble Space Telescope (HST) and JWST and has faint ∼3σ detections in JWST F150W and HST F160W, which signal a Lyα break near the red edge of both filters, implying z ∼ 12. This object (Maisie’s Galaxy) exhibits F115W − F200W > 1.9 mag (2σ lower limit) with a blue continuum slope, resulting in 99.6% of the photo-z probability distribution function favoring z > 11. All data-quality images show no artifacts at the candidate’s position, and independent analyses consistently find a strong preference for z > 11. Its colors are inconsistent with Galactic stars, and it is resolved (r _h = 340 ± 14 pc). Maisie’s Galaxy has log M _*/M _⊙ ∼ 8.5 and is highly star-forming (log sSFR ∼ −8.2 yr⁻¹), with a blue rest-UV color (β ∼ −2.5) indicating little dust, though not extremely low metallicity. While the presence of this source is in tension with most predictions, it agrees with empirical extrapolations assuming UV luminosity functions that smoothly decline with increasing redshift. Should follow-up spectroscopy validate this redshift, our universe was already aglow with galaxies less than 400 Myr after the Big Bang.

The relationship between stellar populations and the ionizing flux with which they irradiate their surroundings has profound implications for the evolution of the intergalactic medium (IGM). We quantify the ionizing flux arising from synthetic stellar populations which incorporate the evolution of interacting binary stars. We determine that these show ionizing flux boosted by 60 per cent at 0.05 ≤ Z ≤ 0.3 Z_⊙ and a more modest 10–20 per cent at near-solar metallicities relative to star-forming populations in which stars evolve in isolation. The relation of ionizing flux to observables such as 1500 Å continuum and ultraviolet spectral slope is sensitive to attributes of the stellar population including age, star formation history and initial mass function (IMF). For a galaxy forming 1 M_⊙ yr⁻¹, observed at >100 Myr after the onset of star formation, we predict a production rate of photons capable of ionizing hydrogen, N_ion = 1.4 × 10⁵³ s⁻¹ at Z = Z_⊙ and 3.5 × 10⁵³ s⁻¹ at 0.1 Z_⊙, assuming a Salpeter-like IMF. We evaluate the impact of these issues on the ionization of the IGM, finding that the known galaxy populations can maintain the ionization state of the Universe back to z ∼ 9, assuming that their luminosity functions continue to M_UV = −10, and that constraints on the IGM at z ∼ 2–5 can be satisfied with modest Lyman-continuum photon escape fractions of 4–24 per cent depending on assumed metallicity.

Using the Binary Population and Spectral Synthesis code, bpass, we have calculated the rates, time-scales and mass distributions for binary black hole (BH) mergers as a function of metallicity. We consider these in the context of the recently reported first Laser Interferometer Gravitational-Wave Observatory (LIGO) event detection. We find that the event has a very low probability of arising from a stellar population with initial metallicity mass fraction above Z = 0.010 (Z ≳ 0.5 Z_⊙). Binary BH merger events with the reported masses are most likely in populations below 0.008 (Z ≲ 0.4 Z_⊙). Events of this kind can occur at all stellar population ages from 3 Myr up to the age of the Universe, but constitute only 0.1–0.4 per cent of binary BH mergers between metallicities of Z = 0.001 and 0.008. However at metallicity Z = 10⁻⁴, 26 per cent of binary BH mergers would be expected to have the reported masses. At this metallicity, the progenitor merger times can be close to ≈10 Gyr and rotationally mixed stars evolving through quasi-homogeneous evolution, due to mass transfer in a binary, dominate the rate. The masses inferred for the BHs in the binary progenitor of GW 150914 are amongst the most massive expected at anything but the lowest metallicities in our models. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections.