Synergising semi-analytical models and hydrodynamical simulations to interpret JWST data from the first billion years

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
² Kavli Institute for Cosmology and Institute of Astronomy, Madingley Road, CB3 0HA Cambridge, UK
³ Department of Astronomy, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722 Republic of Korea
⁴ Université Claude Bernard Lyon 1, CRAL UMR5574, ENS de Lyon, CNRS, Villeurbanne, F-69622 France
⁵ Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ, 08544 USA

^⋆ Corresponding author; v.mauerhofer@rug.nl

Received: 5 February 2025
Accepted: 10 March 2025

Abstract

Context. The field of high redshift galaxy formation has been revolutionised by the James Webb Space Telescope (JWST), which is yielding unprecedented insights into galaxy assembly at early times. In addition to global statistics, including the redshift evolution of the ultraviolet luminosity function (UV LF) and stellar mass function (SMF), new datasets are providing information on galaxy properties, including the mass-metallicity relation, UV spectral slopes (β), and ionising photon production efficiencies.

Aims. In this work our key aim is to understand the physical mechanisms that can simultaneously explain the unexpected abundance of bright galaxies at z ≳ 11 as well as the metal enrichment and observed spectral properties of galaxies in the early Universe. We also aim to determine the key sources of reionisation in light of recent data.

Methods. We incorporated interstellar medium physics – namely, cold gas fractions and star formation efficiencies – derived from the SPHINX²⁰ high-resolution radiation-hydrodynamics simulation into DELPHI, a semi-analytic model that tracks the assembly of dark matter halos and their baryonic components from z ∼ 4.5 − 40. In addition, we explored two methodologies to boost galaxy luminosities at z ≳ 11: a stellar initial mass function (IMF) that becomes increasingly top-heavy with decreasing metallicity and increasing redshift (eIMF model), and star formation efficiencies that increase with increasing redshift (eSFE model).

Results. Our key findings are the following: (i) Both the eIMF and eSFE models can explain the abundance of bright galaxies at z ≳ 11. (ii) Dust attenuation plays an important role in determining the bright end (M_UV ≲ −21) of the UV LF at z ≲ 11. At higher redshifts, dust is too dispersed to have an impact on the UV luminosity. (iii) The mass-metallicity relation is in place as early as z ∼ 17 in all models, although its slope is model-dependent. (iv) Within the spread of both the models and observations, all of our models are in good agreement with current estimates of β slopes at z ∼ 5 − 17 and Balmer break strengths at z ∼ 6 − 10. (v) In the eIMF model, galaxies at z ≳ 12 or with M_UV ≳ −18 show typical values of ξ_ion ∼ 10^25.55 [Hz erg⁻¹], a factor two larger than in other models. (vi) Star formation in low-mass galaxies (M_∗ ≲ 10⁹ M_⊙) is the key reionisation driver, providing the bulk (∼85%) of ionising photons down to its midpoint at z ∼ 7.