Exploring the nature of UV-bright z ≳ 10 galaxies detected by JWST: star formation, black hole accretion, or a non-universal IMF?

ABSTRACT We use the Cosmic Archaeology Tool (CAT) semi-analytical model to explore the contribution of Population (Pop) III/II stars and active galactic nuclei (AGNs) to the galaxy ultraviolet (UV) luminosity function (LF) evolution at 4 ≤ z ≤ 20. We compare in particular with recent JWST data in order to explore the apparent tension between observations and theoretical models in the number density of bright galaxies at z ≳ 10. The model predicts a star formation history dominated by UV faint (MUV −18) galaxies, with a Pop III contribution of 10~{\ per\ cent} (0. 5~{\ per\ cent}) at z ≃ 20 (z ≃ 10). Stars are the primary sources of cosmic reionization, with 5~{\ per\ cent}\!-\!10~{\ per\ cent} of ionizing photons escaping into the intergalatic medium at 5 ≤ z ≤ 10, while the contribution of unobscured AGNs becomes dominant only at z ≲ 5. The predicted stellar and AGN UV LFs reproduce the observational data at 5 ≲ z ≲ 9–10. At higher redshift, CAT predicts a steeper evolution in the faint-end slope (MUV –18), and a number density of bright galaxies (MUV ≃ −20) consistent with data at z ∼ 10–11, but smaller by 0. 8 dex at z ∼ 12–13, and 1. 2 dex at z ∼ 14–16, when compared to the values estimated by recent studies. Including the AGN emission does not affect the above findings, as AGNs contribute at most to 10~{\ per\ cent} of the total UV luminosity at MUV –19 and z ≳ 10. Interestingly, considering a gradual transition in the stellar initial mass function, modulated by metallicity and redshift as suggested by recent simulations, the model agrees with JWST data at z ∼ 12–13, and the disagreement at z ∼ 14–16 is reduced to 0. 5 dex.