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

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 greater than or similar to 10. The model predicts a star formation history dominated by UV faint (MUV > -18) galaxies, with a Pop III contribution of less than or similar to 10 (less than or similar to 0.5) at z similar or equal to 20 (z similar or equal to 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 less than or similar to 5. The predicted stellar and AGN UV LFs reproduce the observational data at 5 less than or similar to z less than or similar to 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 similar or equal to -20) consistent with data at z similar to 10-11, but smaller by 0.8 dex at z similar to 12-13, and 1.2 dex at z similar to 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 less than or similar to 10 per cent of the total UV luminosity at MUV < -19 and z greater than or similar to 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 similar to 12-13, and the disagreement at z similar to 14-16 is reduced to 0.5 dex.