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 () at z ≃ 20 (z ≃ 10). Stars are the primary sources of cosmic reionization, with 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 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.