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We use the Cosmic Archaeology Tool (CAT) semi-analytical model to explore thecontribution of Population (Pop) III/II stars and active galactic nuclei (AGNs)to the galaxy UV luminosity function (LF) evolution at $4 \leq z \leq 20$. Wecompare in particular with recent JWST data in order to explore the apparenttension between observations and theoretical models in the number density ofbright galaxies at $z \gtrsim 10$. The model predicts a star formation historydominated by UV faint ($M_{\rm UV} > - 18$) galaxies, with a Pop IIIcontribution of $\lesssim 10\%$ ($\lesssim 0.5\%$) at $z \simeq 20$ ($z \simeq10$). Stars are the primary sources of cosmic reionization, with $5 - 10 \%$ ofionizing photons escaping into the intergalatic medium at $5 \leq z \leq 10$,while the contribution of unobscured AGNs becomes dominant only at $z \lesssim5$. The predicted stellar and AGN UV LFs reproduce the observational data at $5\lesssim z \lesssim 9 - 10$. At higher redshift, CAT predicts a steeperevolution in the faint-end slope ($M_{\rm UV} > - 18$), and a number density ofbright galaxies ($M_{\rm UV} \simeq -20$) consistent with data at $z \sim 10 -11$, but smaller by 0.8 dex at $z \sim 12 - 13$, and 1.2 dex at $z \sim 14 -16$, when compared to the values estimated by recent studies. Including the AGNemission does not affect the above findings, as AGNs contribute at most to$\lesssim 10 \%$ of the total UV luminosity at $M_{\rm UV} < - 19$ and $z\gtrsim 10$. Interestingly, considering a gradual transition in the stellarIMF, modulated by metallicity and redshift as suggested by recent simulations,the model agrees with JWST data at $z \sim 12 - 13$, and the disagreement at $z\sim 14 - 16$ is reduced to 0.5 dex.

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