How galaxies acquire their stellar mass at high redshift: High star formation efficiencies and the relative roles of dust and initial mass function

Aims. JWST has measured an unprecedented abundance of galaxies above redshift z ≳ 4 − 5, whose formation and evolution are still difficult to reconcile within traditional galaxy evolution models in a Lambda Cold Dark Matter (ΛCDM) framework. Here we present a study on the star formation histories of these high-redshift galaxies between z ≃ 5 − 12 via a cutting-edge data-driven semi-empirical model that uses the observed ultra-violet (UV) luminosity functions (LFs) as input to retrieve star formation rates (SFRs), naturally bypassing any uncertain modelling of cooling, feedback and/or stochastic processes. Methods. Galaxy stellar masses are progressively built in time by integrating their SFRs assigned along their progenitor haloes via the SFR-halo accretion rate relation, derived from abundance matching between the input observed UV LFs with the dark matter halo accretion rate distributions at each redshift. Our original method reverse-engineers empirical estimates of the high-z galactic SFRs directly from observations via abundance matching rather than fitting observed LFs with parametric star formation efficiencies (SFEs). This makes the SFEs a full prediction of the model rather than a tuned input, serving as a solid baseline to test burstiness, dust attenuation, or initial mass function variations. Results. Our approach reproduces the total stellar mass function, the large-scale clustering, and the star-forming main sequence. We find that massive galaxies grew their stellar mass with a bursty star formation at z ∼ 9 − 10, broadly in agreement with the star formation histories inferred from spectral energy distribution fitting, with the SFE reaching high peaks of 0.8 − 0.9 at z > 9 and lowering to standard values of 0.2 − 0.3 below z ≲ 9. We find that the presence of dust could enhance the predicted SFRs at z ≲ 8, better reproducing the observed SFRs of massive dusty galaxies, and increase the SFEs to values close to or even above unity at z ≳ 8. Finally, switching to top-heavy initial mass functions reduces the SFEs by a factor of 2 − 3, highlighting the need for a variable initial mass function as an inevitable ingredient in the evolution of galaxies at high redshifts to avoid unphysical SFEs, especially in the presence of dust.