COSMOS-Web: Stellar mass assembly in relation to dark matter halos across 0.2 <

We study the stellar mass assembly of galaxies via the stellar mass function (SMF) and the coevolution with dark matter halos via abundance matching in the largest redshift range to date, 0.2 < z < 12. We used the 0.53 deg2 imaged by JWST from the COSMOS-Web survey, in combination with ancillary imaging in over 30 photometric bands, to select highly complete samples (down to log M⋆/M⊙ = 7.5 − 8.8) in 15 redshift bins. Our results show that the normalization of the SMF monotonically decreases from z = 0.2 to z = 12 with strong mass-dependent evolution. At z > 5, we find increased abundances of massive (log M⋆/M⊙ > 10.5) systems compared to predictions from semi-analytical models and hydrodynamical simulations. These findings challenge traditional galaxy formation models by implying integrated star formation efficiencies (SFEs) of $ \epsilon_{\star}\equiv M_{\star}\, f_{\mathrm{b}}^{-1} M_{\mathrm{halo}}^{-1} \gtrsim 0.5 $. We find a flattening of the SMF at the high-mass end that is better described by a double power law at z > 5.5, after correcting for the Eddington bias. At z ≲ 5.5, it transitions to a Schechter law, which coincides with the emergence of the first massive quiescent galaxies in the Universe, indicating that physical mechanisms that suppress galaxy growth start to take place at z ∼ 5.5 on a global scale. By integrating the SMF, we trace the cosmic stellar mass density and infer the star formation rate density, which at z > 7.5 agrees remarkably with recent JWST UV luminosity function-derived estimates. This agreement solidifies the emerging picture of rapid galaxy formation leading to increased abundances of bright and massive galaxies in the first ∼0.7 Gyr. However, at z ≲ 3.5, we find significant tension (∼0.3 dex) with the cosmic star formation (SF) history from instantaneous SF measures, the causes of which remain poorly understood. We infer the stellar-to-halo mass relation (SHMR) and the SFE from abundance matching out to z = 12, finding a non-monotonic evolution. The SFE has the characteristic strong dependence with mass in the range of 0.02 − 0.2, and mildly decreases at the low-mass end out to z ∼ 3.5. At z ∼ 3.5, there is an upturn and the SFE increases sharply from ∼0.1 to approach a high SFE of 0.8 − 1 by z ∼ 10 for log(Mh/M⊙)≈11.5, albeit with large uncertainties. Finally, we use the SHMR to track the SFE and stellar mass growth throughout the halo history and find that they do not grow at the same rate – from the earliest times up until z ∼ 3.5 the halo growth rate outpaces galaxy assembly, but at z > 3.5 halo growth stagnates and accumulated gas reservoirs keep the SF going and galaxies outpace halos.