Galaxy build-up in the first 1.5 Gyr of cosmic history: insights from the stellar mass function at z ∼ 4–9 from JWST NIRCam observations

Combining the public JWST/NIRCam imaging programs CEERS, PRIMER, and JADES, spanning a total area of ∼ 500 arcmin², we obtain a sample of >30 000 galaxies at z_phot ∼ 4–9 that allows us to perform a complete, rest-optical-selected census of the galaxy population at z > 3. Comparing the stellar mass M_∗ and the UV-slope β distributions between JWST- and HST-selected samples, we generally find very good agreement and no significant biases. Nevertheless, JWST enables us to probe a new population of UV-red galaxies that was missing from previous HST-based Lyman-break galaxy (LBG) samples. We measure galaxy stellar mass functions (SMFs) at z ∼ 4–9 down to limiting masses of 10^7.5–10^8.5 M, finding steep low-mass slopes over the entire redshift range, reaching values of α ≈ −2 at z ≿ 6. At the high-mass end, UV-red galaxies dominate at least out to z ∼ 6. The implied redshift evolution of the SMF suggests a rapid build-up of massive dust-obscured or quiescent galaxies from z ∼ 6 to z ∼ 4 as well as an enhanced efficiency of star formation towards earlier times (z ≿ 6). Finally, we show that the galaxy mass density grows by a factor ∼ 20× from z ∼ 9 to z ∼ 4. Our results emphasize the importance of rest-frame optically selected samples in inferring accurate distributions of physical properties and studying the mass build-up of galaxies in the first 1.5 Gyr of cosmic history.