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 zₚₕₒₜ~ 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 107.5\!-\!108.5\, 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.