Galaxy Build-up in the first 1. 5 Gyr of Cosmic History: Insights from the Stellar Mass Function at z4-9 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 z4-9 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 distributions between JWST- and HST-selected samples, we generally find very good agreement and no significant biases. Nevertheless, JWST enables us to probe a small population of UV-red galaxies that was missing from previous HST-based LBG samples. We measure galaxy stellar mass functions (SMFs) at z4-9 and show that they are broadly consistent with existing literature results. However, UV-red galaxies dominate the high-mass end of the SMF at least out to z6. In particular the most massive galaxies typically show very red colors between ₎₁ₒ1. 5m and 4. 5m, and thus JWST's unprecedented resolution and sensitivity at these wavelengths yields more accurate constraints on their abundance and masses. The implied redshift evolution of the high-mass end of the SMF suggests a rapid build-up of massive dust-obscured as well as quiescent galaxies from z6 to z4 as well as an enhanced efficiency of star formation towards earlier times (z6). We find the SMFs to be steep over the entire redshift range, and slightly steepening with redshift from z 4-6, reaching values of -2 at z6. Finally, we show that the galaxy mass density grows by a factor 20 in the 1 Gyr of cosmic time from z9 to z4. 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.