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~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 small population of UV-red galaxies that was missing from previous HST-based LBG samples. We measure galaxy stellar mass functions (SMFs) at z~4-9 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 z~6. In particular the most massive galaxies typically show very red colors between λobs~1.5μm and ~4.5μm, 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 z~6 to z~4 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 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.