Galaxy Build-up in the first 1.5 Gyr of Cosmic History: Insights from the Stellar Mass Function at $z\sim 4-9$ from JWST NIRCam Observations

Abstract

Combining the public JWST/NIRCam imaging programs CEERS, PRIMER and JADES, spanning a total area of $\sim 500{\mathrm{arcmin}}^2$, we obtain a sample of $>$30,000 galaxies at $z_{\mathrm{phot}}\sim 4-9$ that allows us to perform a complete, rest-optical selected census of the galaxy population at $z>3$. Comparing the stellar mass $M_{\ast }$ and the UV-slope ${\hat{I}}^2$ 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 $z\sim 4-9$ down to limiting masses of $10^{7.5}-10^{8.5}{\mathrm{M}}_{\odot }$, finding steep low mass slopes over the entire redshift range, reaching values of $\hat{I}\pm \approx -2$ at $z\gtrsim 6$. At the high-mass end, UV-red galaxies dominate at least out to $z\sim 6$. The implied redshift evolution of the SMF suggests a rapid build-up of massive dust-obscured or quiescent galaxies from $z\sim 6$ to $z\sim 4$ as well as an enhanced efficiency of star formation towards earlier times ($z\gtrsim 6$). Finally, we show that the galaxy mass density grows by a factor $\sim 20\times$ from $z\sim 9$ to $z\sim 4$. 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.

31 pages, 17 figures, published in MNRAS