Galaxy build-up in the first 1.5 Gyr of cosmic history: insights from the stellar mass function at <i>z</i> ~ 4–9 from <i>JWST</i> NIRCam observations
Andrea Weibel, Pascal A. Oesch, Laia Barrufet, Rashmi Gottumukkala, Richard S Ellis, P. Santini, John R. Weaver, Natalie Allen, R. J. Bouwens, R. A. A. Bowler, Gabe Brammer, Adam C. Carnall, Fergus Cullen, Pratika Dayal, Mark Dickinson, Callum T. Donnan, J. S. Dunlop, Mauro Giavalisco, Norman A. Grogin, G. D. Illingworth, Anton M. Koekemoer, Ivo Labbé, Danilo Marchesini, D J McLeod, R. J. McLure, Rohan P. Naidu, Pablo G. Pérez‐González, Marko Shuntov, Mauro Stefanon, Sune Toft, Mengyuan Xiao
Abstract
ABSTRACT Combining the public JWST/NIRCam imaging programs CEERS, PRIMER, and JADES, spanning a total area of $\sim 500\, {\rm arcmin}^2$, we obtain a sample of $\gt $30 000 galaxies at $z_{\rm phot}\sim 4\!-\!9$ that allows us to perform a complete, rest-optical-selected census of the galaxy population at $z\gt 3$. Comparing the stellar mass $M_*$ and the UV-slope $\beta$ 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 $10^{7.5}\!-\!10^{8.5}\, {\rm M_\odot }$, finding steep low-mass slopes over the entire redshift range, reaching values of $\alpha \approx -2$ 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.