Stress Testing $Λ$CDM with High-redshift Galaxy Candidates

Abstract: Early data from JWST have revealed a bevy of high-redshift galaxy candidates with unexpectedly high stellar masses. I examine these candidates in the context of the most massive galaxies expected in $\Lambda$CDM-like models, wherein the stellar mass of a galaxy is limited by the available baryonic reservoir of its host dark matter halo. For a given cosmology, the abundance of dark matter halos as function of mass and redshift sets an absolute upper limit on the number density $n(>M_{\star},z)$ and stellar mass density $\rho_{\star}(>M_{\star},z)$ of galaxies above a stellar mass limit of $M_{\star}$ at any epoch $z$. The reported masses of the most massive galaxy candidates at $z \sim 10$ in JWST observations are in tension with these limits, indicating an issue with well-developed techniques for photometric selection of galaxies, galaxy stellar mass or effective survey volume estimates, or the $\Lambda$CDM model. That the strongest tension appears at $z \sim 10$, and not (yet?) at the highest redshifts probed by JWST galaxy candidates ($z \sim 16-20$), is promising for tests of the $\Lambda$CDM model using forthcoming wider-area JWST surveys.