ΛCDM is still not broken: empirical constraints on the star formation efficiency at z ~ 12-30

The James Webb Space Telescope continues to push back the redshift frontier to ever earlier cosmic epochs, with recent announcements of galaxy candidates at redshifts of 15 z 30. We leverage the recent GUREFT suite of dissipationless N-body simulations, which were designed for interpreting observations in the high redshift Universe, and provide predictions of dark matter halo mass functions and halo growth rates for a state-of-the-art cosmology over a wide range of halo masses from 6 < z< 30. We combine these results with an empirical framework that maps halo growth rate to galaxy star formation rate and then to rest-frame UV luminosity. We find that even if all of the photometrically selected 15 z 30 galaxy candidates are real and actually at these extreme redshifts, there is no fundamental tension with ΛCDM, nor are exotic explanations required. With stellar light-to-mass ratios similar to those in well-studied lower redshift galaxies, our simple model can account for the observed extreme ultra-high redshift populations with star formation efficiencies that peak at values of 20-65 percent. Bursty star formation, or higher light-to-mass ratios such as are expected for lower metallicity stellar populations or a top-heavy Initial Mass Function, would result in even lower required star formation efficiencies, comparable to values predicted by high resolution numerical simulations of high-surface density star forming clouds.