No evidence (yet) for increased star-formation efficiency at early times

Abstract

Early JWST observations have revealed substantial numbers of galaxies out to redshifts as high as $z \simeq 14$, reflecting a slow evolution of the galaxy UV luminosity function (LF) not anticipated by many models of galaxy evolution. The discovery of fairly massive galaxies at early times has again been viewed as a challenge to our understanding of early galaxy growth or even ${\rm \Lambda}$CDM cosmology. Here we develop and test a simple theoretical model which shows that these observations are unsurprising, but instead are arguably as expected if one assumes a non-evolving halo-mass dependent galaxy-formation efficiency consistent with that observed today. Crucially, this model matches the observed galaxy UV LF at $z \simeq 6-13$ and the galaxy stellar mass function (GSMF) at $z \simeq 6-8$. Using new constraints on Lyman continuum escape and the ionizing photon production efficiency, we also predict the progress of cosmic hydrogen reionization consistent with current observations. The requirement to fit both the UV LF and the GSMF breaks the degeneracy between mass-to-light ratio and star-formation efficiency, where the typical mass-to-light ratio of galaxies increases systematically with redshift beyond $z \simeq 6$. However, at present this does not require changes to the IMF, cosmic dust, or any other new astrophysics. Rather, the current data can be reproduced simply by assuming ever-younger stellar populations consistent with a formation epoch at $z \simeq 15$. A key prediction of our model therefore is that there should be a more rapid drop-off in the galaxy number density beyond $z \simeq 15$, where one can no longer appeal to ever younger ages to offset the precipitous descent of the halo mass function.