Insights from HST into Ultra-Massive Galaxies and Early-Universe Cosmology

The early-science observations made by the James Webb Space Telescope (JWST) have revealed an excess of ultra-massive galaxy candidates that appear to challenge the standard cosmological model ($\mathrm{\Lambda }$CDM). Here, we argue that any modifications to $\mathrm{\Lambda }$CDM that can produce such ultra-massive galaxies in the early Universe would also affect the UV galaxy luminosity function (UV LF) inferred from the Hubble Space Telescope (HST). The UV LF covers the same redshifts ($z\approx 7-10$) and host-halo masses $(M_{\mathrm{h}}\approx {10}^{10}-{10}^{12}{M}_{\odot }$) as the JWST candidates, but tracks star-formation rate rather than stellar mass. We consider beyond-$\mathrm{\Lambda }$CDM power-spectrum enhancements and show that any departure large enough to reproduce the abundance of ultra-massive JWST candidates is in conflict with the HST data. Our analysis, therefore, severely disfavors a cosmological explanation for the JWST abundance problem. Looking ahead, we determine the maximum allowable stellar-mass function and provide projections for the high-$z$ UV LF given our constraints on cosmology from current HST data.