James Webb Space Telescope Feed Post

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

Paper abstract: 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 (\LambdaCDM). Here, we argue that any modifications to \LambdaCDM 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-\LambdaCDM 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.