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

TL;DR

This study uses HST data to show that modifications to the standard cosmological model cannot explain the ultra-massive galaxy candidates observed by JWST, challenging alternative cosmological theories.

Contribution

The paper demonstrates that cosmological modifications cannot account for JWST's ultra-massive galaxy observations without conflicting with existing HST data.

Findings

Cosmological modifications conflict with HST UV luminosity function data.

Standard $ m extLambda$CDM remains consistent with galaxy abundance observations.

Provides projections for galaxy formation constraints at high redshift.

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 ($\Lambda$CDM). Here, we argue that any modifications to $\Lambda$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-$\Lambda$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.