Abstract
The distribution of dark matter halo masses can be accurately predicted in
the $Łambda$CDM cosmology. The presence of a single massive halo or galaxy at
a particular redshift, assuming some baryon and stellar fraction for the
latter, can therefore be used to test the underlying cosmological model. A
number of recent measurements of very large galaxy stellar masses at high
redshift ($z > 8$) motivate an investigation into whether any of these objects
are in tension with $Łambda$CDM. We use extreme value statistics to generate
confidence regions in the mass-redshift plane for the most extreme mass haloes
and galaxies. Tests against numerical models show no tension, neither in their
dark matter halo masses nor their galaxy stellar masses. However, we find
tentative $> 3\sigma$ tension with recent observational determinations of
galaxy masses at high redshift from both HST & JWST, despite using conservative
estimates for the stellar fraction ($f_\star 1$). Either these galaxies
are in tension with $Łambda$CDM, or there are unaccounted for uncertainties in
their stellar mass or redshift estimates.
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