The Most Massive Galaxies and Black Holes Allowed by $\Lambda$CDM

TL;DR

This paper uses galaxy and black hole mass functions at high redshift to test the limits of $ ext{Lambda}$CDM cosmology, providing methods to evaluate the likelihood of observing massive objects that challenge standard models.

Contribution

It introduces a framework to calculate maximum galaxy and black hole masses at high redshift based on $ ext{Lambda}$CDM halo number densities and provides a public code for assessing the probability of observing such massive objects.

Findings

High-redshift galaxy stellar mass functions constrain massive halo densities.

Some high-redshift quasars have black hole to stellar mass ratios exceeding local universe averages.

A public code is provided to evaluate the likelihood of observing $ ext{Lambda}$CDM}-inconsistent halos.

Abstract

Given a galaxy's stellar mass, its host halo mass has a lower limit from the cosmic baryon fraction and known baryonic physics. At z>4, galaxy stellar mass functions place lower limits on halo number densities that approach expected $\Lambda$CDM halo mass functions. High-redshift galaxy stellar mass functions can thus place interesting limits on number densities of massive haloes, which are otherwise very difficult to measure. Although halo mass functions at z<8 are consistent with observed galaxy stellar masses if galaxy baryonic conversion efficiencies increase with redshift, JWST and WFIRST will more than double the redshift range over which useful constraints are available. We calculate maximum galaxy stellar masses as a function of redshift given expected halo number densities from $\Lambda$CDM. We apply similar arguments to black holes. If their virial mass estimates are accurate, number density constraints alone suggest that the quasars SDSS J1044-0125 and SDSS J010013.02+280225.8 likely have black hole mass -- stellar mass ratios higher than the median z=0 relation, confirming the expectation from Lauer bias. Finally, we present a public code to evaluate the probability of an apparently $\Lambda$CDM-inconsistent high-mass halo being detected given the combined effects of multiple surveys and observational errors.