Stress testing Λ CDM with high-redshift galaxy candidates

Abstract

Early data from the James Webb Space Telescope (JWST) have revealed a bevy of high-redshift galaxy candidates with unexpectedly high stellar masses. An immediate concern is the consistency of these candidates with galaxy formation in the standard ΛCDM cosmological model, wherein the stellar mass (M_⋆) of a galaxy is limited by the available baryonic reservoir of its host dark matter halo. The mass function of dark matter haloes therefore imposes an absolute upper limit on the number density n (>M_⋆, z) and stellar mass density ρ_⋆ (>M_⋆, z) of galaxies more massive than M_⋆ at any epoch z. Here I show that the most massive galaxy candidates in JWST observations at z ≈ 7-10 lie at the very edge of these limits, indicating an important unresolved issue with the properties of galaxies derived from the observations, how galaxies form at early times in ΛCDM or within this standard cosmology itself.

Keywords: Cosmology; Galaxies and clusters.

Fig. 1. Limits on the abundance of galaxies as a function of redshift.

Curves show the relationship between M_⋆ and z at fixed cumulative halo abundance (left) and fixed ρ_b (>M_halo), or equivalently fixed peak height ν (right). The most extreme L23 galaxy candidates are shown as blue stars, with uncertainties indicating 68% intervals (symmetric about the median) of the posterior probability distribution. The existence of a galaxy with M_⋆ at redshift z requires that such galaxies have a cumulative co-moving number density that is, at most, the number density shown in the left panel, as those galaxies must reside in host halo of mass M_halo = M_⋆/(f_bϵ). The cumulative co-moving number density corresponding to an observed M_⋆ will probably be (much) smaller than is indicated here, as the curves are placed on the plot by assuming the physically maximal ϵ = 1.0. For smaller values of ϵ, the curves in each panel move down relative to the points by a factor of ϵ (as indicated by the black downward-facing arrows). The right panel demonstrates that even for the most conservative assumption of ϵ = 1.0, the data points correspond to very rare peaks in the density field, implying a limited baryonic reservoir that is in tension with the measured stellar masses of the galaxies.

Fig. 2. Stellar mass density limits.

The co-moving stellar mass density contained within galaxies more massive than M_⋆ at z ≈ 9.1 (left) and z ≈ 7.5 (right) for three values of the assumed conversion efficiency ϵ of a halo’s cosmic allotment of baryons into stars. Only if all available baryons in all haloes with enough baryons to form the galaxies reported by L23 have indeed been converted into stars by that point—an unrealistic limit—is it possible to produce the stellar mass density in the highest M_⋆ bin at z ≈ 9 measured by L23 in a typical volume of a ΛCDM Universe with the Planck 2020 cosmology. Results are similar at z ≈ 7.5. For more realistic values of ϵ, the required baryon reservoir is substantially larger than the theoretical maximum in this cosmology. When considering 1 σ shot noise and sample variance errors added in quadrature (which comprise the uncertainties on the L23 data points in each panel), the measurements are consistent with the base ΛCDM model if ϵ > 0.57, which would still imply incredibly efficient star formation in the high-redshift Universe.