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Late-time growth weakly affects the significance of high-redshift massive galaxies

Qianran Xia, Dragan Huterer, Nhat-Minh Nguyen

TL;DR

This paper investigates whether late-time cosmic growth can meaningfully modify the predicted abundance of massive galaxies seen by JWST at high redshift. By parameterizing growth with a tunable growth index $\gamma$ in $f(a)\approx\Omega_m(a)^{\gamma}$ and computing extreme-value statistics for the most massive halo in a survey, calibrated to Planck cosmology and using the Warren et al. mass function with Eddington-bias corrections, the authors assess how sensitive high-redshift rareness is to late-time growth. Across multiple mass functions and JWST-like samples, they find only a modest change in the significance of the most massive high-redshift galaxies when $\gamma$ is varied within observational bounds (e.g., from $\sim2.7\sigma$ to $\sim2.2\sigma$ for a key case). This suggests that late-time growth history alone is unlikely to resolve apparent tensions between JWST observations and standard cosmology, though the analysis relies on smooth growth and Gaussian initial conditions; more exotic early-universe scenarios could, in principle, yield different implications.

Abstract

Recent observations by the James Webb Space Telescope have revealed massive galaxies at very high redshift ($z\simeq 7-15$). The question of whether the existence of such galaxies is expected in the corresponding JWST surveys has received a lot of attention, though the answer straddles areas of cosmology and complex astrophysical details of high-redshift galaxy formation. The growth rate of density fluctuations determines the amplitude of overdensities that collapse to form galaxies. Late-time modifications of growth, combined with measurements at both $z\sim 1$ from large-scale structure and $z\sim 1000$ from the cosmic microwave background, affect the predictions for the abundance of first galaxies in the universe. In this paper, we point out that the late-time growth rate of structure affects the statistical significance of high-redshift, high-mass objects very weakly. Consequently, if the existence and abundance of these objects are confirmed to be unexpected, the variations in the late-time growth history are unlikely to explain these anomalies.

Late-time growth weakly affects the significance of high-redshift massive galaxies

TL;DR

This paper investigates whether late-time cosmic growth can meaningfully modify the predicted abundance of massive galaxies seen by JWST at high redshift. By parameterizing growth with a tunable growth index in and computing extreme-value statistics for the most massive halo in a survey, calibrated to Planck cosmology and using the Warren et al. mass function with Eddington-bias corrections, the authors assess how sensitive high-redshift rareness is to late-time growth. Across multiple mass functions and JWST-like samples, they find only a modest change in the significance of the most massive high-redshift galaxies when is varied within observational bounds (e.g., from to for a key case). This suggests that late-time growth history alone is unlikely to resolve apparent tensions between JWST observations and standard cosmology, though the analysis relies on smooth growth and Gaussian initial conditions; more exotic early-universe scenarios could, in principle, yield different implications.

Abstract

Recent observations by the James Webb Space Telescope have revealed massive galaxies at very high redshift (). The question of whether the existence of such galaxies is expected in the corresponding JWST surveys has received a lot of attention, though the answer straddles areas of cosmology and complex astrophysical details of high-redshift galaxy formation. The growth rate of density fluctuations determines the amplitude of overdensities that collapse to form galaxies. Late-time modifications of growth, combined with measurements at both from large-scale structure and from the cosmic microwave background, affect the predictions for the abundance of first galaxies in the universe. In this paper, we point out that the late-time growth rate of structure affects the statistical significance of high-redshift, high-mass objects very weakly. Consequently, if the existence and abundance of these objects are confirmed to be unexpected, the variations in the late-time growth history are unlikely to explain these anomalies.

Paper Structure

This paper contains 4 sections, 11 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Methodology and Data
  3. Results
  4. Conclusions

Figures (1)

  • Figure 1: Logarithm of the most massive object’s mass ($y$-axis) expected in the survey, based on the specifications of the 2024Natur.635..311X sample, as a function of the growth index $\gamma$ ($x$-axis). The horizontal colored bands show the 68.3%, 95.4%, and 99.7% credible intervals for the mass of the highest-mass object in that sample, as a function of $\gamma$. The orange error bar (independent of the theory parameter on the x-axis, and shown multiple times for viewing convenience) shows the actual measurement of the highest-mass object in this sample at $z=5.58$ in this sample, corrected for Eddington bias. The vertical band shows the $\pm$5$\sigma$ range of values of $\gamma$ allowed by present data.