[Paper Review] Are the ultra-high-redshift galaxies at z > 10 surprising in the context of standard galaxy formation models?

L. Y. Aaron Yung, Rachel S. Somerville+3 more|arXiv (Cornell University)|Apr 10, 2023

Galaxies: Formation, Evolution, Phenomena175 references16 citations

TL;DR

The paper uses Santa Cruz semi-analytic models with the GUREFT N-body merger trees to predict ultra-high-redshift galaxy populations and compares rest-frame UV LFs and stellar mass functions to JWST observations, finding significant tensions that can be reconciled with modest UV luminosity boosts or stochasticity.

ABSTRACT

A substantial number of ultra-high redshift (8 < z < 17) galaxy candidates have been detected with JWST, posing the question: are these observational results surprising in the context of current galaxy formation models? We address this question using the well-established Santa Cruz semi-analytic models, implemented within merger trees from the new suite of cosmological N-body simulations GUREFT, which were carefully designed for ultra-high redshift studies. Using our fiducial models calibrated at z=0, we present predictions for stellar mass functions, rest-frame UV luminosity functions, and various scaling relations. We find that our (dust-free) models predict galaxy number densities at z~11 (z~13) that are an order of magnitude (a factor of ~30) lower than the observational estimates. We estimate the uncertainty in the observed number densities due to cosmic variance, and find that it leads to a fractional error of ~20-30% at z=11 (~30-80% at z=14) for a 100 sq arcmin field. We explore which processes in our models are most likely to be rate-limiting for the formation of luminous galaxies at these early epochs, considering the halo formation rate, gas cooling, star formation, and stellar feedback, and conclude that it is mainly efficient stellar-driven winds. We find that a modest boost of a factor of ~4 to the UV luminosities, which could arise from a top-heavy stellar initial mass function, would bring our current models into agreement with the observations. Adding a stochastic component to the UV luminosity can also reconcile our results with the observations.

Motivation & Objective

• Assess whether current ultra-high-redshift galaxy candidates are compatible with standard galaxy formation models.
• Predict rest-frame UV luminosity functions and stellar mass functions at z~8–17 using updated N-body merger trees.
• Quantify observational uncertainties (cosmic variance, photometric/redshift errors) and model uncertainties.
• Explore physics bottlenecks in early galaxy formation and the impact of UV luminosity boosts or stochasticity on reconciliation with observations.

Proposed method

• Employ Santa Cruz semi-analytic model (SAM) of galaxy formation integrated with GUREFT N-body merger trees to z~6–20.
• Use dust-free, rest-frame UV magnitudes derived from convolving SFH and chemical histories with BPASS-based SPS to predict MUV (no nebular emission).
• Compare predicted UV luminosity functions and stellar mass functions to JWST-based measurements from CEERS, NGDEEP, COSMOS-Web, and other surveys.
• Quantify cosmic variance using Analytic calculators and results from large-volume simulations (Bluetides) to estimate field-to-field uncertainties.
• Test the impact of a moderate UV luminosity boost (factor ~4) and of stochastic UV luminosity with Gaussian scatter on reconciling predictions with observations.

Experimental results

Research questions

• RQ1 How do the Santa Cruz SAM predictions for UV luminosity functions and stellar mass functions at z>10 compare with JWST-derived observations?
• RQ2 What are the dominant physical bottlenecks limiting the formation of luminous galaxies at ultra-high redshifts in standard models?
• RQ3 Can modest modifications to UV luminosities (e.g., IMF variations) or stochasticity in star formation bring models into agreement with observations?
• RQ4 What is the role of cosmic variance and observational uncertainties in interpreting tension between theory and data?

Key findings

• Predicted galaxy number densities at z~11 are an order of magnitude lower than observational estimates; at z~13 the discrepancy is a factor of ~30.
• Cosmic variance for a 100 arcmin^2 field yields fractional errors of ~20-30% at z=11 and ~30–80% at z=14, underscoring large field-to-field uncertainties.
• Efficient stellar-driven winds are identified as the main rate-limiting process for forming luminous galaxies at ultra-high redshift within the fiducial model.
• A modest UV-luminosity boost by a factor of ~4 (e.g., from a top-heavy IMF) would bring predictions into agreement with observations; adding a stochastic UV component can also reconcile results.
• Bursty/star-formation stochasticity with Gaussian scatter sigma ~1.5–2 magnitudes in UV luminosity is required to match z=11–13 observations.
• Dust is neglected in the fiducial models to maximize optimistic predictions, noting that dust attenuation could ease bright-end tensions at z~8–9.