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Matching JWST UV Luminosity Functions with Refined $Λ$CDM Halo Models

Saeed Fakhry, Maryam Shiravand, Antonino Del Popolo

TL;DR

This study shows that the apparent JWST overabundance of luminous high-redshift galaxies can be reconciled within the standard $\Lambda$CDM framework by adopting physically informed halo mass functions. By incorporating angular momentum, dynamical friction, and redshift-dependent collapse barriers into DP1 and DP2, the authors predict a richer high-mass halo population at $z \gtrsim 7$ than the conventional ST formalism, especially in the high-mass tail relevant for the bright UV luminosity function. A semi-empirical mapping from halo mass to star formation rate and UV luminosity demonstrates that DP2 achieves good agreement with JWST UVLF observations across $z=7$–$14$ with moderate star formation efficiencies ($f_\star \sim 0.1$–$0.25$), while ST requires implausibly high efficiencies. The results argue that the JWST results do not require new physics beyond $\Lambda$CDM but rather a more accurate treatment of small-scale dissipative dynamics during collapse, underscoring the need for physically motivated, mass- and redshift-dependent collapse criteria in early structure formation models.

Abstract

The James Webb Space Telescope (JWST) has unveiled a population of unexpectedly massive and luminous galaxies at redshifts $z \gtrsim 7$, posing a significant challenge to the standard $Λ$CDM cosmological paradigm. In this work, we address the tension between early JWST observations of luminous high-redshift galaxies and predictions of the standard $Λ$CDM model by revisiting the physics of dark matter halo formation. Employing refined halo mass functions derived by Del Popolo \textit{et al.} (DP1 and DP2) that incorporate angular momentum, dynamical friction, and redshift-dependent collapse barriers, we demonstrate a significant enhancement in the abundance of massive halos at $z \gtrsim 7$ compared to the conventional Sheth-Tormen (ST) formalism. Using a semi-empirical framework linking halo mass to UV luminosity, we show that the DP2 model reproduces the observed UV luminosity functions from $z = 7$ to $14$ with moderate star formation efficiencies, whereas the ST model requires implausibly high efficiencies. Our results suggest that the JWST overabundance problem stems not from new physics beyond $Λ$CDM, but from oversimplified treatments of gravitational collapse, highlighting the critical role of small-scale dissipative dynamics in early structure formation.

Matching JWST UV Luminosity Functions with Refined $Λ$CDM Halo Models

TL;DR

This study shows that the apparent JWST overabundance of luminous high-redshift galaxies can be reconciled within the standard CDM framework by adopting physically informed halo mass functions. By incorporating angular momentum, dynamical friction, and redshift-dependent collapse barriers into DP1 and DP2, the authors predict a richer high-mass halo population at than the conventional ST formalism, especially in the high-mass tail relevant for the bright UV luminosity function. A semi-empirical mapping from halo mass to star formation rate and UV luminosity demonstrates that DP2 achieves good agreement with JWST UVLF observations across – with moderate star formation efficiencies (–), while ST requires implausibly high efficiencies. The results argue that the JWST results do not require new physics beyond CDM but rather a more accurate treatment of small-scale dissipative dynamics during collapse, underscoring the need for physically motivated, mass- and redshift-dependent collapse criteria in early structure formation models.

Abstract

The James Webb Space Telescope (JWST) has unveiled a population of unexpectedly massive and luminous galaxies at redshifts , posing a significant challenge to the standard CDM cosmological paradigm. In this work, we address the tension between early JWST observations of luminous high-redshift galaxies and predictions of the standard CDM model by revisiting the physics of dark matter halo formation. Employing refined halo mass functions derived by Del Popolo \textit{et al.} (DP1 and DP2) that incorporate angular momentum, dynamical friction, and redshift-dependent collapse barriers, we demonstrate a significant enhancement in the abundance of massive halos at compared to the conventional Sheth-Tormen (ST) formalism. Using a semi-empirical framework linking halo mass to UV luminosity, we show that the DP2 model reproduces the observed UV luminosity functions from to with moderate star formation efficiencies, whereas the ST model requires implausibly high efficiencies. Our results suggest that the JWST overabundance problem stems not from new physics beyond CDM, but from oversimplified treatments of gravitational collapse, highlighting the critical role of small-scale dissipative dynamics in early structure formation.

Paper Structure

This paper contains 5 sections, 23 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Matter Power Spectrum in the $\Lambda$CDM Model
  3. Halo Mass Functions
  4. UV Luminosity Function and JWST Observations
  5. Conclusions

Figures (3)

  • Figure 1: ST, DP1 and DP2 halo mass functions at redshifts $z=7\hbox{-}14$.
  • Figure 2: The UVLF as a function of UV magnitude for different star formation efficiencies, evaluated using the ST (left panel), DP1 (middle panel), and DP2 (right panel) mass functions at redshifts $z=7\hbox{-}10$. The black lines represent the UVLFs predicted by the different models incorporating the redshift- and halo-mass-dependent star formation efficiency. The dataset with error bars represent recent observational constraints on the UV luminosity function at the same redshift from multiple studies: filled squares denote measurements from 2015ApJ...803...34B2021AJ....162...47B2022ApJ...927...81B; filled stars correspond to 2023ApJ...946L..13F; upward filled triangles represent 2023MNRAS.520.4554D; and dash indicate 2021ApJ...922...29S.
  • Figure 3: Similar to Fig. \ref{['Fig2']}, but for $z=11\hbox{-}14$. The dataset, displayed with error bars, presents recent observational constraints on the UV luminosity function at a common redshift, synthesized from a range of independent studies: filled squares correspond to the measurements reported in 2015ApJ...803...34B2021AJ....162...47B2022ApJ...927...81B; upward filled triangles denote data from 2023MNRAS.520.4554D; downward filled triangles represent results from 2023ApJS..265....5H; rightward filled triangles indicate findings from 2024ApJ...965..169A; filled stars correspond to 2023ApJ...946L..13F; filled pentagons signify the measurements presented in 2024MNRAS.527.5004M; and filled diamonds denote the constraints derived by 2024ApJ...970...31R.