[Paper Review] Accelerated Structure Formation: the Early Emergence of Massive Galaxies and Clusters of Galaxies

Stacy McGaugh, James M. Schombert+2 more|arXiv (Cornell University)|Jun 25, 2024

Astronomy and Astrophysical Research4 citations

TL;DR

This paper argues that massive galaxies and galaxy clusters formed earlier than predicted by the standard ΛCDM model, presenting observational evidence from JWST and kinematic data supporting accelerated structure formation. It shows that these early massive systems align with long-standing predictions of MOND, particularly the rapid growth of stellar mass and large-scale structure by z ≳ 10.

ABSTRACT

Galaxies in the early universe appear to have grown too big too fast, assembling into massive, monolithic objects more rapidly than anticipated in the hierarchical $Λ$CDM structure formation paradigm. The available photometric data are consistent with there being a population of massive galaxies that form early ($z \gtrsim 10$) and quench rapidly over a short ($\lesssim 1$ Gyr) timescale, consistent with the traditional picture for the evolution of giant elliptical galaxies. Similarly, kinematic observations as a function of redshift show that massive spirals and their scaling relations were in place at early times. Explaining the early emergence of massive galaxies requires either an extremely efficient conversion of baryons into stars at $z>10$ or a more rapid assembly of baryons than anticipated in $Λ$CDM. The latter possibility was explicitly predicted in advance by MOND. We discuss some further predictions of MOND, such as the early emergence of clusters of galaxies and early reionization.

Motivation & Objective

• To investigate the discrepancy between observed early formation of massive galaxies and the predictions of the hierarchical ΛCDM structure formation model.
• To evaluate whether the observed early emergence of massive galaxies and clusters can be explained by alternative gravity theories like MOND.
• To test the consistency of high-redshift galaxy kinematics with the baryonic Tully-Fisher relation and dark matter fraction–surface brightness relation at z ≳ 6.
• To assess whether MOND's long-standing predictions about early cosmic structure formation are now empirically supported by JWST data.
• To examine the implications of accelerated structure formation for cosmological models, particularly the tension between ΛCDM and observed early-time galaxy evolution.

Proposed method

• Analyzes high-redshift galaxy data from JWST and pre-JWST observations to assess stellar mass assembly and morphological maturity at z ≳ 10.
• Applies an exponential star formation history model (ψ(t) = ψ₀e⁻ᵘ) with short e-folding timescales (τ ≲ 1 Gyr) to simulate rapid mass growth in early galaxies.
• Uses the time-redshift relation in vanilla ΛCDM cosmology to calculate luminosity evolution and compare with observed galaxy magnitudes.
• Employs kinematic data from spiral galaxies at high redshift to test the baryonic Tully-Fisher relation and dark matter fraction–surface brightness relation.
• Compares observational constraints on galaxy formation timelines with theoretical predictions from MOND, particularly those of Sanders (1998) and McGaugh (2015).
• Evaluates the consistency of MOND with large-scale structure formation, including the cosmic web and cluster emergence by z ≈ 2.

Figure 1: Merger trees for two model galaxies from the TNG50-1 simulation (Pillepich et al., 2019 ; Nelson et al., 2019 ) selected to have $M_{*}\approx 9\times 10^{10}\;\mathrm{M}_{\sun}$ at $z=0$ ; i.e., the stellar mass of a local $L^{*}$ early type galaxy (Driver et al., 2022 ) . Mass assembles

Experimental results

Research questions

• RQ1 Do high-redshift galaxies (z ≳ 10) exhibit signs of rapid, monolithic-like mass assembly inconsistent with hierarchical ΛCDM formation?
• RQ2 To what extent do the kinematics of massive spiral galaxies at z ≳ 6 support the existence of mature scaling relations like the baryonic Tully-Fisher relation at early cosmic times?
• RQ3 Can the early emergence of massive galaxies and clusters be explained by MOND’s non-linear dynamics, which accelerate structure formation compared to ΛCDM?
• RQ4 How do observational constraints from JWST and other telescopes challenge the standard timeline of galaxy and cluster formation in ΛCDM?
• RQ5 What are the implications of MOND’s successful a priori predictions for early structure formation in light of current high-redshift data?

Key findings

• Galaxies at z ≳ 10 show evidence of rapid mass assembly consistent with an exponential star formation history and e-folding timescale of τ ≲ 1 Gyr, reaching M* ≈ 10¹¹ M☉ by z = 0.
• Kinematic observations of high-redshift spirals (z ≳ 6) reveal circular velocities >250 km s⁻¹ and adherence to the baryonic Tully-Fisher relation, indicating mature, massive disks formed early.
• The dark matter fraction–surface brightness relation is already in place at z ≳ 6, implying that dynamical mass growth is not merely a luminosity evolution effect.
• The early emergence of massive galaxies and clusters is consistent with MOND predictions made over two decades ago, including massive galaxies at z ≳ 10 and clusters by z ≈ 2.
• MOND predicts accelerated structure formation due to non-linear dynamics, leading to earlier collapse and growth than in linear ΛCDM growth models.
• Despite the success of ΛCDM in fitting cosmological observables, it fails to explain the early formation of massive systems, while MOND provides a coherent, predictive alternative.

Figure 2: The redshift dependence of the Spitzer [4.5] apparent magnitude $m^{*}$ of Schechter function fits to populations of galaxies in clusters and candidate protoclusters. Data from Mancone et al. ( 2010 , black circles) , Wylezalek et al. ( 2014 , dark blue circles) , and Franck & McGaugh ( 20