[Paper Review] Characterising ultra-high-redshift dark matter halo demographics and assembly histories with the GUREFT simulations

L. Y. Aaron Yung, Rachel S. Somerville+4 more|arXiv (Cornell University)|Sep 25, 2023

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This paper presents the GUREFT simulation suite—high-resolution, dark matter-only cosmological simulations spanning redshifts z ≈ 20 to 6 with 1024³ particles across four box sizes (5–90 Mpc/h)—to characterize ultra-high-redshift halo demographics and assembly histories. It delivers updated, accurate halo mass functions, concentration, spin, and accretion rate fitting functions that significantly deviate from prior analytic models at z ≳ 10.

ABSTRACT

Dark matter halo demographics and assembly histories are a manifestation of cosmological structure formation and have profound implications for the formation and evolution of galaxies. In particular, merger trees provide fundamental input for several modelling techniques, such as semi-analytic models (SAMs), sub-halo abundance matching (SHAM), and decorated halo occupation distribution models (HODs). Motivated by the new ultra-high-redshift (z > 10) frontier enabled by JWST, we present a new suite of Gadget at Ultrahigh Redshift with Extra-Fine Timesteps (GUREFT) dark matter-only cosmological simulations that are carefully designed to capture halo merger histories and structural properties in the ultra-z universe. The simulation suite consists of four 1024^3-particle simulations with box sizes of 5, 15, 35, and 90 Mpc h-1, each with 170 snapshots stored between 40 > z > 6. With the unprecedented number of available snapshots and strategically chosen dynamic range covered by these boxes, gureft uncovers the emerging dark matter halo populations and their assembly histories in the earliest epochs of cosmic history. In this work, we present the halo mass functions between z ~ 20 to 6 down to log(Mvir/Msun) ~ 5, and show that at high redshift, these robust halo mass functions can differ substantially from commonly used analytic approximations or older fitting functions in the literature. We also present key physical properties of the ultra-z halo population, such as concentration and spin, as well as their mass growth and merger rates, and again provide updated fitting functions.

Motivation & Objective

• To model the demographics and assembly histories of dark matter halos at ultra-high redshift (z ≳ 10), where observations from JWST are beginning to probe the early universe.
• To address the lack of high-resolution, high-snapshot-density simulations covering the early cosmic epoch (z ≈ 20 to 6), which are essential for accurate galaxy formation modeling.
• To provide robust, simulation-based fitting functions for halo properties such as mass function, concentration, spin, and accretion rates that are valid across a wide dynamic range in mass and redshift.
• To improve the accuracy of semi-analytic models, sub-halo abundance matching, and halo occupation distribution techniques by supplying high-fidelity merger tree and structural data for the first galaxies.
• To perform a convergence study assessing the impact of mass resolution on extreme halo properties like high concentration and spin, ensuring reliability of results.

Proposed method

• Running four high-resolution, dark matter-only simulations with 1024³ particles and box sizes of 5, 15, 35, and 90 Mpc/h, each with 170 snapshots between z = 40 and z = 6.
• Using Gadget-2 with extra-fine timesteps to ensure accurate tracking of halo formation and merger histories in the ultra-high-redshift regime.
• Applying standard halo finding algorithms (e.g., phase-space friends-of-friends or FoF) to identify halos and measure virial mass, concentration, spin, and circular velocity.
• Deriving halo mass functions (HMF) and fitting them with updated functional forms, including power-law and exponential corrections, to match the full redshift and mass range.
• Calculating merger rates, accretion rates (dM/dt and ḡM/M), and scaling relations (e.g., Mvir–Vmax, Mvir–c, Mvir–λ) across redshifts and masses.
• Performing a convergence study by comparing overlapping mass and redshift regions between adjacent simulation boxes to assess resolution effects on extreme halo properties.

Experimental results

Research questions

• RQ1 How do halo mass functions at z ≳ 10 compare to analytic approximations and older fitting functions in the literature?
• RQ2 What are the redshift trends of halo concentration, spin, and maximum circular velocity in the ultra-high-redshift universe?
• RQ3 How do halo accretion rates and merger histories evolve from z ≈ 6 to z ≈ 20, and how do they compare to extrapolations from lower-redshift simulations?
• RQ4 To what extent do existing fitting functions for Mvir–Vmax, Mvir–c, and Mvir–dM/dt break down at high redshift and low mass, and what new fits are needed?
• RQ5 How does mass resolution affect the population of extreme halos (e.g., high-concentration or high-spin objects) in the early universe?

Key findings

• The GUREFT halo mass function (HMF) spans from log(Mvir/M☉) ≈ 5 to 12 and z ≈ 6 to 20, with a new fitting function that accurately describes this wide dynamic range.
• Commonly used analytic HMF models, including those based on the Extended Press-Schechter formalism, can differ from GUREFT results by up to 1 dex at z ≳ 10, highlighting their inaccuracy in the ultra-high-redshift regime.
• The Mvir–Vmax relation increases by approximately 0.25 dex from z = 0 to z = 20, indicating that halos at higher redshifts have higher circular velocities for a given mass.
• Halo spin (λ) decreases with increasing redshift, with λB showing mild evolution and λP showing stronger evolution; fitting functions for these distributions are provided for the first time at z ≳ 10.
• The Mvir–cVir relation remains flat across z ≈ 6 to 20, with a mild normalization increase at earlier times, meaning halos at z ≈ 20 are more concentrated than at lower redshifts for the same mass.
• Specific accretion rates (Ṁ/M) increase monotonically with redshift from z ≈ 6 to 20, and while the slope of the Ṁ/M–z relation flattens slightly at higher redshift, the trend continues, with existing low-z fits failing to extrapolate accurately to this regime.