A Comparison of Galacticus and COZMIC WDM Subhalo Populations

TL;DR

This study compares warm dark matter subhalo populations predicted by the Galacticus semi-analytic model and COZMIC N-body simulations across $m_\text{WDM}$ values from $3$ to $10$ keV, focusing on the subhalo mass function, spatial distribution, and internal structure metrics $V_\text{max}$ and $R_\text{max}$. Both approaches show suppression of low-mass subhalos with decreasing $m_\text{WDM}$ and predict lower $V_\text{max}$ and larger $R_\text{max}$ for WDM halos at fixed mass, consistent with reduced concentration. Galacticus reproduces the COZMIC trends within statistical uncertainties and offers computational efficiency, though some discrepancies appear for extreme $m_\text{WDM}$ that may arise from halo finding, resolution, or modeling differences. The results validate the use of SAMs to explore WDM implications for structure formation and related astrophysical phenomena while highlighting the need for more WDM N-body benchmarks to strengthen statistics.

Abstract

We present a comparative analysis of warm dark matter (WDM) subhalo populations generated by the semi-analytic model {\sc Galacticus} and the COZMIC suite of dark matter-only $N$-body simulations. Using a range of thermal relic WDM particle masses (3--10 keV), we examine key summary statistics -- including the subhalo mass function, spatial distribution, maximum circular velocity $V_\text{max}$, and its corresponding radius $ R_\text{max} $ -- to evaluate the consistency between these two modeling frameworks. Both models predict a suppression of low-mass subhalos correlated with decreasing WDM particle mass, and that WDM subhalos tend to have lower $V_\text{max} $ and larger $ R_\text{max} $ values than their CDM counterparts at fixed mass. While {\sc Galacticus} provides more statistically precise results due to a larger sample size, the COZMIC simulations display similar qualitative trends. We discuss how differences in halo finder algorithms, simulation resolution, and modeling assumptions affect subhalo statistics. Our findings demonstrate that {\sc Galacticus} can reliably reproduce WDM subhalo distributions seen in $N$-body simulations, offering a computationally efficient tool for exploring the implications of WDM across astrophysical phenomena.

Figures (9)

• Figure 1: Left panel: Subhalo bound mass functions for CDM (black) and WDM3, 4, 5, 6, 6.5, 10 models (red through purple). Solid curves denote the average mass function over the $N$-body simulation data from COZMIC for WDM models and Symphony/Milky Way-est for CDM. Dashed curves show the average mass function over all Galacticus merger trees. Shaded regions around the N-body results represent uncertainty on the average mass function arising from the halo-to-halo scatter (estimated from the Galacticus data) and the finite number of N-body realizations. Right panel: The ratio of Symphony/Milky Way-est to Galacticus subhalo bound mass functions (black), and ratios of COZMIC to Galacticus subhalo bound mass functions (colored lines), as a function of the subhalo-to-host mass ratio. The dashed horizontal line shows the $y = 1$ line. The black dotted line indicates ratio of subhalo bound mass functions between the Symphony and Caterpillar $N$-body simulation suites.
• Figure 2: Normalized radial distributions for $N$-body and Galacticus subhalo populations. Results are shown for CDM (black) and WDM3, 4, 5, 6, 6.5, 10 models (red through purple). Solid curves denote the average radial distribution over the $N$-body simulation data from COZMIC for WDM models and Symphony/Milky Way-est for CDM. Dashed curves show the average radial distribution over all Galacticus merger trees. Shaded regions correspond to $1\sigma$ uncertainties on the mean of the N-body data rising from halo-to-halo variance.
• Figure 3: Inverted cumulative distribution functions of $V_\text{max}$ for $N$-body (solid) and Galacticus (dashed) subhalo populations. Results are shown for both CDM (black) and WDM3 through WDM10 (red through purple) models. Shaded regions correspond to $1\sigma$ uncertainties on the mean of the N-body data rising from halo-to-halo variance. The vertical grey line indicates the mean $V_\text{max}$ of halos at the high-resolution simulation threshold of $m = 1.0 \times 10^8 \textrm{M}_\odot$ .
• Figure 4: Inverted cumulative distribution functions of $R_\text{max}$ for COZMIC (left; solid curves) and Galacticus (right; dashed curves) models. Results are shown for CDM (black) and WDM3, 4, 5, 6, 6.5, 10 models (red through purple). Shaded regions correspond to $1\sigma$ uncertainties on the mean of the N-body data rising from halo-to-halo variance. The grey dashed line in each panel shows the mean $R_\text{max}$ of halos at the $m = 1.0 \times 10^8 \mathrm{M}_\odot$ high-resolution simulation threshold.
• Figure 5: Averaged subhalo $V_\text{max}$ (top) and averaged subhalo $R_\text{max}$ (bottom) versus halo mass for Galacticus WDM3 through WDM10 + CDM models. The bottom panel shows two additional curves corresponding to WDM 1 keV (brown) and WDM 2 keV (cyan) models.