[Paper Review] Constraints on Decaying Dark Matter

Annika H. G. Peter, Christopher E. Moody+2 more|arXiv (Cornell University)|Nov 22, 2010

Dark Matter and Cosmic Phenomena6 references3 citations

TL;DR

This study uses N-body simulations to constrain decaying dark matter in a two-component WIMP model where a heavier dark matter particle decays into a lighter one and a massless particle, with recoil speeds $v_k$ and half-life $ au$. It finds that $ au < 40$ Gyr is ruled out for $v_k ightarrow 20$ km s$^{-1}$ or higher, primarily from constraints on dwarf galaxy and cluster halo structures.

ABSTRACT

We explore a dark-matter model in which there are two dark-matter species nearly degenerate in mass, with epsilon = Delta M/M &lt;&lt; 1. The heavier particle undergoes two-body decay with a half-life tau, to the lighter dark-matter particle and a noninteracting massless particle. Unlike previous work on decaying dark matter, we explore the regime tau &gt; 100 Myr and non-relativistic kick speeds vk / c = epsilon. Using a set of N-body simulations of isolated dark-matter halos, we show how halos change as a function of tau and vk. We find that tau &lt; 40 Gyr is ruled out for vk &gt; 20 km s^{-1} (epsilon &gt; 10^{-4}) when we compare the simulations to observations of dwarf-galaxy- to cluster-mass dark matter halos. We highlight which set of observations should provide better future constraints for decays and other types of dark-matter physics.

Motivation & Objective

• To investigate the impact of dark matter decay on the structural evolution of dark matter halos, particularly in the regime of long half-lives ($\tau > 100$ Myr) and non-relativistic recoil speeds.
• To determine observational constraints on the decay parameter space ($\tau$, $v_k$) using observed properties of dwarf galaxies, galaxy clusters, and Milky Way satellite systems.
• To assess how halo mass loss and kinetic energy differences from decay affect density profiles and subhalo survival, especially in the context of cold dark matter halos.
• To identify which astrophysical observations—particularly cluster mass functions and satellite central densities—offer the tightest future constraints on decaying dark matter models.

Proposed method

• Simulated isolated dark matter halos using the N-body code GADGET-2, modified to include particle decay with specified half-life $\tau$ and recoil speed $v_k$.
• Used Navarro-Frenk-White (NFW) profiles for initial halo density structure, with concentration $c$ as a free parameter, and initialized velocity distributions assuming isotropy for dynamical equilibrium.
• Performed 100 simulations with $v_k = 10$ to $500$ km s$^{-1}$, $\tau = 0.1$ to $100$ Gyr, and $c = 5$ to $30$, focusing on $M_{\text{vir}} = 10^{12} M_\odot$ halos.
• Mapped simulated halo evolution to observable properties via comparison with observed cluster mass functions, mass-concentration relations, and Milky Way satellite central densities.
• Constructed synthetic Milky Way satellite populations using analytic merger trees and decay-modified halo properties, applying star-formation prescriptions tied to reionization redshift $z_{\text{re}} = 7$.
• Evaluated constraints by requiring that simulated populations reproduce the observed number and central density of Milky Way satellites, with sensitivity to dynamical friction models.

Experimental results

Research questions

• RQ1 How do long-lived decaying dark matter particles ($\tau > 100$ Myr) affect the structural evolution of cold dark matter halos?
• RQ2 What constraints can be placed on the decay half-life $\tau$ and recoil speed $v_k$ using observed cluster mass functions and mass-concentration relations?
• RQ3 To what extent do the central densities of Milky Way satellite galaxies constrain the decay parameters of a nearly degenerate two-component dark matter model?
• RQ4 How sensitive are the constraints to uncertainties in subhalo evolution, dynamical friction, and star-formation history after accretion?
• RQ5 Which future observational probes—especially from deep sky surveys—hold the most promise for improving constraints on decaying dark matter?

Key findings

• The model with $\tau < 40$ Gyr is ruled out for $v_k \gtrsim 20$ km s$^{-1}$, based on comparisons with observed dwarf galaxy and cluster halo properties.
• Constraints from the galaxy cluster mass function and mass-concentration relation are strongest for $v_k \gtrsim 200$ km s$^{-1}$, while Milky Way satellite central densities provide tighter limits for $v_k \lesssim 200$ km s$^{-1}$.
• The pale red region to the right of the dashed curve and under the solid curve in Fig. 2 is excluded by cluster mass function and mass-concentration data, indicating strong sensitivity to decay-induced halo mass loss and structural changes.
• The red region in Fig. 2 represents additional exclusion from Milky Way satellites, especially when dynamical friction is modeled as in $\Lambda$CDM, showing that subhalo survival is sensitive to decay physics.
• Constraints are relatively insensitive to the star-formation prescription for satellites but highly sensitive to the post-accretion evolution of subhalos and their central density profiles.
• Future deep and wide sky surveys are expected to significantly improve constraints on decaying dark matter, particularly by measuring the distribution of central densities in Local Group satellites.