[Paper Review] Milky Way Satellite Census. IV. Constraints on Decaying Dark Matter from Observations of Milky Way Satellite Galaxies

S. Mau, Ethan O. Nadler+68 more|arXiv (Cornell University)|Jan 1, 2022

Dark Matter and Cosmic Phenomena117 references39 citations

TL;DR

This paper constrains decaying dark matter (DDM) using the Milky Way satellite galaxy population, modeling how DDM decays impart a 'kick velocity' to daughter particles, depleting low-mass subhalos. Using high-resolution simulations and a forward modeling approach that accounts for observational biases, it finds DDM models with lifetimes shorter than 18 Gyr (for 20 km s⁻¹ kick) or 29 Gyr (for 40 km s⁻¹ kick) are excluded at 95% confidence, providing some of the tightest constraints on DDM from small-scale structure.

ABSTRACT

We use a recent census of the Milky Way (MW) satellite galaxy population to constrain the lifetime of particle dark matter (DM). We consider two-body decaying dark matter (DDM) in which a heavy DM particle decays with lifetime τ comparable to the age of the universe to a lighter DM particle (with mass splitting ϵ) and to a dark radiation species. These decays impart a characteristic "kick velocity,"V kick = ϵ c, on the DM daughter particles, significantly depleting the DM content of low-mass subhalos and making them more susceptible to tidal disruption. We fit the suppression of the present-day DDM subhalo mass function (SHMF) as a function of τ and V kick using a suite of high-resolution zoom-in simulations of MW-mass halos, and we validate this model on new DDM simulations of systems specifically chosen to resemble the MW. We implement our DDM SHMF predictions in a forward model that incorporates inhomogeneities in the spatial distribution and detectability of MW satellites and uncertainties in the mapping between galaxies and DM halos, the properties of the MW system, and the disruption of subhalos by the MW disk using an empirical model for the galaxy-halo connection. By comparing to the observed MW satellite population, we conservatively exclude DDM models with τ < 18 Gyr (29 Gyr) for V kick = 20 kms-1 (40 kms-1) at 95% confidence. These constraints are among the most stringent and robust small-scale structure limits on the DM particle lifetime and strongly disfavor DDM models that have been proposed to alleviate the Hubble and S 8 tensions.

Motivation & Objective

• To test whether decaying dark matter (DDM) can explain cosmological tensions like the Hubble and S8 tensions.
• To assess the impact of DDM decay on the subhalo mass function (SHMF) of Milky Way satellites.
• To constrain the lifetime and kick velocity of DDM particles using observed satellite abundances and spatial distributions.
• To develop a robust forward modeling framework that accounts for observational incompleteness, galaxy-halo connection uncertainties, and disk-induced tidal disruption.

Proposed method

• Simulate DDM decay in high-resolution zoom-in simulations of Milky Way-mass halos to model the suppression of the subhalo mass function (SHMF) as a function of decay lifetime τ and kick velocity Vkick.
• Validate the simulated SHMF suppression against new DDM simulations of systems resembling the Milky Way.
• Implement the DDM SHMF predictions in a forward modeling framework that includes inhomogeneous satellite detectability, galaxy-halo connection uncertainties, and empirical disk disruption models.
• Constrain DDM parameters by comparing the modeled satellite population to the observed MW satellite system using Bayesian inference.
• Use Markov Chain Monte Carlo (MCMC) sampling via emcee to explore parameter space and compute 95% confidence limits.
• Incorporate observational data from the Dark Energy Survey (DES), including spatial distribution, magnitude limits, and completeness corrections.

Experimental results

Research questions

• RQ1 What is the maximum allowed lifetime of a decaying dark matter particle that is consistent with the observed Milky Way satellite population?
• RQ2 How does the kick velocity imparted to daughter dark matter particles affect the survival of low-mass subhalos?
• RQ3 To what extent do observational biases and uncertainties in the galaxy-halo connection affect constraints on DDM?
• RQ4 Can DDM models proposed to resolve the Hubble and S8 tensions be ruled out by small-scale structure observations?
• RQ5 How do the constraints from Milky Way satellites compare to those from large-scale structure and expansion history probes?

Key findings

• The study excludes decaying dark matter models with a lifetime τ < 18 Gyr at 95% confidence for a kick velocity Vkick = 20 km s⁻¹.
• For a higher kick velocity of 40 km s⁻¹, the constraint tightens to τ < 29 Gyr at 95% confidence.
• These constraints are among the most stringent to date on the lifetime of decaying dark matter particles from small-scale structure.
• The results strongly disfavor DDM models proposed to alleviate the Hubble and S8 tensions, as they typically require lifetimes below the excluded range.
• The forward modeling approach successfully accounts for observational incompleteness, galaxy-halo connection uncertainties, and tidal disruption, enhancing the robustness of the constraints.
• The findings demonstrate that Milky Way satellite abundances provide a powerful and complementary probe of DDM physics to large-scale structure and expansion history.