[Paper Review] Dark matter local density determination: recent observations and future prospects

P.F. de Salas, Axel Widmark|arXiv (Cornell University)|Dec 21, 2020

Dark Matter and Cosmic PhenomenaPhysics and Astronomy207 references163 citations

TL;DR

This paper reviews recent advances in estimating the local dark matter density (ρDM,⊙) using data from the Gaia satellite, comparing methods like vertical Jeans equations, moment methods, and global mass modelling. It finds a consensus range of 0.4–0.6 GeV/cm³ from local studies and 0.3–0.5 GeV/cm³ from global studies, while highlighting that systematic biases from violating the Ideal Galaxy assumptions—such as non-steady state dynamics, broken symmetries, and uncertain baryonic distributions—limit precision, urging more sophisticated models for future Gaia data releases.

ABSTRACT

This report summarises progress made in estimating the local density of dark matter ($ ho_{\mathrm{DM,\odot}}$), a quantity that is especially important for dark matter direct detection experiments. We outline and compare the most common methods to estimate $ ho_{\mathrm{DM,\odot}}$ and the results from recent studies, including those that have benefited from the observations of the ESA/Gaia satellite. The result of most local analyses coincide within a range of $ ho_{\mathrm{DM,\odot}} \simeq ext{0.4--0.6}\,\mathrm{GeV/cm^3} = ext{0.011--0.016}\,\mathrm{M_\odot / pc^3}$, while a slightly lower range of $ ho_{\mathrm{DM,\odot}} \simeq ext{0.3--0.5}\,\mathrm{GeV/cm^3} = ext{0.008--0.013}\,\mathrm{M_\odot / pc^3}$ is preferred by most global studies. In light of recent discoveries, we discuss the importance of going beyond the approximations of what we define as the Ideal Galaxy (a steady-state Galaxy with axisymmetric shape and a mirror symmetry across the mid-plane) in order to improve the precision of $ ho_{\mathrm{DM,\odot}}$ measurements. In particular, we review the growing evidence for local disequilibrium and broken symmetries in the present configuration of the Milky Way, as well as uncertainties associated with the Galactic distribution of baryons. Finally, we comment on new ideas that have been proposed to further constrain the value of $ ho_{\mathrm{DM,\odot}}$, most of which would benefit from Gaia's final data release.

Motivation & Objective

• . The paper aims to synthesize recent progress in estimating the local dark matter density (ρDM,⊙), a key parameter for direct detection experiments.
• It identifies systematic uncertainties in current ρDM,⊙ estimates arising from assumptions of the Ideal Galaxy model—steady state, axisymmetry, and mirror symmetry across the Galactic plane.
• The study evaluates how recent Gaia observations have improved constraints on ρDM,⊙, especially through improved kinematic data on local and distant stars.
• It calls for future models to relax the Ideal Galaxy assumptions, particularly regarding dynamical disequilibrium and non-axisymmetric structures, to reduce systematic errors.
• The objective includes assessing new methods—such as using stellar streams and radial velocity time derivatives—to break degeneracies in ρDM,⊙ and cross-section measurements.

Proposed method

• . The paper compares multiple methods for estimating ρDM,⊙, including modelling the phase-space distribution function and using moment-based approaches like the vertical Jeans equation.
• It evaluates the use of the circular velocity curve and its fitting to infer the gravitational potential and, by extension, ρDM,⊙.
• The study reviews global mass modelling techniques, such as Jeans anisotropic modelling and distribution function fitting of disc and halo stars.
• It examines the role of Gaia data—especially EDR3 and the upcoming final data release—in improving kinematic constraints on stellar motions and accelerations.
• It discusses novel approaches such as using stellar streams to infer disc mass and gravitational potential independently of steady-state assumptions.
• It considers future techniques involving time-derivatives of radial velocities and space-based accelerometers to measure Galactic acceleration and break degeneracies in ρDM,⊙ and cross-section estimates.

Experimental results

Research questions

• RQ1 . What is the current consensus range for the local dark matter density ρDM,⊙ based on recent observational data?
• RQ2 How do different estimation methods—local, very local, and global—compare in their ρDM,⊙results and what causes discrepancies between them?
• RQ3 To what extent do violations of the Ideal Galaxy assumptions (non-steady state, broken symmetry, non-axisymmetric structures) bias ρDM,⊙estimates?
• RQ4 How can future Gaia data and new observational techniques improve the precision of ρDM,⊙measurements?
• RQ5 Can the degeneracy between ρDM,⊙ and the spin-independent cross-section σ be broken using time-dependent signals like diurnal modulation or acceleration measurements?

Key findings

• . Recent local analyses of stellar kinematics yield a preferred ρDM,⊙range of 0.4–0.6 GeV/cm³, corresponding to 0.011–0.016 M⊙/pc³.
• Most global mass modelling approaches prefer a slightly lower range of ρDM,⊙= 0.3–0.5 GeV/cm³ (0.008–0.013 M⊙/pc³), indicating a tension between local and global estimates.
• . The paper identifies that systematic uncertainties from violating the Ideal Galaxy assumptions—especially dynamical disequilibrium and broken mirror symmetry—are major contributors to discrepancies in ρDM,⊙estimates.
• Gaia data have revealed time-varying structures and non-steady state dynamics, challenging the long-standing assumption of a steady-state Galaxy in current ρDM,⊙modelling.
• The paper highlights that current dynamical models are now limited by systematic errors rather than statistical uncertainties, especially due to unmodelled non-axisymmetric features and baryonic distribution uncertainties.
• Future improvements are expected from the final Gaia data release and novel methods such as using stellar streams or measuring radial velocity time derivatives to constrain Galactic acceleration and break degeneracies in ρDM,⊙and σ.