[Paper Review] Probing the dark matter haloes of external galaxies with stellar streams

Madison Walder, Denis Erkal+2 more|arXiv (Cornell University)|Feb 20, 2024

Astronomy and Astrophysical Research4 citations

TL;DR

This paper demonstrates that stellar streams in external galaxies can constrain dark matter halo properties—such as radial profile, scale radius, and enclosed mass—even with limited data. Using mock streams under realistic observational conditions, it shows that multiple wraps around the host galaxy enable precise radial profile recovery without radial velocities, while a single radial velocity measurement significantly improves constraints across all halo parameters.

ABSTRACT

Stellar streams have proven to be powerful tools for measuring the Milky Way's gravitational potential and hence its dark matter halo. In the coming years, Vera Rubin, Euclid, ARRAKIHS, and NGRST will uncover a plethora of streams around external galaxies. Although great in number, observations of these distant streams will often be limited to only the on-sky position of the stream. In this work, we explore how well we will be able to measure the dark matter haloes of these galaxies by fitting simplified mock streams with a variety of intrinsic and orbital properties in a range of data availability scenarios. We find that streams with multiple wraps around their host galaxy can constrain the overall radial profile and scale radius of the potential without radial velocities. In many other cases, a single radial velocity measurement often provides a significant boost to constraining power for the radial profile, scale radius, and enclosed mass of the dark matter halo. Given the wealth of data expected soon, this suggests that we will be able to measure the dark matter haloes of a statistically significant sample of galaxies with stellar streams in the coming years.

Motivation & Objective

• To assess the feasibility of measuring dark matter halo properties in external galaxies using stellar streams under realistic data limitations.
• To investigate how different stream properties (e.g., eccentricity, inclination, apocenter, number of wraps) influence the constraining power on dark matter halo parameters.
• To evaluate the impact of radial velocity measurements on improving constraints for radial profile, scale radius, and enclosed mass in dark matter haloes.
• To determine which stream characteristics maximize information gain for different halo parameters under varying data availability.
• To lay the groundwork for applying stream-fitting techniques to upcoming surveys like LSST, NGRST, and ARRAKIHS, enabling a statistical sample of extragalactic halo measurements.

Proposed method

• Simulated a diverse set of mock stellar streams with varying intrinsic and orbital properties (eccentricity, inclination, apocenter, number of wraps) around host galaxies with NFW-like dark matter potentials.
• Generated stream tracks using orbital integration in realistic gravitational potentials, assuming different levels of observational uncertainty in sky position (2D) and with added radial velocity measurements (3D).
• Fitted the mock streams to the host potential using a Bayesian inference framework, sampling posterior distributions for halo parameters (γ, rs, M_NFW) under different data availability scenarios.
• Quantified constraining power via credible intervals and posterior constraints, comparing results across stream types and data completeness levels.
• Used a range of stream track uncertainties (1 kpc to 0.1 kpc) to reflect current and future survey capabilities, assessing performance under realistic observational limits.
• Evaluated the influence of stream geometry (e.g., number of wraps, inclination, eccentricity) on the precision and accuracy of halo parameter recovery.

Experimental results

Research questions

• RQ1 Can the radial profile of a dark matter halo be constrained using only the on-sky position of extragalactic stellar streams, without radial velocity data?
• RQ2 How does a single radial velocity measurement improve constraints on the dark matter halo's radial profile, scale radius, and enclosed mass?
• RQ3 Which stream properties—such as number of wraps, eccentricity, inclination, or apocenter—maximize information gain for different halo parameters?
• RQ4 To what extent do stream track uncertainties (from current vs. future surveys) affect the precision of halo parameter recovery?
• RQ5 Can streams with apocenters beyond the halo scale radius provide stronger constraints on the radial profile and enclosed mass?

Key findings

• With current-level stream track uncertainties (σ_track = 1 kpc), multiple wraps (L = 2) around the host galaxy enable precise recovery of the dark matter halo's radial profile without any radial velocity data.
• For streams with multiple wraps, the enclosed mass can be bounded, though not precisely measured, due to the precision of the radial profile constraint rather than direct mass measurement.
• With future photometric precision (σ_track = 0.1 kpc), the radial profile of the dark matter potential can be recovered for a wide range of observable stream scenarios using only sky position.
• A single radial velocity measurement significantly enhances constraining power across all halo parameters, reducing degeneracies and improving accuracy for radial profile, scale radius, and enclosed mass.
• Longer streams (multiple wraps) provide the most information on the radial profile, followed by inclination, eccentricity, and apocenter distance.
• For measuring enclosed dark matter mass, streams with low eccentricity are most informative, followed by moderate eccentricity and then those with apocenters in the outer halo.