[Paper Review] Movie of a Simulated Stellar Stream from a Globular Cluster on a Circular Orbit in the Milky Way

Newlin C. Weatherford|arXiv (Cornell University)|Oct 2, 2023

Stellar, planetary, and galactic studiesPhysics and Astronomy3 citations

TL;DR

This study uses Monte Carlo simulations to model tidal tail and stellar stream formation from a globular cluster on a circular orbit in the Milky Way, revealing that returning tidal tails can form after the stream circumnavigates the Galaxy, increasing stream velocity dispersion by several km s⁻¹. The work identifies sharp plateaus in the escape timescale distribution linked to chaotic scattering, and shows that cluster evolution reduces median escape times from ~10 Gyr to <100 Myr due to mass loss and relaxation.

ABSTRACT

This is a movie of stellar streams forming from stars ejected from a globular cluster (GC) simulation run with the Cluste Monte Carlo code (CMC). The movie is also featured on YouTube (youtu.be/zJKCvAf6U3E}{youtu.be/zJKCvAf6U3E) corresponds to Figure 13 of the following submission to The Astrophysical Journal (this description will be updated when published).Weatherford, N. C., Rasio, F. A., Chatterjee, S., Fragione, G., Kıroğlu, F., and Kremer, K., arXiv e-prints, arXiv:2310.01485, doi: 10.48550/arXiv.2310.01485The movie is uploaded here (rather than directly embedded in the online journal article) due to its size. Each point point represents the location of an individual stellar body that has acquired enough energy to escape from the GC (but has not necessarily done so yet). Point color indicates the time elapsed since the body first acquired suffient energy to eventually escape. The lower left panel shows the projected positions in the static center-of-mass coordinates of the GC orbiting the center of the Milky Way at a Galactocentric distance Rgc = 8 kpc., a face-on view of the GC orbit. The other panels show the three orthographic projections along each of the cardinal directions in the rotating clustercentric coordinates, in which the center of the GC is located at the origin while the Galactic center is located on the x-axis at xRgc = –8 kpc (defined this way because the coordinates XYZ and xyz are dimensionless in units of Rgc). The views in the upper row are edge-on to the GC's orbit, looking along (upper left) and perpendicular to (upper right) the ray connecting the cluster center to the Galactic center, while the view in the lower right panel is face-on to the cluster orbit. In the three orthographic panels, the blue circles have radii r =[1,2,3,4,5] times the GC's tidal radius. In the lower left panel, the red circles have radii R/kpc =[2,4,6,8,10,12], and the blue circle radius r = 5 times the GC's tidal radius, with guiding center at R = Rgc.

Motivation & Objective

• To model tidal tail and stellar stream formation from globular clusters using Monte Carlo methods in a smooth Galactic potential.
• To investigate the formation and detectability of returning tidal tails after the stream completes a full orbit around the Galaxy.
• To analyze the escape timescale distribution of potential escapers (PES) and its dependence on Jacobi energy and cluster evolution.
• To improve the physical accuracy of escape criteria in Monte Carlo cluster codes by accounting for delayed escape and ongoing relaxation.
• To enable future large-parameter-space studies of stellar streams and direct comparisons with Gaia observations.

Proposed method

• Applies the Monte Carlo cluster modeling method (CMC) to simulate a globular cluster on a circular orbit in a smooth, spherical Galactic potential.
• Treats energetically unbound stars (potential escapers) as collisionless, enabling fast simulation while capturing asymmetric tidal features.
• Uses a delayed escape criterion based on full trajectory integration rather than analytic scalings with Jacobi energy to improve physical realism.
• Integrates the CMC code with the Gala and COSMIC frameworks to model orbital dynamics and stellar evolution.
• Maps stream velocity profiles relative to circular speed and surface density, focusing on regions near the cluster and along the stream.
• Excludes returning tails post-escape to isolate their effect on velocity dispersion and stream morphology.

Experimental results

Research questions

• RQ1 Can returning tidal tails form after a stellar stream completes a full orbit around the Milky Way in a smooth potential?
• RQ2 How does the distribution of escape timescales for potential escapers reflect underlying chaotic dynamics in the three-body problem?
• RQ3 How does cluster evolution—specifically mass loss and relaxation—affect the median escape timescale of potential escapers?
• RQ4 To what extent do returning tails contribute to stream velocity dispersion, and how does this affect constraints on dark matter subhalos?
• RQ5 Can improved escape physics in Monte Carlo codes enable accurate, large-scale simulations of stellar streams for comparison with Gaia data?

Key findings

• Returning tidal tails form robustly after ~10 Gyr of orbital motion in a smooth, circular orbit, with stars re-entering the cluster's vicinity after circumnavigating the Galactic center.
• The escape timescale distribution exhibits sharp plateaus corresponding to distinct, locally smooth regions of the chaotic saddle in phase space, linked to the three-body problem's dynamics.
• The median escape timescale decreases from ~10 Gyr in a static cluster to <100 Myr when cluster mass loss and internal evolution are included, with scaling Δt ∼ E_J^(-0.1) for low-E_J and E_J^(-0.4) for high-E_J.
• Returning tails increase the velocity dispersion of stellar streams by several km s⁻¹, an effect that mimics heating from external perturbers like giant molecular clouds or dark matter subhalos.
• The study identifies a need to implement delayed escape criteria based on full trajectory integration in CMC to better capture the complex energy dependence of escape in asymmetric cluster regions.
• Future work will implement these improvements to enable large-parameter-space simulations of tidal tails and direct comparison with Gaia observations.