Charting Galactic Accelerations with Stellar Streams and Machine Learning

Abstract

We present a data-driven method for reconstructing the galactic acceleration field from phase-space (position and velocity) measurements of stellar streams. Our approach is based on a flexible and differentiable fit to the stream in phase-space, enabling a direct estimate of the acceleration vector along the stream. Reconstruction of the local acceleration field can be applied independently to each of several streams, allowing us to sample the acceleration field due to the underlying galactic potential across a range of scales. Our approach is methodologically different from previous works, as a model for the gravitational potential does not need to be adopted beforehand. Instead, our flexible neural-network-based model treats the stream as a collection of orbits with a locally similar mixture of energies, rather than assuming that the stream delineates a single stellar orbit. Accordingly, our approach allows for distinct regions of the stream to have different mean energies, as is the case for real stellar streams. Once the acceleration vector is sampled along the stream, standard analytic models for the galactic potential can then be rapidly constrained. We find our method recovers the correct parameters for a ground-truth triaxial logarithmic halo potential when applied to simulated stellar streams. Alternatively, we demonstrate that a flexible potential can be constrained with a neural network, and standard multipole expansions can also be constrained. Our approach is applicable to simple and complicated gravitational potentials alike and enables potential reconstruction from a fully data-driven standpoint using measurements of slowly phase-mixing tidal debris.

Constraining the Gravitational Potential from the Projected Morphology of Extragalactic Tidal Streams

“…If the 3D track of the orbit is known along with its speed, then the acceleration can be calculated directly (Nibauer et al 2022). If the speed is not known and the orbit is assumed to be circular, then the correct acceleration vector must point in the same direction as the curvature vector.…”

“…From an information theoretic perspective, Bonaca & Hogg (2018) found that kinematically cold streams provide a localized constraint on the galactic potential. Indeed, in Nibauer et al (2022), it is shown that local galactic accelerations can be recovered from streams directly, without explicit reference to any potential model. The results from these analyses both imply that there must be some amount of information about the potential encoded in the geometry of tidal features (e.g., the light-blue contours in Figure 3 of Bonaca & Hogg 2018 and Equation (6) of Nibauer et al 2022), though it has remained unclear which properties of the potential can actually be inferred from projected stream morphology.…”

“…Indeed, in Nibauer et al (2022), it is shown that local galactic accelerations can be recovered from streams directly, without explicit reference to any potential model. The results from these analyses both imply that there must be some amount of information about the potential encoded in the geometry of tidal features (e.g., the light-blue contours in Figure 3 of Bonaca & Hogg 2018 and Equation (6) of Nibauer et al 2022), though it has remained unclear which properties of the potential can actually be inferred from projected stream morphology. Using the generative "streakline" technique, Pearson et al (2015) found that a spherical halo reproduces the curvature of Palomar 5 better than a triaxial halo.…”

A Ghost in Boötes: The Least-Luminous Disrupted Dwarf Galaxy

“…These surveys have likewise unearthed a patchwork of ex situ stellar streams encircling the Galaxy, tidally stretched remnants of past satellites and their globular clusters (e.g., Majewski et al 2003;Belokurov et al 2007a;Grillmair 2009;Shipp et al 2018;Bonaca et al 2021). Although these emissaries are fascinating in their own right due to their extragalactic formation, they also teach us about the distribution of our own Galaxy's stellar and dark matter (e.g., Murali & Dubinski 1999;Eyre & Binney 2011;Bonaca et al 2014;Bovy et al 2016;Bonaca & Hogg 2018;Malhan et al 2021;Nibauer et al 2022).…”

Mapping Dark Matter with Extragalactic Stellar Streams: The Case of Centaurus A

“…Time dependence: We have assumed that the potential of Cen A is static. From recent work on stellar streams in the Milky Way (e.g.,Shipp et al 2021;Dillamore et al 2022;Lilleengen et al 2022), we know that both thin GC streams and dwarf galaxy streams are sensitive to perturbations from accreted dwarf galaxies (see also the LMC's effect on the Milky Way's dark matter halo in Garavito-Camargo et al 2019) Nibauer et al (2022). argue that not all streams are sensitive to such perturbations (depending on the streams' orbits with respect to the infalling satellite's orbit) and that some streams can still provide powerful constraints on dark matter halo parameters despite their time dependence.…”