Slant, Fan, and Narrow: The Response of Stellar Streams to a Tilting Galactic Disk

Abstract

Stellar streams are sensitive tracers of the gravitational potential, which is typically assumed to be static in the inner Galaxy. However, massive mergers like Gaia-Sausage-Enceladus can impart torques on the stellar disk of the Milky Way that result in the disk tilting at rates of up to 10°–20° Gyr−1. Here, we demonstrate the effects of disk tilting on the morphology and kinematics of stellar streams. Through a series of numerical experiments, we find that streams with nearby apocenters (r apo ≲ 20 kpc) are sensitive to disk tilting, with the primary effect being changes to the stream’s on-sky track and width. Interestingly, disk tilting can produce both more diffuse streams and more narrow streams, depending on the orbital inclination of the progenitor and the direction in which the disk is tilting. Our model of Pal 5's tidal tails for a tilting rate of 15° Gyr−1 is in excellent agreement with the observed stream’s track and width, and reproduces the extreme narrowing of the trailing tail. We also find that failure to account for a tilting disk can bias constraints on shape parameters of the Milky Way’s local dark matter distribution at the level of 5%–10%, with the direction of the bias changing for different streams. Disk tilting could therefore explain discrepancies in the Milky Way’s dark matter halo shape inferred using different streams.

Trojan Globular Clusters: Radial Migration via Trapping in Bar Resonances

“…GC orbits may also be affected by a variety of other factors not considered in this study, such as a time-dependent potential or changing disk orientation (e.g., Dillamore et al 2022;Nibauer et al 2024). Recent accretion of satellites is less likely to be important, since there have been no significant merger events in the inner Galaxy since the formation of the bar (Sanders et al 2024;Deason & Belokurov 2024).…”