LMC-driven anisotropic boosts in stream--subhalo interactions

TL;DR

This paper demonstrates that the presence of a Large Magellanic Cloud analog significantly alters the encounter rates between stellar streams and dark matter subhalos, affecting stream morphology and dynamics in the Milky Way.

Contribution

It reveals how an LMC analog influences stream--subhalo interactions through increased subhalo numbers, host displacement, and halo wake effects, providing new insights into stream perturbations.

Findings

Encounter rates increase by up to 40% near the LMC analog.

Asymmetry in subhalo distribution causes 50-70% variation in encounter rates across the sky.

Retrograde streams experience 50% more encounters in the opposite hemisphere.

Abstract

Dark Matter (DM) subhalos are predicted to perturb stellar streams; stream morphologies and dynamics can constrain the mass distribution of subhalos. Using FIRE-2 simulations of Milky Way-mass galaxies, we show that presence of a Large Magellanic Cloud (LMC)--analog significantly changes stream-subhalo encounter rates. Three key factors drive these changes. First, the LMC--analog brings in many subhalos, increasing encounter rates for streams near the massive satellite by up to 20--40%. Second, the LMC--analog displaces the host from its center-of-mass (inducing reflex motion), causing a north-south asymmetry in the density and radial velocity distribution of subhalos. This asymmetry results in encounter rates varying by 50--70% across the sky at the same distance. Finally, the LMC--mass satellite induces a density wake in the host's DM halo, further boosting the encounter rates near the LMC--analog. We also explore the influence of stream orbital properties, finding a 50% increase in encounters for streams moving retrograde to the LMC--analog's orbit in the opposite hemisphere. The dependence of encounter rates on stream location and orbit has important implications for where to search for new streams with spurs and gaps in the Milky Way.