Subhalos-stream interaction in the presence of massive satellites

Dark matter subhalos potentially can be detected when they interact with stellar streams, leaving perturbations along the stream track. Massive in-falling satellites such as the Large Magellanic Cloud (LMC) can create a dark matter wake that could lead to significant perturbations in the subhalo population of the host galaxy, complicating efforts to interpret possible interaction signatures. Using the FIRE-2 zoomed cosmological baryonic simulations, we explore the evolution of encounter rates between subhalos with masses 10⁵–10⁹ M_⊙ and a stellar stream orbiting at a distance of 20–40 kpc from the galactic center during a LMC-like merger. We use an analytic, axisymmetric treatment of flybys, assuming an isotropic Maxwellian distribution for the relative radial velocity and a uniform number density of subhalos at each time step, to estimate the number of encounters in this radial range as a function of sky position. While the LMC drags in a large number of subhalos at infall, most of them are quickly destroyed by tides; the survivors account for only about 10% of the total subhalo population after the first pericenter passage of the LMC analog. However, while the direct subhalo contribution does not substantially affect the global number density profile of subhalos, the merger significantly perturbs the global distribution of relative radial velocities for the subhalos, imparting a more circular nature to their orbits and improving the common assumption of a Maxwellian velocity distribution. Furthermore, the contributed subhalos are not uniformly distributed across the halo, but are concentrated near the pericenter of the interaction, where they can increase the encounter rate by up to a factor 5 as the LMC approaches pericenter.