Subhalo Spreading of Thin Tidal Star Streams

Abstract

Dark matter subhalos that pass a thin tidal star stream change the velocities of the stars near the point of closest encounter. Subsequent orbital evolution increases the stream width and spreads the changes along the stream. We measure the average widths and velocity dispersions of stream populations in three Milky Way–like cold dark matter cosmology simulations containing star particle globular clusters and galactic disks of 0, 1, and 2 times a baseline model. Power-law fits to the velocity dispersion with stream longitude, ϕ, for the overlaid streams in the 10–60 kpc range find σ ≃ 5–15 ϕ 0.2–0.5 km s−1, with the coefficients depending on the subhalo numbers, as well as the stream measurement details. The velocity distributions summed along the entire length of the streams do not require the progenitor location. They also rise with subhalo number and are significantly non-Gaussian, with the ratios of the 6σ to the 3σ clipped velocity dispersions being ∼1.5 ± 0.3 and ∼2.5 ± 1 for measurements within 1° and 5° of the streams, respectively. Streams longer than 50° have an average radial velocity dispersion of 2.1 km s−1 with a disk and 4.2 km s−1 without a disk. However, a few similar thin, low-velocity dispersion streams are present in all three simulations. Statistically reliable conclusions require velocity data extending several degrees from the stream centerline for a set of streams.