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.
Dark matter sub-halos that pass near or through a thin tidal star stream locally increase its velocity dispersion. Subsequent orbital evolution further increases the velocity dispersion and stream width, lowering the surface density of a stream. The kinematic properties of streams are measured in cosmological Milky Way-like halo simulations. The distance along a stream is a proxy for the time a star has spent in the stream, although there are a range of ages at any distance. Power law fits to the velocity dispersion with angular distance for the average of the streams in the 10-60 kpc range finds σ θ = 6φ 0.25 km s −1 , σ φ = 8φ 0.39 km s −1 , and σ r = 10φ 0.44 km s −1 for |φ| < 34 • , for stars within θ = ±5 • of the stream equator. The errors of the coefficients are about 10% for these streams, with comparable systematic errors depending on exactly which streams are selected and the stream width and length fitted. The stream velocity dispersions close to the clusters generally increase with the sub-halo numbers.
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