Star Stream Velocity Distributions in Cold Dark Matter and Warm Dark Matter Galactic Halos

Abstract The dark matter subhalos orbiting in a galactic halo perturb the orbits of stars in thin stellar streams. Over time, the random velocities in the streams develop non-Gaussian wings. The rate of velocity increase is approximately a random walk at a rate proportional to the number of subhalos, primarily those in the mass range ≈10 6−7 M ⊙ . The distribution of random velocities in long streams is measured in simulated Milky Way–like halos that develop in representative warm dark matter (WDM) and cold dark matter (CDM) cosmologies. The radial velocity distributions are well modeled as the sum of a Gaussian and an exponential. The resulting Markov Chain Monte Carlo fits find Gaussian cores of 1−2 km s −1 and exponential wings that increase from 3 km s −1 for 5.5 keV WDM, 4 km s −1 for 7 keV WDM, to 6 km s −1 for a CDM halo. The observational prospects to use stream measurements to constrain the nature of galactic dark matter are discussed.