Dark matter sub-halo counts via star stream crossings

Abstract

Dark matter sub-halos create gaps in the stellar streams orbiting in the halos of galaxies. We evaluate the sub-halo stream crossing integral with the guidance of simulations to find that the linear rate of gap creation, R u, in a typical cold dark matter (CDM) galactic halo at 100 kpc is Ru ≈ 0.0066 M̂?0.358 kpc?1 Gyr?1, where M̂8(M̂/108M) is the minimum mass halo that creates a visible gap. The relation can be recast entirely in terms of observables, as Ru ≈ 0.059ω ?0.85 kpc?1 Gyr ?1, for w in kpc, normalized at 100 kpc. Using published data, the density of gaps is estimated for M31s NW stream and the Milky Way Pal 5 stream, Orphan stream, and Eastern Banded Structure. The estimated rates of gap creation all have errors of 50% or more due to uncertain dynamical ages and the relatively noisy stream density measurements. The gap-ratewidth data are in good agreement with the CDM-predicted relation. The high density of gaps in the narrow streams requires a total halo population of 105 sub-halos above a minimum mass of 105M⊙. © 2012 The American Astronomical Society. All rights reserved.