The Pal 5 Star Stream Gaps

Abstract

Pal 5 is a low mass, low velocity dispersion, globular cluster with spectacular tidal tails. We use the SDSS DR8 data to extend the density measurements of the trailing star stream to 23 degrees distance from the cluster, at which point the stream runs off the edge of the available sky coverage. The size and the number of gaps in the stream are measured using a filter which approximates the structure of the gaps found in stream simulations. We find 5 gaps that are at least 99% confidence detections with about a dozen gaps at 90% confidence. The statistical significance of a gap is estimated using bootstrap re-sampling of the control regions on either side of the stream. The density minimum closest to the cluster is likely the result of the epicyclic orbits of the tidal outflow and has been discounted. To create the number of 99% confidence gaps per unit length at the mean age of the stream requires a halo population of nearly a thousand dark matter sub-halos with peak circular velocities above 1 km/s within 30kpc of the galactic center. These numbers are a factor of about 3 below cold stream simulation at this sub-halo mass or velocity, but given the uncertainties in both measurement and more realistic warm stream modeling, are in substantial agreement with the LCDM prediction