Star Streams and the Assembly History of the Galaxy

Abstract

Thin halo star streams originate from the evaporation of globular clusters and therefore provide information about the early epoch globular cluster population. The observed tidal tails from halo globular clusters in the Milky Way are much more shorter than expected from a star cluster orbiting for 10~Gyr. The discrepancy is likely the result of the assumption that the clusters have been orbiting in a non-evolving galactic halo for a Hubble time. As a first step towards more realistic stream histories, a toy model that combines an idealized merger model with a simplified model of the internal collisional relaxation of individual star clusters is developed. On the average, the velocity dispersion increases with distance causing the density of the stream to decline with distance. Consequently, the streams visible in current data will normally be some fraction of the entire stream. Nevertheless, the high surface density segment of the stellar streams created from the evaporation of the more massive globular clusters should all be visible in low obscuration parts of the sky if closer than about 30~kpc. The Pan-STARRS1 halo volume is used to compare the numbers of halo streams and globular clusters.