Kinematics in galactic tidal tails - A source for Hypervelocity stars?
  [abridged] In this work we study in detail the kinematics of tidal debris stars to investigate the implications of the new scenario that the observed sample of Hypervelocity stars could partly originate from a dwarf-host galaxy collision. We use a suite of N-body simulations following the encounter of a satellite galaxy with its Milky Way-type host galaxy to gather statistics on the properties of stripped-off stars. We study especially the orbital energy distribution of this population.   We quantify the typical pattern in angular and phase space formed by the debris stars. We further develop a simple stripping model predicting the kinematics of stripped-off stars. We show that the distribution of orbital energies in the tidal debris has a typical form which can be described quite accurately by a simple function. Based on this we develop a method to predict the energy distribution which allows us to evaluate the significance and the implications of high velocity stars in satellite tidal debris.   Generally tidal collisions of satellite galaxy produce stars which escape into the intragalactic space even if the satellite itself is on a bound orbit. The main parameters determining the maximum energy kick a tidal debris star can get is the initial mass of the satellite and only to a lower extent its orbit. Main contributors to an unbound stellar population created in this way are massive satellites (M_sat > 10^9 M_sun). We thus expect intragalactic stars to have a metallicity higher than the surviving satellite population of the Milky Way. However, the probability that the observed HVS population is significantly contaminated by tidal debris stars appears small in the light of our results. 
Tilmann Piffl, Mary Williams, Matthias Steinmetz
astro-ph.GA