Accretion relicts in the solar neighbourhood: debris from omegaCen's parent galaxy

Abstract

We use numerical simulations to investigate the orbital characteristics of tidal debris from satellites whose orbits are dragged into the plane of galactic disks by dynamical friction before disruption. We find that these satellites may deposit a significant fraction of their stars into the disk components of a galaxy, and use our results to motivate the search for accretion relicts in samples of metal-poor disk stars in the vicinity of the Sun. Satellites disrupted on very eccentric orbits coplanar with the disk are expected to shed stars in "trails" of distinct orbital energy and angular momentum during each pericentric passage. To an observer located between the pericenter and apocenter of such orbits, these trails would show as distinct groupings of stars with low vertical velocity and a broad, symmetric, often double-peaked distribution of Galactocentric radial velocities. One group of stars with these characteristics stands out in available compilations of nearby metal-poor stars. These stars have specific angular momenta similar to that of the globular cluster omegaCen, long hypothesized to be the nucleus of a dwarf galaxy disrupted by the Milky Way tidal field. In addition to their kindred kinematics, stars in the omegaCen group share distinct chemical abundance characteristics, and trace a well-defined track in the [alpha/Fe] versus [Fe/H] plane, consistent with simple closed-box enrichment models and a protracted star formation history. The dynamical and chemical coherence of this group suggests that it consists of stars that once belonged to the dwarf that brought omegaCen into the Galaxy. The presence of this and other "tidal relicts" in the solar neighbourhood suggest an extra-Galactic origin for the presence of nearby stars with odd kinematics and chemistry.