On the Origin of High-eccentricity Halo Stars

The present-day chemical and dynamical properties of the Milky Way are signatures of the Galaxy's formation and evolution. Using a self consistent chemodynamical evolution code we examine these properties within the currently favoured paradigm for galaxy formation-hierarchical clustering within a CDM cosmology. Our Tree N-body/Smoothed Particle Hydrodynamics code includes a self-consistent treatment of gravity, hydrodynamics, radiative cooling, star formation , supernova feedback and chemical enrichment. Two models are described which explore the role of small-scale density perturbations in driving the evolution of structure within the Milky Way. The relationship between metallicity and kinematics of halo stars are quantified and the implications for galaxy formation discussed. While high-eccentricity halo stars have previously been considered a signature of " rapid collapse " , we suggest that many such stars may have come from recently accreted satellites. 1. Introduction The " monolithic collapse " versus " satellite accretion " debate surrounding galaxy formation is a classic one, and one which received attention once again at this Euroconference III. The former scenario was best expressed by Eggen, Lynden-Bell & Sandage (1962, hereafter ELS); supporting evidence for the ELS picture came from the apparent positive correlation between eccentricity and metallicity of halo stars. However , current cosmological theories of structure formation have more in common with accretion-style scenarios like that envisioned by Searle & Zinn (1978). Evidence in support of the latter can be found in the observations of stellar phase space substructure in the Galactic halo (e.g. Helmi et al. 1999). We were motivated to run a grid of chemodynamical simulations with the intention of contrasting the effects of the two collapse scenarios on the evolution of the Milky Way. The two models described here vary primarily in their degree of clustering, and we examine the properties of the resulting simulated galaxies, in order to uncover present-day " signatures " of the model initial conditions and evolution. Here, we focus on the distribution of halo star orbital eccentricities.