Formation of the Galactic Stellar Halo. I. Structure and Kinematics

We perform numerical simulations for the formation of the Galactic stellarhalo, based on the currently favored cold dark matter (CDM) theory of galaxyformation. Our numerical models, taking into account both dynamical andchemical evolution processes in a consistent manner, are aimed at explainingobserved structure and kinematics of the stellar halo in the context ofhierarchical galaxy formation. The main results of the present simulations aresummarized as follows. (1) Basic physical processes involved in the formationof the stellar halo, composed of metal-deficient stars with [Fe/H] $\le$ -1.0,are described by both dissipative and dissipationless merging of subgalacticclumps and their resultant tidal disruption in the course of gravitationalcontraction of the Galaxy at high redshift ($z$ $>$ 1). (2) The simulated halohas the density profile similar to the observed power-law form of $\rho (r)$$\sim$ $r^{-3.5}$, and has also the similar metallicity distribution to theobservations. The halo virtually shows no radial gradient for stellar ages andonly small gradient for metallicities. (3) The dual nature of the halo, i.e.,its inner flattened and outer spherical density distribution, is reproduced, atleast qualitatively, by the present model. The outer spherical halo is formedvia essentially dissipationless merging of small subgalactic clumps, whereasthe inner flattened one is formed via three different mechanisms, i.e.,dissipative merging between larger, more massive clumps, adiabatic contractiondue to the growing Galactic disk, and gaseous accretion onto the equatorialplane.Comment: 55 pages 20 figures (figure1,2,3: GIF), 2001, ApJ, in pres