Formation of the Galactic stellar halo. I. Structure and kinematics

We perform numerical simulations for the formation of the Galactic stellar halo, based on the currently favored cold dark matter theory of galaxy form ation. Our numerical models, taking into account both dynamical and chemica l evolution processes in a consistent manner, are aimed at explaining the o bserved structure and kinematics of the stellar halo in the context of hier archical galaxy formation. The main results of the present simulations are summarized as follows : (1) Basic physical processes involved in the format ion of the stellar halo, composed of metal-deficient stars with [Fe/H] less than or equal to -1.0, are described by both dissipative and dissipationle ss merging of subgalactic clumps and their resultant tidal disruption in th e course of gravitational contraction of the Galaxy at high redshift (z > 1 ). (2) The simulated halo has a density profile similar to the observed pow er-law form of rho (r) similar to r(-3.5) and also has a metallicity distri bution similar to the observations. The halo shows virtually no radial grad ient for stellar ages and only a small gradient for metallicities. (3) The dual nature of the halo, i.e., its inner flattened and outer spherical dens ity distribution, is reproduced, at least qualitatively, by the present mod el. The outer spherical halo is formed via essentially dissipationless merg ing of small subgalactic clumps, whereas the inner flattened one is formed via three different mechanisms, i.e., dissipative merging between larger, m ore massive clumps, adiabatic contraction due to the growing Galactic disk, and gaseous accretion onto the equatorial plane. (4) For the simulated met al-poor stars with [Fe/H] less than or equal to -1.0, there is no strong co rrelation between metal abundances and orbital eccentricities, in good agre ement with the recent observations. Moreover, the observed fraction of the low-eccentricity stars is reproduced correctly for [Fe/H] less than or equa l to -1.6 and approximately for the intermediate-abundance range of -1.6 < [Fe/H] <less than or equal to> -1.0. (5) The mean rotational velocity of th e simulated halo, [V-phi], is somewhat positive (prograde) at [Fe/H] < -2.2 and increases linearly with [Fe/H] at [Fe/H] > -2.2. The stars at smaller distance from the disk plane appear to show systematically larger [V-phi]. Based on these results, we discuss how early processes cesses of dissipatio nless and dissipative merging of subgalactic clumps can reproduce plausibly and consistently the recent observational results on the Galactic stellar halo. We also present a possible scenario for the formation of the entire G alaxy structure, including bulge and disk components, in conjunction with h alo formation.