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

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Abstract
We perform numerical simulations for the formation of the Galactic stellar halo, based on the currently favored cold dark matter theory of galaxy formation. Our numerical models, taking into account both dynamical and chemical evolution processes in a consistent manner, are aimed at explaining the observed structure and kinematics of the stellar halo in the context of hierarchical galaxy formation. The main results of the present simulations are summarized as follows: (1) Basic physical processes involved in the formation of the stellar halo, composed of metal-deficient stars with [Fe/H] ≤ -1.0, are described by both dissipative and dissipationless merging of subgalactic clumps and their resultant tidal disruption in the course of gravitational contraction of the Galaxy at high redshift (z > 1). (2) The simulated halo has a density profile similar to the observed power-law form of ρ(r) ~ r-3.5 and also has a metallicity distribution similar to the observations. The halo shows virtually no radial gradient 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 density distribution, is reproduced, at least qualitatively, by the present model. The outer spherical halo is formed via essentially dissipationless merging of small subgalactic clumps, whereas the inner flattened one is formed via three different mechanisms, i.e., dissipative merging between larger, more massive clumps, adiabatic contraction due to the growing Galactic disk, and gaseous accretion onto the equatorial plane. (4) For the simulated metal-poor stars with [Fe/H] ≤ -1.0, there is no strong correlation between metal abundances and orbital eccentricities, in good agreement with the recent observations. Moreover, the observed fraction of the low-eccentricity stars is reproduced correctly for [Fe/H] ≤ -1.6 and approximately for the intermediate-abundance range of -1.6 < [Fe/H] ≤ -1.0. (5) The mean rotational velocity of the simulated halo, , 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 . Based on these results, we discuss how early processes of dissipationless 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 Galaxy structure, including bulge and disk components, in conjunction with halo formation.
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