Fashionably Late? Building up the Milky Way's Inner Halo

TL;DR

This study reveals a new, flattened inner halo component with specific kinematic properties, formed late in the Galaxy's history, and highlights the clumpy, progenitor-driven structure of the Milky Way's inner halo based on precise stellar data.

Contribution

It identifies a distinct, flattened inner halo component with unique kinematics and metallicity distribution, formed during or after disk formation, and analyzes the inner halo's clumpy structure and progenitor origins.

Findings

Discovery of a new flattened inner halo component with c/a ~ 0.2.

Inner halo shows clumpy energy and angular momentum distribution.

Outer halo likely younger with more retrograde motion.

Abstract

Using a sample of 248 metal-poor stars (RR Lyraes, red giants and RHB stars) which is remarkable for the accuracy of its 6-D kinematical data, we find a new component for the local halo which has an axial ratio c/a ~ 0.2, a similar flattening to the thick disk. It has a small prograde rotation but is supported by velocity anisotropy, and contains more intermediate-metallicity stars (with -1.5 < [Fe/H] < -1.0) than the rest of our sample. We suggest that this component was formed quite late, during or after the formation of the disk. It formed either from the gas that was accreted by the last major mergers experienced by the Galaxy, or by dynamical friction of massive infalling satellite(s) with the halo and possibly the stellar disk or thick disk. The remainder of the stars in our sample exhibit a clumpy distribution in energy and angular momentum, suggesting that the early, chaotic conditions under which the inner halo formed were not violent enough to erase the record of their origins. The clumpy structure suggests that a relatively small number of progenitors were responsible for building up the inner halo, in line with theoretical expectations. We find a difference in mean binding energy between the RR Lyrae variables and the red giants in our sample, suggesting that more of the RR Lyraes in the sample belong to the outer halo, and that the outer halo may be somewhat younger, as first suggested by Searle and Zinn (1978). We also find that the RR Lyrae mean rotation is more negative than the red giants, which is consistent with the recent result of Carollo et al.(2007) that the outer halo has a retrograde rotation and with the difference in kinematics seen between RR Lyraes and BHB stars by Kinman et al.(2007).