Metallicity Bias in the Kinematics of the Milky Way Stellar Halo

TL;DR

This study investigates how metallicity influences the kinematic properties of the Milky Way's stellar halo, revealing distinct motion patterns linked to different metallicity populations and supporting a multi-component formation history.

Contribution

It provides new insights into the metallicity-dependent kinematic profiles of halo stars, highlighting differences in velocity and anisotropy between metal-rich and metal-poor populations.

Findings

Metal-rich stars show prograde motion with a 40 km/s azimuthal velocity offset.

Metal-poor stars exhibit retrograde motion and a 65 km/s rotation difference.

Anisotropy decreases with radius, indicating different orbital characteristics at various distances.

Abstract

Here we study the metallicity bias in the velocity dispersions, the derived quantity called anisotropy and the mean azimuthal velocity profiles of the Milky Way stellar halo using Blue Horizontal Branch (BHB) stars taken from SDSS/SEGUE survey. The comparatively metal-rich sample ([Fe/H]>-2) has prograde motion and is found to have an offset of 40 km/s in the mean azimuthal velocity with respect to a metal-poor sample ([Fe/H]<=-2) which has retrograde motion. The difference in rotation between the most metal-poor and most metal-rich population was found to be around 65 km/s. For galactocentric distances r<16 kpc, an offset in velocity dispersion profiles and anisotropy can also be seen. In the inner regions, the metal-poor population is in average tangential orbit; however, anisotropy is found to decrease monotonically with radius independent of metallicity. Beyond r = 16 kpc, both the metal-rich and the metal-poor samples are found to have tangential motion. The metallicity bias in the kinematics of the halo stars qualitatively supports the co-existence of at least two-components in the halo having different formation history e.g. in-situ formation and formation by accretion.