http://arxiv.org/abs/1407.4808
Accurate determination of the local dark matter density is important for understanding the nature and distribution of dark matter in the universe. This requires that the local velocity distribution is characterised correctly. Here, we present a kinematic study of 16,276 SEGUE G-type dwarf stars in the solar neighbourhood, with which we determine the shape of the velocity ellipsoid in the meridional plane. We separate our G-dwarf stars based on their [Fe/H] and [alpha/Fe] abundances and estimate the best-fitting Milky Way model independently for each sub-sample using a maximum-likelihood method that accounts for possible contaminants. We show that the different subpopulations yield consistent results only when we allow the velocity ellipsoid in the disk to be tilted, demonstrating that the common assumption of decoupled radial and vertical motions in the disk is incorrect. Further, we the find that the tilt angle alpha of the velocity ellipsoid increases with height |z| from 5{\deg} at 0.5 kpc to 14{\deg} at 2.0 kpc, consistent with pointing toward the Galactic centre at an angle tan(alpha) ~ |z|/R. We also confirm earlier findings that the subpopulations behave almost isothermally both radially and vertically, about 39 (20) km/s for the chemically-young, metal-rich disk stars to about 60 (48) km/s for the chemically-old, metal-poor disk stars. We conclude that the coupling between radial and vertical motion captured in the velocity ellipsoid tilt cannot be ignored when considering dynamical models of the solar neighbourhood. In a subsequent paper, we will develop a new modelling scheme informed by these results and make an improved determination of the local dark matter density.
A. Budenbender, G. Ven and L. Watkins
Mon, 21 Jul 14
27/55
Comments: submitted to MNRAS; 12 pages
You must be logged in to post a comment.