The peculiar velocities in the Galactic outer disk--hints of the elliptical disk and the perturbation of the spiral structures

TL;DR

This study analyzes peculiar velocities in the Galactic outer disk using LAMOST red clump stars, revealing elliptical disk features and spiral arm perturbations, and estimates the bar's pattern speed based on resonance locations.

Contribution

It identifies two types of peculiar velocities and attributes them to elliptical disk structure and spiral arm perturbations, providing new insights into Galactic dynamics.

Findings

Elliptical disk explains the long-wave velocity mode.

Outer Lindblad resonance radius at 9.24 kpc.

Bar pattern speed estimated at 48 km/s/kpc.

Abstract

We present the peculiar in-plane velocities derived from the LAMOST red clump stars. From the variations of the in-plane velocity with the Galactocentric radius for the young and old red clump stars, we are able to identify two types of peculiar velocities: 1) both the two red clump populations show that the radial velocity is negative within $R=9.0$\,kpc and becomes positive beyond (denoted as the \emph{long-wave} mode); and 2) the young red clump stars show larger mean radial velocity than the old population by about 3\,km$\rm s^{-1}$ between $R=9$ and 12\,kpc (denoted as the \emph{short-wave} mode). We find that the elliptical disk induced by the rotating bar can well explain the \emph{long-wave} mode peculiar velocity. The axis ratio of the elliptical disk is around 0.8-0.95 and the disk keeps circular at $R=9.24\pm0.2$\,kpc, which should be the location of the outer Lindblad resonance radius (OLR). Adopting the circular speed of 238\,km$\rm s^{-1}$, the pattern speed of the bar is then derived as $48\pm3$\,km$\rm s^{-1}$kpc$^{-1}$ from the location of OLR. On the other hand, the \emph{short-wave} mode is likely the perturbation of the spiral arms as density waves.