Rediscovering the Galactic outer disk with LAMOST data

TL;DR

This study uses LAMOST giant star data to analyze the structure of the Galactic outer disk, revealing a smooth transition to the halo, a smaller scale length, disk flaring in all populations, and significant vertical and radial motions.

Contribution

It provides new measurements of the outer disk's scale length, shows the absence of truncation, and identifies vertical oscillations and asymmetric motions across a wide radius range.

Findings

Outer disk smoothly transitions to halo without truncation.

Outer disk scale length is 1.6 kpc, smaller than previous estimates.

Vertical oscillations and disk flaring are present in both old and young populations.

Abstract

From the derived stellar density profile using LAMOST giant stars, we find that the Galactic disk does not show truncation or break, but smoothly transit to the halo from 19 kpc. The scale length of the outer disk is only $1.6\pm0.1$\,kpc, substantially smaller than previous results. This implies that the shapes of the inner and outer disk are different. Meanwhile, the disk flaring is not only found in older populations, but also in younger population. Moreover, the vertical oscillations of the disk are identified in a wide range or $R$ from 8 to 14 kpc. We also find that the velocity dispersion profile as a function of the Galactocentric radius is flat with scale length of $26.3\pm3.2$\,kpc. We confirm that the radial velocity profile in outer disk is significantly affected by asymmetric motion. The bar with either a slower or a faster pattern speed can induce the similar radial asymmetric motion.