Rings and Radial Waves in the Disk of the Milky Way

TL;DR

This paper identifies oscillating asymmetries in star counts across the Milky Way's disk, revealing concentric rings linked to disk oscillations and spiral structure, based on Sloan Digital Sky Survey data.

Contribution

It provides evidence that the Monoceros and TriAnd Rings are caused by disk oscillations and details their spatial relationships and possible connection to spiral arms.

Findings

Oscillating star count asymmetries form concentric rings around the Galactic center.

The Monoceros and TriAnd Rings are likely due to disk oscillations.

The disk exhibits non-azimuthal symmetry and possible links to spiral structure.

Abstract

We show that in the anticenter region, between Galactic longitudes of $110^\circ<l<229^\circ$, there is an oscillating asymmetry in the main sequence star counts on either side of the Galactic plane using data from the Sloan Digital Sky Survey. This asymmetry oscillates from more stars in the north at distances of about 2 kpc from the Sun to more stars in the south at 4-6 kpc from the Sun to more stars in the north at distances of 8-10 kpc from the Sun. We also see evidence that there are more stars in the south at distances of 12-16 kpc from the Sun. The three more distant asymmetries form roughly concentric rings around the Galactic center, opening in the direction of the Milky Way's spiral arms. The northern ring, 9 kpc from the Sun, is easily identified with the previously discovered Monoceros Ring. Parts of the southern ring at 14 kpc from the Sun (which we call the TriAnd Ring) have previously been identified as related to the Monoceros Ring and others have been called the Triangulum Andromeda Overdensity. The two nearer oscillations are approximated by a toy model in which the disk plane is offset by of the order 100 pc up and then down at different radii. We also show that the disk is not azimuthally symmetric around the Galactic anticenter and that there could be a correspondence between our observed oscillations and the spiral structure of the Galaxy. Our observations suggest that the TriAnd and Monoceros Rings (which extend to at least 25 kpc from the Galactic center) are primarily the result of disk oscillations.