On Rings and Streams in the Galactic Anti-Center

TL;DR

This paper identifies and characterizes multiple stellar over-densities near the Milky Way anti-center, including the Monoceros Ring, Anti-Center Stream, and Eastern Banded Structure, providing new measurements of their properties and potential origins.

Contribution

It offers detailed observational analysis of these structures, including distance, velocity, metallicity, and spatial extent, highlighting differences from the canonical thick disk and proposing their possible distinct origins.

Findings

Anti-center over-densities confirmed as Monoceros, ACS, and EBS.

ACS extends to higher latitude at lower longitude with a sharp cutoff.

Monoceros Ring has a consistent distance of 17.6 kpc from Galactic Center.

Abstract

We confirm that there are at least three separate low-latitude over-densities of blue F turnoff stars near the Milky Way anti-center: the Monoceros Ring, the Anti-Center Stream (ACS), and the Eastern Banded Structure (EBS). There might also be a small number of normal thick disk stars at the same location. The ACS is a tilted component that extends to higher Galactic latitude at lower Galactic longitude, 10 kpc from the Sun towards the anti-center. It has a sharp cutoff on the high latitude side. Distance, velocity, and proper motion measurements are consistent with previous orbit fits. The mean metallicity is [Fe/H]$=-0.96 \pm 0.03$, which is lower than the thick disk and Monoceros Ring. The Monoceros Ring is a higher density substructure that is present at $15\arcdeg<b<22\arcdeg$ at all longitudes probed in this survey. The structure likely continues towards lower latitudes. The distances are consistent with a constant distance from the Galactic Center of 17.6 kpc. The mean line-of-sight velocity of the structure is consistent with a thick disk rotation. However, the velocity dispersion of these stars is $\sim 15$ km s$^{-1}$, and the metallicity is [Fe/H]$=-0.80 \pm 0.01$. Both of these quantities are lower than the canonical thick disk. We suggest that this ring structure is likely different from the thick disk, though its association with the disk cannot be definitively ruled out. The Eastern Banded Structure (EBS) is detected primarily photometrically, near $(l,b)=(225\arcdeg,30\arcdeg)$, at a distance of 10.9 kpc from the Sun.