The Complex Structure of Stars in the Outer Galactic Disk as revealed by Pan-STARRS1

TL;DR

This study uses Pan-STARRS1 data to map the complex stellar structure of the Monoceros Ring in the outer Galactic disk, revealing intricate morphology and comparing it with simulations to understand its origin.

Contribution

It provides the first detailed panoptic map of the Monoceros Ring, highlighting its complex structure and comparing observational data with two formation scenario models.

Findings

The structure extends over 130 degrees in Galactic longitude.

Morphology includes stream-like features and sharp edges.

Neither of the two models perfectly matches the observations.

Abstract

We present a panoptic view of the stellar structure in the Galactic disk's outer reaches commonly known as the Monoceros Ring, based on data from Pan-STARRS1. These observations clearly show the large extent of the stellar overdensities on both sides of the Galactic disk, extending between b = -25 and b = +35 degrees and covering over 130 degrees in Galactic longitude. The structure exhibits a complex morphology with both stream-like features and a sharp edge to the structure in both the north and the south. We compare this map to mock observations of two published simulations aimed at explaining such structures in the outer stellar disk, one postulating an origin as a tidal stream and the other demonstrating a scenario where the disk is strongly distorted by the accretion of a satellite. These morphological comparisons of simulations can link formation scenarios to observed structures, such as demonstrating that the distorted-disk model can produce thin density features resembling tidal streams. Although neither model produces perfect agreement with the observations--the tidal stream predicts material at larger distances which is not detected while in the distorted disk model the midplane is warped to an excessive degree--future tuning of the models to accommodate these latest data may yield better agreement.