Cresting the wave: Proper motions of the Eastern Banded Structure

TL;DR

This study uses precise proper motions from SDSS and Gaia to analyze the kinematics of the Eastern Banded Structure and Hydra I, revealing their connection to the Monoceros Ring and suggesting a Galactic disc origin.

Contribution

It demonstrates that the EBS and Hydra I are not separate streams but part of the Monoceros Ring, sharing similar kinematic and chemical properties, indicating a common Galactic disc origin.

Findings

EBS and Hydra I share kinematic properties with Monoceros Ring

Proper motions indicate prograde rotation similar to the thick disc

EBS likely marks the edge of the Monoceros Ring, not a distinct stream

Abstract

We study the kinematic properties of the Eastern Banded Structure (EBS) and Hydra I overdensity using exquisite proper motions derived from the Sloan Digital Sky Survey (SDSS) and Gaia source catalog. Main sequence turn-off stars in the vicinity of the EBS are identified from SDSS photometry; we use the proper motions and, where applicable, spectroscopic measurements of these stars to probe the kinematics of this apparent stream. We find that the EBS and Hydra I share common kinematic and chemical properties with the nearby Monoceros Ring. In particular, the proper motions of the EBS, like Monoceros, are indicative of prograde rotation (V_phi ~ 180-220 km/s), which is similar to the Galactic thick disc. The kinematic structure of stars in the vicinity of the EBS suggest that it is not a distinct stellar stream, but rather marks the "edge" of the Monoceros Ring. The EBS and Hydra I are the latest substructures to be linked with Monoceros, leaving the Galactic anti-centre a mess of interlinked overdensities which likely share a unified, Galactic disc origin.