Mapping the Asymmetric Thick Disk: The Hercules Thick Disk Cloud

TL;DR

This paper investigates the asymmetry in the thick disk of the Milky Way, presenting new stellar density maps that challenge previous ring-like interpretations and suggest alternative origins like a triaxial structure or merger remnant.

Contribution

It provides new stellar density maps from the MAPS Catalog that clarify the nature and extent of the Hercules Thick Disk Cloud, challenging prior ring-based models.

Findings

Overdensity does not extend into the fourth quadrant.

The feature is unlikely to be a ring structure.

Possible origins include disk bar interaction, triaxial thick disk, or merger remnant.

Abstract

The stellar asymmetry of faint thick disk/inner halo stars in the first quadrant first reported by Larsen & Humphreys (1996) and investigated further by Parker et al. (2003, 2004) has been recently confirmed by SDSS (Juric et al. 2008). Their interpretation of the excess in the star counts as a ringlike structure, however, is not supported by critical complimentary data in the fourth quadrant not covered by SDSS. We present stellar density maps from the Minnesota Automated Plate Scanner (MAPS) Catalog of the POSS I showing that the overdensity does not extend into the fourth quadrant. The overdensity is most probably not a ring. It could be due to interaction with the disk bar, evidence for a triaxial thick disk, or a merger remnant/stream. We call this feature the Hercules Thick Disk Cloud.