Mapping the Asymmetric Thick Disk: II Distance, Size and Mass of the Hercules Thick Disk Cloud

Abstract

The Hercules Thick Disk Cloud (Larsen et al. 2008) was initially discovered as an excess in the number of faint blue stars between quadrants 1 and 4 of the Galaxy. The origin of the Cloud could be an interaction with the disk bar, a triaxial thick disk or a merger remnant or stream. To better map the spatial extent of the Cloud along the line of sight, we have obtained multi-color UBVR photometry for 1.2 million stars in 63 fields approximately 1 square degree each. Our analysis of the fields beyond the apparent boundaries of the excess have already ruled out a triaxial thick disk as a likely explanation (Larsen, Humphreys and Cabanela 2010) In this paper we present our results for the star counts over all of our fields, determine the spatial extent of the over density across and along the line of sight, and estimate the size and mass of the Cloud. Using photometric parallaxes, the stars responsible for the excess are between 1 and 6 kiloparsecs from the Sun, 0.5 -- 4 kpc above the Galactic plane, and extends approximately 3-4 kiloparsecs across our line of sight. It is thus a major substructure in the Galaxy. The distribution of the excess along our sight lines corresponds with the density contours of the bar in the Disk, and its most distant stars are directly over the bar. We also see through the Cloud to its far side. Over the entire 500 square degrees of sky containing the Cloud, we estimate more than 5.6 million stars and 1.9 million solar masses of material. If the over density is associated with the bar, it would exceed 1.4 billion stars and more than than 50 million solar masses. Finally, we argue that the Hercules-Aquila Cloud (Belokurov et al. 2007) is actually the Hercules Thick Disk Cloud.

52 pages, 13 figures

References in corpus (6)

• The Sagittarius Dwarf Galaxy: a Model for Evolution in a Triaxial Milky Way Halo
• The Hercules-Aquila Cloud
• Disentangling the Hercules stream
• Improved $u^{\prime }{g}^{\prime }{r}^{\prime }{i}^{\prime }{z}^{\prime }$ to $UBV{R}_C{I}_C$ Transformation Equations for Main Sequence Stars
• Mapping the Asymmetric Thick Disk: The Hercules Thick Disk Cloud
• Mapping the Asymmetric Thick Disk: III. The Kinematics and Interaction with the Galactic Bar

Cited by in corpus (11)

• The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
• Streams, substructures and the early history of the Milky Way
• The wobbly Galaxy: kinematics north and south with RAVE red clump giants
• Stellar Streams and Clouds in the Galactic Halo
• Galactic Archaeology. The dwarfs that survived and perished
• The Gravitational Force Field of the Galaxy Measured From the Kinematics of RR Lyrae in Gaia
• A Hero's Little Horse: Discovery of a Dissolving Star Cluster in Pegasus
• Strong RR Lyrae excess in the Hercules-Aquila Cloud
• A 10,000 star spectroscopic survey of the thick disk-halo interface : Phase-space sub-structure in the thick disk
• Spectroscopic Follow-Up of the Hercules Aquila Cloud
• Mapping the Asymmetric Thick Disk: III. The Kinematics and Interaction with the Galactic Bar