Mapping the asymmetric thick disk. II. Distance, size, and mass of the Hercules Thick Disk Cloud

Jeffrey A. Larsen, Juan E. Cabanela, Roberta M. Humphreys

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10 Scopus citations


The Hercules Thick Disk Cloud 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 each of approximately 1 deg2. Our analysis of the fields beyond the apparent boundaries of the excess has already ruled out a triaxial Thick Disk as a likely explanation. In this paper, we present our results for the star counts over all of our fields, determine the spatial extent of the overdensity 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 6kpc from the Sun, 0.5-4kpc above the Galactic plane, and extend approximately 3-4kpc across our line of sight. The Cloud 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 deg2 of the sky containing the Cloud, we estimate more than 5.6 million stars and 1.9 million solar masses of material. If the overdensity is associated with the bar, it would exceed 1.4 billion stars and more than 50million solar masses. Finally, we argue that the Hercules-Aquila Cloud is actually the Hercules Thick Disk Cloud.

Original languageEnglish (US)
Article number130
JournalAstronomical Journal
Issue number4
StatePublished - Apr 2011


  • Galaxy: kinematics and dynamics
  • Galaxy: structure


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