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Disk Heating, Galactoseismology, and the Formation of Stellar Halos

1
Department of Astronomy, Columbia University, 550 W 120th st., New York, NY 10027, USA
2
Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
3
Max Planck Institute for Astronomy, Heidelberg 69117, Germany
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Fermi National Accelerator Laboratory, P. O. Box 500, Batavia, IL 60510, USA
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Department of Natural Sciences, LaGuardia Community College, City University of New York, 31-10 Thomson Ave., Long Island City, NY 11101, USA
6
Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904, USA
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The Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
8
Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Academic Editors: Duncan A. Forbes and Ericson D. Lopez
Galaxies 2017, 5(3), 44; https://doi.org/10.3390/galaxies5030044
Received: 1 July 2017 / Revised: 13 August 2017 / Accepted: 14 August 2017 / Published: 26 August 2017
(This article belongs to the Special Issue On the Origin (and Evolution) of Baryonic Galaxy Halos)
Deep photometric surveys of the Milky Way have revealed diffuse structures encircling our Galaxy far beyond the “classical” limits of the stellar disk. This paper reviews results from our own and other observational programs, which together suggest that, despite their extreme positions, the stars in these structures were formed in our Galactic disk. Mounting evidence from recent observations and simulations implies kinematic connections between several of these distinct structures. This suggests the existence of collective disk oscillations that can plausibly be traced all the way to asymmetries seen in the stellar velocity distribution around the Sun. There are multiple interesting implications of these findings: they promise new perspectives on the process of disk heating; they provide direct evidence for a stellar halo formation mechanism in addition to the accretion and disruption of satellite galaxies; and, they motivate searches of current and near-future surveys to trace these oscillations across the Galaxy. Such maps could be used as dynamical diagnostics in the emerging field of “Galactoseismology”, which promises to model the history of interactions between the Milky Way and its entourage of satellites, as well examine the density of our dark matter halo. As sensitivity to very low surface brightness features around external galaxies increases, many more examples of such disk oscillations will likely be identified. Statistical samples of such features not only encode detailed information about interaction rates and mergers, but also about long sought-after dark matter halo densities and shapes. Models for the Milky Way’s own Galactoseismic history will therefore serve as a critical foundation for studying the weak dynamical interactions of galaxies across the universe. View Full-Text
Keywords: galaxies: galaxy formation; galactic disks; stellar halos; Milky Way galaxies: galaxy formation; galactic disks; stellar halos; Milky Way
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Johnston, K.V.; Price-Whelan, A.M.; Bergemann, M.; Laporte, C.; Li, T.S.; Sheffield, A.A.; Majewski, S.R.; Beaton, R.S.; Sesar, B.; Sharma, S. Disk Heating, Galactoseismology, and the Formation of Stellar Halos. Galaxies 2017, 5, 44.

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