Disk Heating, Galactoseismology, and the Formation of Stellar Halos

TL;DR

This paper discusses how diffuse stellar structures around the Milky Way, formed in the galactic disk, reveal insights into disk heating, halo formation, and galaxy interactions through the emerging field of Galactoseismology.

Contribution

It introduces the concept of Galactoseismology as a new approach to study galactic dynamics and dark matter through disk oscillations and stellar halo structures.

Findings

Evidence links halo structures to disk oscillations.

Disk oscillations can trace galaxy interaction history.

Potential to measure dark matter halo properties.

Abstract

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.