The Milky Way as a High Redshift Galaxy: The Importance of Thick Disk Formation in Galaxies

TL;DR

This paper compares the Milky Way's early star formation and thick disk formation with distant galaxies, suggesting thick disk formation is a common phase in galaxy evolution driven by intense star formation and turbulence.

Contribution

It demonstrates that the thick disk formation in the Milky Way is a typical phase in disk galaxy evolution, supported by star formation history and turbulence models consistent with observations.

Findings

Milky Way formed half its stars by z~1 during a high SFI phase

Thick disk formation linked to intense star formation and turbulence

Models reproduce stellar velocity dispersions consistent with observations

Abstract

We compare the star-formation history and dynamics of the Milky Way (MW) with the properties of distant disk galaxies. During the first ~4 Gyr of its evolution, the MW formed stars with a high star-formation intensity (SFI), Sigma_SFR~0.6 Msun/yr/kpc2 and as a result, generated outflows and high turbulence in its interstellar medium. This intense phase of star formation corresponds to the formation of the thick disk. The formation of the thick disk is a crucial phase which enables the MW to have formed approximately half of its total stellar mass by z~1 which is similar to "MW progenitor galaxies" selected by abundance matching. This agreement suggests that the formation of the thick disk may be a generic evolutionary phase in disk galaxies. Using a simple energy injection-kinetic energy relationship between the 1-D velocity dispersion and SFI, we can reproduce the average perpendicular dispersion in stellar velocities of the MW with age. This relationship, its inferred evolution, and required efficiency are consistent with observations of galaxies from z~0-3. The high turbulence generated by intense star formation naturally resulted in a thick disk, a chemically well-mixed ISM, and is the mechanism that links the evolution of MW to the observed characteristics of distant disk galaxies.