The origin of vertical structure in a simulated galactic disk




Loewen, Nicholas

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We use the APOSTLE simulation suite to study the formation of galactic disks, in order to better understand the origin of their vertical structure. We select a disk-dominated Milky Way analogue galaxy from the simulation which experiences a minimum of external interaction with its environment as a generalizable test case. The simulated stellar disk is found to form upside-down from a gradually thinning, flared gaseous disk, where the rate of thinning is regulated by feedback from in-situ star formation. No significant sources of vertical heating are present in the disk, allowing the vertical structure of newly-formed stellar populations to be preserved over time. As a result, the properties of the stellar disk as a function of age accurately trace the properties of the gaseous disk as a function of time. This allows us to derive a physical model, in which the disk is isothermal, in quasi-hydrostatic equilibrium, and vertically supported by bulk motions rather than thermal pressure, which relates the present-day vertical age-velocity dispersion relation (AVR) at a given radius to the local star formation history as a simple power-law relation, with a best fit power law index $n=1.82$. This relation is then applied to the observed AVRs in the Milky Way from the recent literature, providing a predicted local star formation history for the Milky Way as a function of radius. We then compare this predicted history to others from the literature, in order to test whether our upside-down model from the simulation is consistent with the observed Galaxy. We also examine the observed ratio of vertical to radial velocity dispersions for consistency with our model. While our predicted history is broadly consistent with other predictions, the range of possible histories in the literature makes a more definitive conclusion difficult.



astronomy, astrophysics, galaxy, disk, simulated, vertical