Inside Out and Upside-Down: The Roles of Gas Cooling and Dynamical
  Heating in Shaping the Stellar Age-Velocity Relation
  Kinematic studies of disk galaxies, using individual stars in the Milky Way
or statistical studies of global disk kinematics over time, provide insight
into how disks form and evolve. We use a high-resolution, cosmological
zoom-simulation of a Milky Way-mass disk galaxy h277 to tie together local disk
kinematics and the evolution of the disk over time. The present-day stellar
age-velocity relationship (AVR) of h277 is nearly identical to that of the
analogous solar-neighborhood measurement in the Milky Way. A crucial element of
this success is the simulation's dynamically cold multi-phase ISM, which allows
young stars to form with a low velocity dispersion
($\sigma_{\mathrm{birth}}$$\sim 6 - 8\ \mathrm{km}\, \mathrm{s}^{-1}$) at late
times. Older stars are born kinematically hotter (i.e., the disk settles over
time in an "upside-down" formation scenario), and are subsequently heated after
birth. The disk also grows "inside-out", and many of the older stars in the
solar neighborhood at $z=0$ are present because of radial mixing. We
demonstrate that the evolution of $\sigma_{\mathrm{birth}}$ in h277 can be
explained by the same model used to describe the general decrease in velocity
dispersion observed in disk galaxies from $z\sim 2-3$ to the present-day, in
which the disk evolves in quasi-stable equilibrium and the ISM velocity
dispersion decreases over time due to a decreasing gas fraction. Thus, our
results tie together local observations of the Milky Way's AVR with observed
kinematics of high $z$ disk galaxies.
