Understand
It has long been known that the vertical motions of Galactic disk stars increase with stellar age, commonly interpreted as vertical heating through orbit scattering.
- Here we map the vertical actions of disk stars as a function of age ($\tau\le$ 8 Gyrs) and across a large range of Galactocentric radii, $\overline{R}_{\rm GC}$, drawing on APOGEE and Gaia data.
- We fit $\widehat{J_z}(\overline{R}_{\rm GC},\tau )$ as a combination of the vertical action at birth, $\widehat{J_{z,0}}$, and subsequent heating $\widehat{\Delta J_z}_{\, {\rm 1Gyr}}(\overline{R}_{\rm GC})$ that scales as $\tau^{\gamma(\overline{R}_{\rm GC})}$.
- The inferred birth temperature, $\widehat{J_{z,0}}(\overline{R}_{\rm GC})$ is 1 kpc km/s for 3 kpc < $\overline{R}_{\rm GC}$ < 10 kpc, consistent with the ISM velocity dispersion; but it rapidly rises outward, to 8 kpc km/s for $\overline{R}_{\rm GC}$ = 14 kpc, likely reflecting the stars' birth in a warped or flared gas disk.