The Age of the Galactic Stellar Halo from Gaia White Dwarfs
  We use 156 044 white dwarf candidates with $\geq5\sigma$ significant parallax
measurements from the Gaia mission to measure the velocity dispersion of the
Galactic disc; $(\sigma_U,\sigma_V,\sigma_W) = (30.8, 23.9, 20.0)$ km s$^{-1}$.
We identify 142 objects that are inconsistent with disc membership at the
$>5\sigma$ level. This is the largest sample of field halo white dwarfs
identified to date. We perform a detailed model atmosphere analysis using
optical and near-infrared photometry and parallaxes to constrain the mass and
cooling age of each white dwarf. The white dwarf cooling ages of our targets
range from 7 Myr for J1657+2056 to 10.3 Gyr for J1049-7400. The latter provides
a firm lower limit of 10.3 Gyr for the age of the inner halo based on the
well-understood physics of white dwarfs. Including the pre-white dwarf
evolutionary lifetimes, and limiting our sample to the recently formed white
dwarfs with cooling ages of $<500$ Myr, we estimate an age of $10.9 \pm 0.4$
Gyr (internal errors only) for the Galactic inner halo. The coolest white
dwarfs in our sample also give similar results. For example, J1049-7400 has a
total age of 10.9-11.1 Gyr. Our age measurements are consistent with other
measurements of the age of the inner halo, including the white dwarf based
measurements of the globular clusters M4, NGC 6397, and 47 Tuc.
