The Age of the Galactic Stellar Halo from Gaia White Dwarfs

TL;DR

This study uses Gaia white dwarf data to measure the Galactic halo's age, finding it to be approximately 11 billion years, consistent with other methods and providing a new, precise age estimate for the inner halo.

Contribution

It presents the largest sample of field halo white dwarfs and uses their cooling ages to estimate the age of the Galactic inner halo with high precision.

Findings

White dwarf cooling ages range from 7 Myr to 10.3 Gyr.

Estimated age of the inner halo is 10.9 ± 0.4 Gyr.

Results align with ages derived from globular clusters.

Abstract

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.