The Mass and Age Distribution of Halo White Dwarfs in the Canada–France Imaging Survey

We present optical spectroscopy for 18 halo white dwarfs identified using photometry from the Canada-France Imaging Survey and Pan-STARRS1 DR1 3π survey combined with astrometry from Gaia DR2. The sample contains 13 DA, 1 DZ, 2 DC, and 2 potentially exotic types of white dwarf. We fit both the spectrum and the spectral energy distribution in order to obtain the temperature and surface gravity, which we then convert into mass and then age using stellar isochrones and the initial-to-final mass relation. We find a large spread in ages that is not consistent with expected formation scenarios for the Galactic halo. We find a mean age of ${9.03}_{-2.03}^{+2.13}$ Gyr and a dispersion of ${4.21}_{-1.58}^{+2.33}$ Gyr for the inner halo using a maximum-likelihood method. This result suggests an extended star formation history within the local halo population.