The age of the Galactic stellar halo fromGaiawhite dwarfs

Abstract

We use 156 044 white dwarf candidates with ≥ 5σ significant parallax measurements from the Gaia mission to measure the velocity dispersion of the Galactic disc; 30.8, 23.9, 20.0) km s −1 . We identify 142 objects that are inconsistent with disc membership at the > 5σ 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 ± 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.

Random Forest identification of the thin disc, thick disc, and halo Gaia-DR2 white dwarf population

“…Finally, in a recent work, Kilic et al (2018) found 142 halo white dwarf candidates by using Gaia-DR2, out of which 55 are within 100 pc from the Sun. Our algorithm has classified 48 of these objects as halo members.…”

“…However, the recent observed Gaia-DR2 provides a new opportunity in the study of the halo white dwarf population (e.g. Kilic et al 2018). Giving its relevance we devote this section to specifically analyze our halo white dwarf sample.…”

“…It is important to note that in the mentioned analysis these objects were discarded as halo members since they were not considered old enough, although two of them are actually older than > 8.5 Gyr. In a more recent analysis, Kilic et al (2018) classified two of these objects (J1312-4728 and J1749+8247) as halo members. In particular, J1749+8247 is a low-mass white dwarf, probably formed in a binary system.…”

“…Winget et al 1987;Garcia-Berro et al 1988;García-Berro et al 1999;Torres et al 2002;Rowell 2013), the halo (e.g. Mochkovitch et al 1990;Isern et al 1998;García-Berro et al 2004;Cojocaru et al 2015;Kilic et al 2018), and the bulge (Calamida et al 2014;Torres et al 2018), or to carry out precise studies of Galactic open and globular clusters (e.g. García-Berro et al 2010;Jeffery et al 2011;Hansen et al 2013;Torres et al 2015).…”

“…These drawbacks forced the traditional membership classification procedure to be based generally on kinematic criteria under the assumption of a zero radial velocity (in some cases equivalently to a zero perpendicular Galactic velocity, W = 0), an assumption that has not been exempt from controversy (e.g. Reid et al 2001;Koopmans & Blandford 2001) due to the existing overlap between the kinematic properties of the different Galactic components. In fact, the kinematic criteria on its own do not guarantee a proper membership classification and a comple-1 Note however that the metallicities of the white dwarf progenitors can be obtained from the main sequence companions if the white dwarfs are in binary systems (Rebassa-Mansergas et al 2016) mentary analysis of the cooling time and age estimate of the white dwarf is also needed (e.g.…”