The Gaia DR2 halo white dwarf population: the luminosity function, mass distribution, and its star formation history

We analyse the volume-limited nearly complete 100 pc sample of 95 halo white dwarf candidates identified by the second data release of Gaia. Based on a detailed population synthesis model, we apply a method that relies on Gaia astrometry and photometry to accurately derive the individual white dwarf parameters. This method is tested with 25 white dwarfs of our sample for which we took optical spectra and performed spectroscopic analysis. We build and analyse the halo white dwarf luminosity function, for which we find for the first time possible evidences of the cut-off, leading to an age estimate of similar or equal to 12 +/- 0.5 Gyr. The mass distribution of the sample peaks at 0.589 M-circle dot, with 71 per cent of the white dwarf masses below 0.6 M-circle dot and just two massive white dwarfs of more than 0.8 M-circle dot. From the age distribution, we find three white dwarfs with total ages above 12 Gyr, of which J1312-4728 is the oldest white dwarf known with an age of 12.41 +/- 0.22 Gyr. We prove that the star formation history is mainly characterized by a burst of star formation that occurred from 10 to 12Gyr in the past, but extended up to 8 Gyr. We also find that the peak of the star formation history is centred at around 11 Gyr, which is compatible with the current age of the Gaia-Enceladus encounter. Finally, 13 per cent of our halo sample is contaminated by high-speed young objects (total age <7Gyr). The origin of these white dwarfs is unclear but their age distribution may be compatible with the encounter with the Sagittarius galaxy.