Aurigaia: mock Gaia DR2 stellar catalogues from the Auriga cosmological
  simulations
  We present and analyse mock stellar catalogues that match the selection
criteria and observables (including uncertainties) of the Gaia satellite data
release 2 (DR2). The source are six cosmological high-resolution
magneto-hydrodynamic $\Lambda$CDM zoom simulations of the formation of Milky
Way analogues from the AURIGA project. Mock data are provided for stars with $V
< 16$ mag, and $V < 20$ mag at $|b|>20$ degrees. The mock catalogues are made
using two different methods: the public SNAPDRAGONS code, and a method based on
that of Lowing et al. that preserves the phase-space distribution of the model
stars. These publicly available catalogues contain 5-parameter astrometry,
radial velocities, multi-band photometry, stellar parameters, dust extinction
values, and uncertainties in all these quantities. In addition, we provide the
gravitational potential and information on the origin of each star. By way of
demonstration, we apply the mock catalogues to analyses of the young stellar
disc and the stellar halo. We show that: i) the young outer stellar disc
exhibits a flared distribution that is detectable in the height and vertical
velocity distribution of A- and B-dwarf stars up to radii of ~15 kpc; and ii)
the spin of the stellar halo out to 100 kpc can be accurately measured with
Gaia DR2 RR Lyrae stars. These catalogues are well suited for comparisons with
observations and should help to: i) develop and test analysis methods for the
Gaia DR2 data; ii) gauge the limitations and biases of the data and iii)
interpret the data in the light of theoretical predictions from realistic $ab$
$initio$ simulations of galaxy formation in the $\Lambda$CDM cosmological
model.
