Gaia Data Release 2: The Celestial reference frame (Gaia-CRF2)
  The second release of Gaia data (Gaia DR2) contains the astrometric
parameters for more than half a million quasars. This set defines a
kinematically non-rotating reference frame in the optical domain referred to as
the Gaia-CRF2. The Gaia-CRF2 is the first realisation of a non-rotating global
optical reference frame that meets the ICRS prescriptions, meaning that it is
built only on extragalactic sources. It consists of the positions of a sample
of 556 869 sources in Gaia DR2, obtained from a positional cross-match with the
ICRF3-prototype and AllWISE AGN catalogues. The sample constitutes a clean,
dense, and homogeneous set of extragalactic point sources in the magnitude
range G from 16 to 21 mag with accurately known optical positions. The median
positional uncertainty is 0.12 mas for G < 18 mag and 0.5 mas at G = 20 mag.
Large-scale systematics are estimated to be in the range 20 to 30 muas. The
accuracy claims are supported by the parallaxes and proper motions of the
quasars in Gaia DR2. The optical positions for a subset of 2820 sources in
common with the ICRF3-prototype show very good overall agreement with the radio
positions, but several tens of sources have significantly discrepant positions.
