Gaia Early Data Release 3: The celestial reference frame (Gaia-CRF3)
  Gaia-CRF3 is the celestial reference frame for positions and proper motions
in the third release of data from the Gaia mission, Gaia DR3 (and for the early
third release, Gaia EDR3, which contains identical astrometric results). The
reference frame is defined by the positions and proper motions at epoch 2016.0
for a specific set of extragalactic sources in the (E)DR3 catalogue.
  We describe the construction of Gaia-CRF3, and its properties in terms of the
distributions in magnitude, colour, and astrometric quality.
  Compact extragalactic sources in Gaia DR3 were identified by positional
cross-matching with 17 external catalogues of quasars (QSO) and active galactic
nuclei (AGN), followed by astrometric filtering designed to remove stellar
contaminants. Selecting a clean sample was favoured over including a higher
number of extragalactic sources. For the final sample, the random and
systematic errors in the proper motions are analysed, as well as the
radio-optical offsets in position for sources in the third realisation of the
International Celestial Reference Frame (ICRF3).
  The Gaia-CRF3 comprises about 1.6 million QSO-like sources, of which 1.2
million have five-parameter astrometric solutions in Gaia DR3 and 0.4 million
have six-parameter solutions. The sources span the magnitude range G = 13 to 21
with a peak density at 20.6 mag, at which the typical positional uncertainty is
about 1 mas. The proper motions show systematic errors on the level of 12
${\mu}$as yr${}^{-1}$ on angular scales greater than 15 deg. For the 3142
optical counterparts of ICRF3 sources in the S/X frequency bands, the median
offset from the radio positions is about 0.5 mas, but exceeds 4 mas in either
coordinate for 127 sources. We outline the future of the Gaia-CRF in the next
Gaia data releases.
