Gaia DR2 celestial reference frame sources

Tables <code>aux_iers_gdr2_cross_id</code> and <code>aux_allwise_agn_gdr2_cross_id</code> list sources whose positions in Gaia DR2 define the celestial reference frame of Gaia DR2 (GaiaCRF2). <code>aux_iers_gdr2_cross_id</code> lists 2820 sources in Gaia DR2 identified as the optical counterparts of VLBI sources with accurate VLBI positions in the ICRF. The first column is the IERS name of the VLBI source, the second column is the source identifier in Gaia DR2 of the optical counterpart. <code>aux_allwise_agn_gdr2_cross_id</code> lists 555934 sources in Gaia DR2 cross-matched to sources in the AllWISE AGN catalogue . The first column is the AllWISE identifier, the second column is the source identifier in Gaia DR2. 1885 of the Gaia DR2 source identifiers in table <code>aux_iers_gdr2_cross_id</code> appear also in table <code>aux_allwise_agn_gdr2_cross_id</code>.