The properties of the quasars astrometric solution in Gaia DR2

Gaia data release 2 (DR2) provides the best non-rotating optical frame aligned with the radio frame (ICRF) thanks to the inclusion of about half-million quasars in the 5-parameter astrometric solution. Given their crucial diagnostic role for characterizing the properties of the celestial reference frame, we aim to make an independent assessment of the astrometry of quasars in DR2. We cross-match with Gaia DR2 the quasars from LQAC3, SDSS and LAMOST, obtaining 208743 new sources (denominated as KQCG). With the quasars already identified in DR2, we estimate the global offsets of parallaxes and proper motions of different quasar subsets to check their astrometric consistency. The features of the proper motion field are analyzed by means of vectorial spherical harmonics (VSH); the scalar field of parallaxes is expanded into spherical harmonics to investigate their spatial correlation. We find a bias of $\sim$ $-0.030$ $mas$ in KQCG parallaxes, and a bias of $-9.1$ $\mu as/yr$ in $\mu_{\alpha\ast}$; a $\sim$ +10 $\mu as/yr$ bias in $\mu_{\delta}$ consistently found in the entire quasar sample. The results of the VSH analysis on different subsets indicate good agreement between them. The proper motion field exhibits a ((-6,-5,-5) $\pm$ 1) $\mu as/yr$ rotation in the northern hemisphere and a rotation of ((0,+1,+3) $\pm$ 1 ) $\mu as /yr$ in the southern one. The spherical harmonics expansion of the parallax field reveals an angular scale of systematics of $\approx$ 18 degrees with a RMS amplitude of 13 $\mu$as. The positional comparison shows that the axes of the current Gaia Celestial Reference Frame and the ICRF2 are aligned within 30 $\mu$as.