The properties of the known QSOs astrometric solutions in Gaia DR2

Shilong Liao, Zhaoxiang Qi, Beatric Bucciarelli, Sufen Guo, Zihuang Cao, Zhenghong Tang

Published 2018-05-06Version 1

We aim to evaluate the properties of the QSOs astrometric solution in the second release of Gaia Release 2 (Gaia DR2). Besides the QSOs from the AllWISE AGN catalog, we also cross match the QSOs sample from LQAC3, SDSS DRQ14 and LAMOST DR5 with the Gaia DR2 to create the largest quasar sample. With this largest QSOs sample, we estimate the global bias in parallax and proper motion, analysis the proper motion field with the vector spherical harmonics (VSH) functions and compare with the QSOs sample used to materialize the Gaia-CRF2 (hereafter GCRF2 sample). The parallaxes of the two samples are expanded with the scalar spherical harmonics. And finally we carry out the positional comparison between the ICRF2 sources and their counterparts in Gaia DR2. The mean global parallaxes bias for the KQCG quasars is -0.0308 mas, and the median parallax is -0.0283 mas. The roughly quadratic variation with \sim 0.010 mas smaller parallaxes towards the ecliptic poles does not show in KQCG sample. The VSH analysis result of KQCG shows a different rotation compared with the GCRF2 sample in the proper motion filed. The rotations of different samples are not larger than 10\mu as/yr for each axis. The glide vector of KQCG is around (-8,+10,+9)\pm 2.5 \mu as /yr. There is a \sim +0.01 mas/yr bias in the declination direction of proper motion, and the typical glide of the proper motion field is (-9, 8, 0)\pm2\mu as/yr after subtracting the bias in \mu_{\delta}. The quasars sample used to define the Gaia reference frame shows a different rotation in northern \left((2.2, 2.8,-1.5)\pm2.0\mu as/yr\right) and southern \left((9.5, 8.7,0.8)\pm2.3\mu as/yr\right) hemisphere. The scalar harmonics spherical expansion show a significant RMS value about 23 \mu as for the harmonic degree l=4. No significant rotation is found between ICRF2 sources and their counterparts in Gaia DR2.