Probing the Properties of Gaia-CRF2 Quasars at the Faintest Magnitudes

Abstract

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 five-parameter astrometric solution. We cross-match with Gaia DR2 the quasars from Large Quasar Astrometric Catalogue, Sloan Digital Sky Survey and LAMOST, obtaining 208,743 new sources (denominated as KQCG), and most of them are fainter than G = 19. Given their crucial diagnostic role for characterizing the properties of the celestial reference frame, we aim to probe more accurately the properties of the reference frame at the faintest magnitudes. Combining them with the quasars already identified in DR2, we estimate the moving mean of parallaxes and proper motions. The features of the proper motion are analyzed by means of vectorial spherical harmonics; the scalar field of parallaxes is expanded into spherical harmonics to investigate their spatial correlation, which reveals an angular scale of systematics of ≈18° with a rms amplitude of 13 μas. The global moving mean of the parallaxes and proper motions are calculated. The amplitude of proper motion bias can be up to ∼10 μas yr−1 in some sky regions for both components, and these biases seem to be related to the patterns of the scanning law. The reference frame exhibits a (−6, −5, −5) ± 1 μas yr−1 spin in the northern hemisphere and a spin of (0, +1, +3) ± 1 μas yr−1 in the southern one, which may be caused by the star contamination. The quadrupolar anisotropy of the proper motion filed is analyzed, where a significant quadrupolar signal (the largest component’s amplitude is up to −15 μas yr−1) is found.