GaiaData Release 2

Abstract

Context. The second release of Gaia data (Gaia DR2) contains the astrometric parameters for more than half a million quasars. This set defines a kinematically non-rotating reference frame in the optical domain. A subset of these quasars have accurate VLBI positions that allow the axes of the reference frame to be aligned with the International Celestial Reference System (ICRF) radio frame. Aims. We describe the astrometric and photometric properties of the quasars that were selected to represent the celestial reference frame of Gaia DR2 (Gaia-CRF2), and to compare the optical and radio positions for sources with accurate VLBI positions. Methods. Descriptive statistics are used to characterise the overall properties of the quasar sample. Residual rotation and orientation errors and large-scale systematics are quantified by means of expansions in vector spherical harmonics. Positional differences are calculated relative to a prototype version of the forthcoming ICRF3. Results. Gaia-CRF2 consists of the positions of a sample of 556 869 sources in Gaia DR2, obtained from a positional cross-match with the ICRF3-prototype and AllWISE AGN catalogues. The sample constitutes a clean, dense, and homogeneous set of extragalactic point sources in the magnitude range G ≃ 16 to 21 mag with accurately known optical positions. The median positional uncertainty is 0.12 mas for G < 18 mag and 0.5 mas at G = mag. Large-scale systematics are estimated to be in the range 20 to 30 μas. The accuracy claims are supported by the parallaxes and proper motions of the quasars in Gaia DR2. The optical positions for a subset of 2820 sources in common with the ICRF3-prototype show very good overall agreement with the radio positions, but several tens of sources have significantly discrepant positions. Conclusions. Based on less than 40% of the data expected from the nominal Gaia mission, Gaia-CRF2 is the first realisation of a non-rotating global optical reference frame that meets the ICRS prescriptions, meaning that it is built only on extragalactic sources. Its accuracy matches the current radio frame of the ICRF, but the density of sources in all parts of the sky is much higher, except along the Galactic equator.

Cosmology with extragalactic proper motions: harmonic formalism, estimators, and forecasts

“…Additionally, a large population of quasars will be observed to pin down the celestial reference frame (see, e.g. Gaia Collaboration et al 2018). There are several potential uses of such a dataset for cosmology (see Darling et al 2018 for a review).…”

Geometric optics in relativistic cosmology: New formulation and a new observable

“…Another issue we would like bring up is connected with the problem of the fixed reference frame. Recall that the parallax is currently measured using the position variation with respect to the non-rotating frame given by a set of distant "fixed quasars" [17,22]. On the other hand, strictly speaking, the definition of parallax in (28) calls for the comparison of the apparent positions using the parallel transport between the observation events.…”

The Grism Lens-Amplified Survey from Space (GLASS) – XIII. G800L optical spectra from the parallel fields

“…GRIZLI first identifies cosmic rays in the F814W and G800L exposures using standard settings with the AstroDrizzle software (Gonzaga & et al 2012), and fits and removes a two dimensional master sky image from the grism exposures. 8 We refine the astrometric alignment including both a fine relative alignment between exposures and a global alignment to an absolute reference frame, here defined by the GAIA DR2 catalogue (Gaia Collaboration et al 2018). As there are no telescope offsets between the paired F814W direct and G800L grism exposures, the alignment of the former is applied directly to the latter.…”