Gaia Early Data Release 3

Abstract

Context. Gaia-CRF3 is the celestial reference frame for positions and proper motions in the third release of data from the Gaia mission, Gaia DR3 (and for the early third release, Gaia EDR3, which contains identical astrometric results). The reference frame is defined by the positions and proper motions at epoch 2016.0 for a specific set of extragalactic sources in the (E)DR3 catalogue. Aims. We describe the construction of Gaia-CRF3 and its properties in terms of the distributions in magnitude, colour, and astrometric quality. Methods. Compact extragalactic sources in Gaia DR3 were identified by positional cross-matching with 17 external catalogues of quasi-stellar objects (QSO) and active galactic nuclei (AGN), followed by astrometric filtering designed to remove stellar contaminants. Selecting a clean sample was favoured over including a higher number of extragalactic sources. For the final sample, the random and systematic errors in the proper motions are analysed, as well as the radio-optical offsets in position for sources in the third realisation of the International Celestial Reference Frame (ICRF3). Results. Gaia-CRF3 comprises about 1.6 million QSO-like sources, of which 1.2 million have five-parameter astrometric solutions in Gaia DR3 and 0.4 million have six-parameter solutions. The sources span the magnitude range G = 13 to 21 with a peak density at 20.6 mag, at which the typical positional uncertainty is about 1 mas. The proper motions show systematic errors on the level of 12 µas yr −1 on angular scales greater than 15 deg. For the 3142 optical counterparts of ICRF3 sources in the S/X frequency bands, the median offset from the radio positions is about 0.5 mas, but it exceeds 4 mas in either coordinate for 127 sources. We outline the future of Gaia-CRF in the next Gaia data releases. Appendices give further details on the external catalogues used, how to extract information about the Gaia-CRF3 sources, potential (Galactic) confusion sources, and the estimation of the spin and orientation of an astrometric solution.

VLBI astrometry on the white dwarf pulsar AR Scorpii

“…The other astrometric parameters were also derived in the same way and the results are listed in Table 4. Note that we used both the Gaia EDR3 position and our VLBI position of J1621−2241 to calculate the position of AR Sco, so each result is with respect to the celestial reference frame of Gaia (Gaia-CRF, Gaia Collaboration et al 2022) and ICRF3, respectively. The position uncertainties of J1621−2241 were taken into account for estimating the position uncertainties of AR Sco.…”

“…The position uncertainties of J1621−2241 were taken into account for estimating the position uncertainties of AR Sco. For the Gaia EDR3 position of J1621−2241, a median radio-optical positional offset of 0.5 mas from Gaia Collaboration et al (2022) was added to the position uncertainties.…”

Finding Quasars behind the Galactic Plane. II. Spectroscopic Identifications of 204 Quasars at ∣b∣ < 20°

“…Quasars are key to understanding the formation and evolution of supermassive black holes and their host galaxies (e.g., Di Matteo et al 2005;Kormendy & Ho 2013), probes of the interstellar and intergalactic mediums at different redshifts (e.g., Weymann et al 1981;Rees 1986;Trump et al 2006), and building blocks of large-scale structures of the universe (e.g., Eisenstein et al 2011;Dawson et al 2013;Blanton et al 2017). Located at cosmic distances and being point sources with small parallaxes and proper motions, quasars are also ideal references for astrometry, with which celestial reference frames can be defined (e.g., Ma et al 2009;Mignard et al 2016;Gaia Collaboration et al 2018, 2022.…”