6 December 2022
Gaia is an astrometric space mission dedicated to measuring the position, distance and motion of stars, developed by the European Space Agency. The challenges of Gaia are too numerous to quote them all. Additionnaly to the impressive progresses were made on the mapping of the Milky Way, the SYRTE teams are working on several segments of this space mission that are crucial for achieving very high astrometric accuracy. In the extragalactic domain of particular interest to us, the maps produced with the satellite show discrepancies between the positions of compact objects (active galactic nuclei or AGN) when reflected at various wavelengths from the radio to the optical domain. These discrepancies, ranging from a few microarcseconds to several tens of milliseconds, reflect the complex nature of AGNs and lead us into unexplored territory.
Indeed, for the first time to our knowledge, it is possible to localise the optical emission in the complex architecture of AGNs: in the disc, in the jet, further away... Understanding the articulation of this radio-optical geometry with the very high energy emission for which the localization is debated,the dynamics of the jet, the spectral class or the photometric and astrometric variability, are all crucial contributions for the realization of reference systems.
The SYRTE Celestial Reference Systems team contributes to Gaia in several ways.
SYRTE researchers and engineers are leading the optical tracking of the Gaia satellite from the ground with the GBOT service, and more specifically in the reduction of these observations. This unusual satellite tracking method combined with traditional techniques allows to know Gaia’s position and velocity with an accuracy better than 150 m in position and 2.5 mm/s in velocity. Because of their experimental nature, the GBOT measurements have undergone a long evaluation campaign and were finally validated by ESA in 2020. They are now used for the Gaia orbit reconstruction that will further improve the astrometric accuracy of a large number of objects in future versions of the Gaia catalogue (DR4 er DR5).
Figure 1. The Gaia CRF3. The "empty" part is the Milky Way...
On the other hand, the SYRTE Celestial Reference Systems team is working on the extragalactic benchmark (see Figure 1) and the identification of quasars among the billions of objects collected by this formidable celestial eye. In particular, the team has contributed to validating the methods for separating quasars from galaxies, making it possible to obtain the purest possible sample of one or the other (see Figure 2). The team also contributes upstream via the establishment of astrometric catalogues such as LQAC (compilation of heterogeneous surveys) or ICRF3 (radio catalogue of comparable precision to Gaia) and used in Gaia for the cross-identification of objects and for statistical studies.
Figure 2. Distribution according to their redshift for candidate galaxies and quasars (QSOs) in Gaia DR3. The blue dashed line corresponds to the selection obtained with the qsoc filter to obtain a contamination-free sample.
The Gaia data, together with the data obtained by VLBI, seem to be a kind of missing link which, by opening the door to multi-wavelength absolute astrometry, will allow our teams to probe in depth these still mysterious celestial objects which nevertheless constitute our best reference points.
For more information:
The Berkeley Prize
Multi-wavelength astrometry
The IERS website
Gaia Collaboration, Klioner SA et al, Gaia Early Data Release 3 - The celestial reference frame (Gaia-CRF3), Astronomy and Astrophysics 667, 148, 2022
A scientific publication on LQAC
On redshift distribution: Babusiaux C et al, Gaia Data Release 3: Catalogue Validation, Astronomy and Astrophysics, 2022