The processing of Gaia observations has been optimised for the detection and measurement of point source profiles as produced by distant stars. Nevertheless, Gaia also observes extended extragalactic sources, i.e. unresolved galaxies and host galaxies of quasars. Depending on their luminosity and distance Gaia may be able to resolve their structure if an adequate treatment of the data is applied. This is why a dedicated pipeline has been developed within the DPAC to specifically analyse the surface brightness light profile of these extended extragalactic objects. This pipeline applies different treatment to galaxies and to quasars (with their host galaxy). A detailed description of the morphological treatment of extended extragalactic sources can be found in Ducourant et al. (2023). The data products of this pipeline are stored in the qso_candidates and galaxy_candidates tables that present all data about extragalactic objects as derived by the Gaia DPAC together with qso_catalogue_name that provides for each quasar information on its catalogues of origin.
All observations of Gaia are not transmitted to the on-ground processing centre. The decision to send or not the observed windows is taken by the on-board video processing unit (Gaia Collaboration et al. 2016b), transmitting only light profiles considered ‘steep’ enough. All extended objects will therefore not be transmitted to the ground. The selection function of Gaia with respect to extragalactic objects has been theoretically assessed in de Bruijne et al. (2015) and de Souza et al. (2014). It is expected that the satellite will mainly observe elliptical galaxies. Few discs may also be observed as long as they encompass a bright bulge or host an active galactic nuclei.