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gaia data release 3 documentation

9 Surface brightness profiles of extragalactic sources

9.2 Lists of sources

Input lists of objects to be processed by the pipeline were beforehand established.

Galaxies: The list of galaxies to be analysed by our pipeline was established by Krone-Martins et al. (2022). It is based on a fully unsupervised heuristic method based on the usage of a stochastic iterative scheme specifically tailored for the star/galaxy separation problem, and relies on random sampling of the data points and a random selection of the analysed data-space dimension at each iteration, a hierarchical density-based clustering method (HDBSCAN) and the application of an automatically optimised supervised method (a Radial Basis Support Vector Machine) trained on the initial unsupervised solution obtained at each iteration. The method analyses Gaia DR2 mostly combined with the AllWISE reduction of the WISE survey (Cutri et al. 2013). The adopted input list contains 1 883 128 galaxy candidates. The sky density distribution of the objects in this list is presented in galactic coordinates in Figure 9.2, and the resulting systematic patterns in the coverage results from the Gaia scanning law, that has a non-homogeneous spread of the transits over the sky.

Quasars: The list of quasars and quasar candidates was set up by combining the following major catalogues of quasars and candidates: AllWISE (R90 sample) (Assef et al. 2018), AllWISE (Secrest et al. 2015), HMQ (Flesch 2015), LQAC3 (Souchay et al. 2015), SDSS-DR12Q (Pâris et al. 2017), ICRF2 (Ma et al. 2009). A selection of unpublished classifications of Gaia DR2 quasars based on photometric variability(Rimoldini et al. 2019) shared within the Gaia Data Processing and Analysis was aggregated to the compile list. Altogether this represents a list of 1 392 788 sources. Figure 9.1 presents the distribution of the Gaia G magnitude for galaxies and QSOs and Figure 9.2 their distribution on the sky as density plot (galactic coordinates) for galaxies and QSOs. The reader shall be aware that for technical reasons due to the long processing cycles of Gaia pipelines, this compilation of surveys had to be prepared before mid 2018. This explains why more recent surveys or catalogues were not included in this input list. Table qso_catalogue_name provides for each source used in this study information on its catalogues of origin.

Figure 9.1: Distribution of G magnitude of galaxies (left) and QSOs (right) (blue for all QSOs and magenta for quasars with a host galaxy detected by the algorithm).
Figure 9.2: Density plot of the distribution on the sky (galactic coordinates) of galaxies (left) and QSOs (right).