9.7 Validation
The comparison of the parameters of surface brightness profiles given in Gaia DR3 to others surveys is difficult for several reasons. The first one is that most of these surveys are ground-based and so the blurring due to the atmosphere has a strong impact on the survey’s fitted profiles which make them unsuitable for comparison with Gaia parameters. For instance, considering the Sérsic profile, the adjusted profile from ground will favour larger Sérsic index (see for instance Balcells et al. (2003)). Then the comparison of radii will be quite impossible since the effective radius of a Sérsic profile is dependent on the Sérsic index.
Several studies used Hubble Space Telescope (HST) data to analyse the surface brightness profiles of extragalactic sources, but usually the sources concerned are very faint and thus not detected by Gaia.
Another important point to be considered is the small size of the Gaia windows ( 2.5). They truncate the flux of large objects compared to ground-based surveys that use images encompassing the total flux of the source. The size of the windows of Gaia will favour parameters that well describe the inner part of these large sources.
A large fraction of galaxies host a bulge. For those that are larger than the Gaia window, the fitting done with Gaia mostly reflects the bulge structure. This effect depends on the fraction of the total flux of the source that is included in the observed windows.
The situation is even more complicated for the quasars since, up to our knowledge, none of the major actual surveys (SDSS DR16 (Ahumada et al. 2020), DES DR2 (Abbott et al. 2021), GAMMA DR3 (Driver et al. 2011; Baldry et al. 2018), …) perform a decomposition into quasar plus host galaxy. Usually they provide Sérsic profiles that consider the central quasar and the host galaxy as a whole.
A detailed description of the validation process of the surface brightness profiles given in Gaia DR3 is provided in Ducourant et al. (2023).