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gaia data release 3 documentation

12.3 Characteristics of the extragalactic sources

12.3.5 Sources with stellar parameters

A number of sources in the qso_candidates and galaxy_candidates tables were also processed by modules aimed at determining stellar parameters, and the latter are available in various Gaia DR3 tables. This is the case for instance for sources with stellar parameters obtained with GSP-Phot and published in the astrophysical_parameters table (Chapter 11 and (Creevey et al. 2023), or those with radial velocities and published in the gaia_source table (Chapter 6 and (Katz et al. 2023)).

Concerning the astrophysical parameters derived from GSP-Phot (Section 11.3.3), there are 255 948 sources with a derived effective temperature in qso_candidates table, and 7069 in the galaxy_candidates table. This module, described in Section 11.3.3, considered all sources with magnitude brighter than 19 mag, having a parallax measurement, and at least 10 and 15 transits in the GBP and GRP bands respectively. The results are then filtered on the basis of convergence criteria of the fits or consistency tests, resulting in about 470 759 263 sources (83.65% of those processed), irrespective of any classification criterion. As such, the overlap of a large number of quasar candidates with this list is not unexpected. Checking various parameters such as magnitude, sky distribution, and effective temperature itself, nothing peculiar comes out. Their presence is an inevitable consequence of the known stellar contamination and it is therefore assumed that they are mostly of stellar nature.

Concerning the radial velocities, those are computed based on spectra templates with a range of stellar parameters.

There are only 4027 such sources in the qso_candidates table, and 160 in the galaxy_candidates table. These small numbers are also due to the fact that radial velocities cannot be determined from faint objects (the GRVS limit in Gaia DR3 is 14), which is what is mostly tabulated in the extragalactic tables. In both tables, all sources are entering the source list though the DSC module. In the qso_candidates table, 2589 of those match the QSOC sample, 40 sources are found in the Surface brightness sample, and 2 belong to the Vari-Classification. In the galaxy_candidates table, 8 of the 160 sources also feature a UGC redshift.

The G magnitude distribution of the corresponding sources is shown in Figure 12.14. As expected, the bulk of the sources featuring a radial velocity in the extragalactic table stem from the DSC sample, and many of them have magnitude too bright to be compatible with genuine extragalactic sources. This is not surprising considering the level of stellar contamination in that sample.

Figure 12.14: Distribution of the G magnitude of the sources with a published radial velocities in the qso_candidates (upper panel) and galaxy_candidates tables (lower panel), for all sources (orange histograms), and separated by the contributing modules. The blue histograms show the distribution of all sources in the respective extragalactic tables (irrespective of having or not a published radial velocity).