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gaia data release 3 documentation

9.6 Data products

9.6.2 Quasars

Here is the list of morphological parameters provided in the qso_candidates table.

  • n_transits: number of transits used for the fitting.

  • intensity_quasar: Intensity of the quasar at the centre obtained with the exponential profile using a fixed scale length (39.4 mas).

  • intensity_quasar_error: Uncertainty on the intensity of the quasar intensity_quasar with the exponential profile.

  • intensity_hostgalaxy: Intensity of the host galaxy at the fitted effective radius (radius_hostgalaxy) [e- s-1]

  • intensity_hostgalaxy_error: Uncertainty on intensity of the host galaxy intensity_hostgalaxy [e- s-1]

  • radius_hostgalaxy: Effective radius of the Sérsic profile containing half of the total luminosity of the host galaxy [mas].

  • radius_hostgalaxy_error: Uncertainty of the effective radius radius_hostgalaxy of the Sérsic profile [mas]

  • sersic_index: Sérsic index of the Sérsic profile describing the host galaxy.

  • sersic_index_error: Uncertainty on the Sérsic index sersic_index of the Sérsic profile.

  • ellipticity_hostgalaxy: Ellipticity (defined as 1-(b/a) where b/a is the axis ratio) of the host galaxy fitted with the Sérsic profile. This parameter is not provided for extremely faint objects for which its determination was not possible.

  • ellipticity_hostgalaxy_error: Uncertainty in the ellipticity ellipticity_hostgalaxy of the host galaxy fitted with the Sérsic profile.

  • posangle_hostgalaxy: Position angle of the host galaxy for the fitted Sérsic profile from North to East []. This parameter is not provided for extremely faint objects for which its determination was not possible.

  • posangle_hostgalaxy_error: Uncertainty in the position angle posangle_hostgalaxy for the Sérsic profile [].

  • host_galaxy_detected: flag indicating the presence (TRUE) or absence (FALSE) of a host galaxy as detected by the pipeline.

  • l2_norm: L2 norm for the combined exponential and Sérsic profiles. This value represents the mean squared error between the integrated flux of all observed samples (from the Sky Mapper and Astrometric Field) and the integrated flux of synthetic samples produced with the fitted profile.

  • morph_params_corr_vec: Vector form of the correlation matrix of the fitted profile parameters, as obtained by morphological fitting of an exponential profile combined with a Sérsic profile.

  • host_galaxy_flag: This flag provides information about the processing or scientific quality of the results of the Galaxy morphology analysis chain for the de Vaucouleurs Profile.

The distribution of these quantities is given in Figure 9.7.

Figure 9.7: Distribution of the parameters fitted for the host galaxies of QSOs: Sérsic index, effective radius, effective intensity, position angle and ellipticity.

Quasars flags

Two flags, host_galaxy_detected and host_galaxy_flag, are given in the table to indicate, respectively, if a host galaxy is detected by Gaia and to indicate the specificity of the profile fitted.

The flags host_galaxy_detected=’false’ indicates that either a host galaxy was not detected or that there was no convergence of fitting and that it is not possible to assess the presence or absence of host galaxy. To select specifically the objects for which there is no host galaxy one should select object with host_galaxy_flag=’3’.

The values taken by host_galaxy_flag and their signification are the following:

  • 1:

    Host Galaxy well measured with circular Sérsic profile

  • 2:

    Host Galaxy well measured with elliptical Sérsic profile

  • 3:

    No host galaxy detected

  • 4:

    Poor solution measured with elliptical Sérsic profile. Some parameters are published while the doubtful are removed.

  • 5:

    No convergence of fitting

  • 6:

    Doubtful solution due to the presence of a secondary source at d < 5 ′′

Only host galaxy radii smaller than 2.5′′have been published ensuring that at least one effective radius of the galaxy is encompassed in the SM windows to avoid to be in the extrapolation regime of the pipeline.

For sources with host_galaxy_flag=5 the fitting did not converge. Some of these sources have a host galaxy and some do not have (as indicated by host_galaxy_detected).

Sources with host_galaxy_flag=6 have a companion source close by that can perturb the fitting and even mimic the presence of a host galaxy. By security, for these sources host_galaxy_detected=’false’ although some sources exhibit clear host galaxy in the visual inspection.

Selection of sub-samples of quasars

To easily retrieve some sub-samples of the quasars analysed, here is a list of typical queries expressed as combination of the two flags host_galaxy_detected and host_galaxy_flag.

Selection of quasars with a host galaxy detected host_galaxy_detected =’true’ Most of the sources selected will have no information about the profile of the host galaxy

Selection of quasars with a host galaxy detected and with measured parameters host_galaxy_detected = ’true’ AND (host_galaxy_flag= ’1’ OR host_galaxy_flag=’2’ OR host_galaxy_flag=’4’)

Selection of quasars with no host galaxy detected host_galaxy_detected =’false’& host_galaxy_flag <’6’