9.7.2 Quasars

To our knowledge, there are no HST-based studies analysing the brightness profile of host galaxies with objects in common with the Gaia list of quasars processed here, essentially due to the magnitude ranges between HST works and Gaia that hardly overlap. We could find comparison data with ground-based survey adjusting a Sérsic as the GAMMA survey, NASATLAS data and in the work of Simard et al. (2011) which is based on SDSS data. But these surveys consider the quasar plus the host galaxy as a whole. The quasar will largely influence the fitted profile observed from ground. The blurring of images resulting from the atmosphere has an impact on the Sérsic index that is very sensible to the slope of the inner profile and consequently on the derive effective radius of the fitted profile. Then the comparison of radii is quite impossible since the effective radius of a Sérsic profile is dependent on the Sérsic index.

There are $\sim \mathrm{226\hspace{0.25em}000}$ quasars processed here that benefit from a Milliquas (Flesch 2021) spectroscopic redshift (TYPE = ’Q’), 2084 from these have a host galaxy detected by Gaia. We present in Figure 9.13 the normalised distribution of the Milliquas redshifts for the quasars with and without a host galaxy detected. Quasars with a host galaxy have small redshifts (mean z=0.54) and quasars for which no host galaxy could be resolved by Gaia have larger redshifts (mean z=1.71), as expected. In very few cases ($\sim$40 sources), the host is detected for larger redshifts. These sources are very faint ($G$ $>$ 20 mag) and suffer either from uncertainties in the light profile fit or in the redshift measurement.

A way to detect the presence of a host galaxy around quasars consists in comparing the mean integrated flux of the sources in the small AF observed window (707 mas $\times$ 2121 mas) with their mean integrated flux in the large SM window (4715 mas $\times$ 2121 mas) (see Figure 9.13). An excess of flux in the SM window when compared to AF window indicates that a host galaxy is detectable. In this diagram, most quasars with a host galaxy lie above the diagonal, in the same locus as the galaxies, confirming the presence of a structure around them.

The distribution of the Sérsic index of the surface brightness profiles of the host galaxies is presented in Figure 9.13. The distribution exhibits a peak around index $\sim$ 0.8 and a mean value of $\sim$ 1.9 which is consistent with quasars being hosted by galaxies with disk-like profiles. This result is in coherence with a recent study of the sizes of quasar host-galaxies in the Hyper Suprime-Cam Subaru Strategic Program (Li et al. 2021).