Euclid Quick Data Release (Q1) XL. The impact of AGN emission on SED-derived physical properties

The Euclid Quick Data Release (Q1) is a powerful dataset to study active galactic nuclei (AGN) and their host galaxies. Deriving their physical properties through multi-component spectral energy distribution (SED) fitting is a challenging task for AGN, but it is greatly aided by the Euclid near-infrared photometry. Here we present a new method of quantifying the reliability of SED-derived parameters, such as AGN bolometric and monochromatic luminosities, host's stellar mass (M-*), star-formation rate (SFR), and specific star-formation rate (sSFR), by using mock SEDs of AGN built by combining observed SEDs of QSOs and galaxies. We applied this methodology to the similar to 1 million Q1 AGN candidates, constructing a catalogue of AGN and host galaxy properties, alongside their respective reliability values. With a reliability threshold at 0.5, we find 88% of sources to have robust stellar masses and 76% to have reliable AGN luminosities. Moreover, through SED fitting we also measured the AGN fraction (f(AGN)) of the total mid-infrared flux and we used its lower-limit to select AGN. A(fAGN, low) > 0:075 threshold yields 85% completeness and purity. Comparable to colour-colour AGN selections, this method has the advantage of being less a ffected by redshift evolution and exploring fainter magnitudes. Additionally, by comparing the AGN and host galaxy parameters across di fferent identification methods, we find that the probed range in stellar mass and AGN luminosity can be quite di fferent. This highlights the importance of combining di fferent approaches and accounting for their selection biases when studying AGN and their role in galaxy evolution. Finally, for the X-ray detected sample, we present the X-ray to mid-IR luminosity relation, and the correlation between stellar mass and bolometric luminosity as a function of redshift, in good agreement with previous results.