The link among X-ray spectral properties, AGN structure, and the host galaxy

Abstract

In this work, we compare the supermassive black hole (SMBH) and host galaxy properties of X-ray obscured and unobscured AGN. For that purpose, we used ∼35 000 X-ray detected AGN in the 4XMM-DR11 catalogue for which there are available measurements for their X-ray spectral parameters, such as the hydrogen column density, NH, and photon index, Γ, from the XMM2Athena Horizon 2020 European project. We constructed the spectral energy distributions (SEDs) of the sources, and we calculated the host galaxy properties via SED fitting analysis, utilising the CIGALE code. We applied strict photometric requirements and quality selection criteria to include only sources with robust X-ray and SED fitting measurements. Our sample consists of 1443 AGN. In the first part of our analysis, we used different NH thresholds (10²³ cm−² or 10²² cm−²) while also taking into account the uncertainties associated with the NH measurements in order to classify these sources as obscured and unobscured (or mildly obscured). We find that obscured AGN tend to live in more massive systems (by ∼0.1 dex) that have a lower star-formation rate, SFR, (by ∼0.25 dex) compared to their unobscured counterparts. However, only the difference in stellar mass, M∗, appears statistically significant (>2σ). The results do not depend on the NH threshold used to classify AGN. The differences in M∗ and SFR are not statistically significant for luminous AGN (log (LX,₂−₁₀ KeV/erg s−¹) > 44). Our findings also show that unobscured AGN have, on average, higher specific black hole accretion rates, λsBHAR, compared to their obscured counterparts, a parameter which is often used as a proxy of the Eddington ratio. In the second part of our analysis, we cross-matched the 1443 X-ray AGN with the SDSS DR16 quasar catalogue of Wu and Shen to obtain information on the SMBH properties of our sources. This resulted in 271 type 1 AGN at z < 1.9. Our findings show that type 1 AGN with increased NH (>10²² cm−²) tend to have higher black hole masses, MBH, compared to AGN with lower NH values at similar M∗. The MBH/M∗ ratio remains consistent for NH values below 10²² cm−², but it exhibits signs of increasing at higher NH values. Finally, we detected a correlation between Γ and Eddington ratios, but only for type 1 sources with NH < 10²² cm−².