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

¹ Instituto de Fisica de Cantabria (CSIC-Universidad de Cantabria), Avenida de los Castros, 39005 Santander, Spain
e-mail: gmountrichas@gmail.com
² INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
³ Department of Physics, University of Helsinki, PO Box 64 00014 Helsinki, Finland
⁴ Jülich Supercomputing Centre, Forschungszentrum Jülich, Wilhelm-Johnen-Straße, 52428 Jülich, Germany
⁵ Institute of Astronomy, National Tsing Hua University, No. 101 Sect. 2 Kuang-Fu Road, 30013 Hsinchu, Taiwan
⁶ National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Ioannou Metaxa and Vasileos Pavlou, 15236 Athens, Greece

Received: 9 October 2023
Accepted: 27 December 2023

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, N_H, 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 N_H thresholds (10²³ cm⁻² or 10²² cm⁻²) while also taking into account the uncertainties associated with the N_H 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 N_H threshold used to classify AGN. The differences in M_* and SFR are not statistically significant for luminous AGN (log (L_{X,2−10 KeV}/erg s⁻¹) > 44). Our findings also show that unobscured AGN have, on average, higher specific black hole accretion rates, λ_sBHAR, 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 N_H (> 10²² cm⁻²) tend to have higher black hole masses, M_BH, compared to AGN with lower N_H values at similar M_*. The M_BH/M_* ratio remains consistent for N_H values below 10²² cm⁻², but it exhibits signs of increasing at higher N_H values. Finally, we detected a correlation between Γ and Eddington ratios, but only for type 1 sources with N_H < 10²² cm⁻².