Obscured and unobscured X-ray active galactic nuclei

I. Host-galaxy properties

¹ Instituto de Astronomía Teórica y Experimental, (IATE, CONICET-UNC), Córdoba, Argentina
² Universidad Nacional de Córdoba, Observatorio Astronómico de Córdoba, Laprida 854, X5000BGR, Córdoba, Argentina
³ Consejo de Investigaciones Científicas y Técnicas de la República Argentina – CONICET, Godoy Cruz 2290, C1425FQB, CABA, Argentina
⁴ Facultad de Ciencias Exactas, Físicas y Naturales, Departamento de Geofísica y Astronomía, CONICET Universidad Nacional de San Juan, Av. Ignacio de la Roza 590 (O), J5402DCS, Rivadavia, San Juan, Argentina

^⋆ Corresponding author: cbornancini@unc.edu.ar

Received: 17 March 2025
Accepted: 8 July 2025

Abstract

Context. Active galactic nuclei (AGNs) play a crucial role in galaxy evolution by influencing the observational properties of their host galaxies.

Aims. We aim to investigate the host-galaxy properties of X-ray selected AGNs, focusing on differences between obscured and unobscured AGNs, and between high-excitation sources (log([O III]λ5007/Hβ ≥ 0.5) and low-excitation sources (log([O III]λ5007/Hβ < 0.5).

Methods. We selected a sample of AGNs from the spectroscopic zCOSMOS survey with 0.5 ≤ z_sp ≤ 0.9 based on the mass-excitation (MEx) diagram and X-ray emission. AGNs were classified as obscured or unobscured using hydrogen column density and as high- or low-excitation based on the [O III]λ5007/Hβ ratio. We analysed various AGN properties, including the hardness ratio, X-ray luminosity, and emission-line ratios such as the ionisation-level-sensitive parameter O32 = log([O III]λ5007/[O II]λ3727) and the metallicity-sensitive parameter R23 = log(([O III]λ5007+[O II]λ3727)/Hβ), and the specific black-hole accretion rate (λ_sBHAR).

Results. Unobscured AGNs exhibit a more evident correlation between the [O III]λ5007/[O II]λ3727 ionisation ratio and X-ray luminosity than obscured AGNs, while high-excitation obscured AGNs reach, on average, higher X-ray luminosities. Furthermore, high-excitation AGNs typically show high values of R23, suggesting low metallicities, similar to that observed in high-redshift galaxies (4 <  z <  6). We find a positive correlation among the λ_sBHAR and N_H, R23, and O32 parameters. The correlation suggests that AGNs with a high specific accretion rate not only have a higher production of high-energy photons, which ionise the surrounding medium more intensely, but are also usually associated with environments less enriched in heavy elements. These results provide insights into the complex interplay among AGN activity, host-galaxy properties, and the role of obscuration in shaping galaxy evolution.