Obscured and unobscured X-ray AGNs I: Host galaxy properties

TL;DR

This study explores how the properties of host galaxies differ between obscured and unobscured X-ray AGNs, revealing correlations between accretion rates, ionization, metallicity, and obscuration that inform galaxy evolution models.

Contribution

It provides new insights into the relationship between AGN obscuration, host galaxy metallicity, and accretion activity using a spectroscopic X-ray selected sample.

Findings

Unobscured AGNs show stronger correlation between ionization ratio and X-ray luminosity.

High-excitation obscured AGNs reach higher X-ray luminosities.

High-excitation AGNs tend to have low metallicities similar to high-redshift galaxies.

Abstract

Active galactic nuclei (AGN) play a crucial role in galaxy evolution by influencing the observational properties of their host galaxies. We investigate the host galaxy properties of X-ray selected AGNs, focusing on differences between obscured and unobscured AGNs, and between high-(log([OIII]5007/H$\beta$}$>$0.5) and low-excitation sources (log([OIII]5007/H$\beta$$<$0.5). We selected a sample of AGNs from the spectroscopic zCOSMOS survey with 0.5 $< zsp <$ 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 [OIII]5007/H$\beta$ ratio. We analysed various AGN properties, including the hardness ratio, X-ray luminosity, emission line ratios such as the ionisation-level sensitive parameter O32=log([OIII]5007/[OII]3727), and the metallicity sensitive parameter R23=log(([OIII]5007+[OII]3727/H$\beta$), and the specific black hole accretion rate. Unobscured AGNs exhibit a more evident correlation between the [OIII]5007/[OII]3727 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 between the parameters $\lambda$sBHAR and N$_H$, R23 and O32 parameters. The correlation suggests that AGNs with a high specific accretion rate have not only 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 between AGN activity, host galaxy properties, and the role of obscuration in shaping galaxy evolution.