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

Carlos G. Bornancini, Gabriel A. Oio, Georgina Coldwell

First listed 2025-07-11 | Last updated 2025-07-11

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$β$}$>$0.5) and low-excitation sources (log([OIII]5007/H$β$$<$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$β$ 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$β$), 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 $λ$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.

Short digest

X-ray selected AGNs from zCOSMOS (0.5<z<0.9) are split by obscuration (NH) and excitation (log[OIII]/Hβ>0.5) to compare host properties. Unobscured sources show a clearer rise of the ionization index O32=[OIII]/[OII] with L2–10 keV than obscured ones, while high-excitation obscured AGNs reach higher X-ray luminosities on average. High-excitation systems also have elevated R23, implying lower gas metallicities reminiscent of z~4–6 galaxies. λsBHAR positively correlates with NH, O32, and R23, linking high specific accretion to harder radiation fields in less enriched environments.

Key figures to inspect

• Figure 1: Check how the adopted cuts (log NH=22 and HR=0.2) partition the sample and whether NH and HR agree; note the tail of heavily absorbed sources and the treatment of non-absorbed objects placed at HR=1.
• Figure 2: Inspect where X-ray detections land on the MEx diagram relative to AGN/composite/SF zones and the horizontal log([OIII]/Hβ)=0.5 line; gauge contamination by SF-classified X-ray emitters.
• Figure 3: Read the HR–L2–10 keV plane with the NH color bar to verify that high-excitation obscured objects cluster at higher Lx and how the AGN/quasar Lx thresholds slice the distribution.
• Figure 4: Test the paper’s key claim by comparing O32 vs L2–10 keV for obscured vs unobscured classes; look for a tighter, steeper relation in unobscured sources and where the high-ionization threshold sits.