Nature of Compton-thick Active Galactic Nuclei in "Non-merging'' Luminous Infrared Galaxies UGC 2608 and NGC 5135 Revealed with Broadband X-ray Spectroscopy

TL;DR

This study reveals that non-merging luminous infrared galaxies host Compton-thick AGNs with small tori, consistent with radiation pressure effects, contrasting with buried AGNs in merging systems.

Contribution

First broadband X-ray spectral analysis of CT AGNs in non-merging LIRGs using XCLUMPY model, showing typical AGN properties and torus characteristics.

Findings

Both galaxies host Compton-thick AGNs with high column densities.

Torus angular widths are small, indicating thin tori.

Torus covering factors align with radiation pressure models.

Abstract

We have analyzed the broadband X-ray spectra of active galactic nuclei (AGNs) in two "non-merging" luminous infrared galaxies (LIRGs) UGC 2608 and NGC 5135, utilizing the data of NuSTAR, Suzaku, XMM-Newton, and Chandra. Applying the X-ray clumpy-torus model (XCLUMPY: Tanimoto et al. 2019), we find that both sources have similar spectra characterized by Compton-thick (CT) absorption ($N_{\rm H} \sim$ 5-7 $\times$ $10^{24}$ cm$^{-2}$) and small torus angular width ($\sigma$ $<$ 20$^{\circ}$). The intrinsic 2-10 keV luminosities are $3.9^{+2.2}_{-1.7}$ $\times$ $10^{43}$ erg s$^{-1}$ (UGC 2608) and $2.0^{+3.3}_{-1.0}$ $\times$ $10^{43}$ erg s$^{-1}$ (NGC 5135). The [O IV]-to-nuclear-12 $\mu$m luminosity ratios are larger than those of typical Seyferts, which are consistent with the torus covering factors ($C_{\rm T} \lesssim$ 0.7) estimated from the torus angular widths and column densities by X-ray spectroscopy. The torus covering factors and Eddington ratios ($\lambda_{\rm Edd} \sim$ 0.1) follow the relation found by Ricci et al. (2017c) for local AGNs, implying that their tori become geometrically thin due to significant radiation pressure of the AGN that blows out some part of the tori. These results indicate that the CT AGNs in these "non-merger" LIRGs are just a normal AGN population seen edge-on through a large line-of-sight column density. They are in contrast to the buried CT AGNs in late-stage mergers that have large torus covering factors even at large Eddington ratios.