A hard X-ray view of luminous and ultra-luminous infrared galaxies in GOALS – I. AGN obscuration along the merger sequence

Abstract

The merger of two or more galaxies can enhance the inflow of material from galactic scales into the close environments of active galactic nuclei (AGNs), obscuring and feeding the supermassive black hole (SMBH). Both recent simulations and observations of AGN in mergers have confirmed that mergers are related to strong nuclear obscuration. However, it is still unclear how AGN obscuration evolves in the last phases of the merger process. We study a sample of 60 luminous and ultra-luminous IR galaxies (U/LIRGs) from the GOALS sample observed by NuSTAR. We find that the fraction of AGNs that are Compton thick (CT; N_H ≥ 10²⁴cm⁻²⁠) peaks at 74⁺¹⁴₋₁₉ per cent at a late merger stage, prior to coalescence, when the nuclei have projected separations (d_(sep)) of 0.4–6 kpc. A similar peak is also observed in the median N_H[(1.6 ± 0.5) × 10²⁴ cm⁻²]. The vast majority (⁠85⁺⁷₋₉ per cent) of the AGNs in the final merger stages (d_(sep) ≲ 10 kpc) are heavily obscured (⁠N_H ≥ 10²³ cm⁻²⁠), and the median N_H of the accreting SMBHs in our sample is systematically higher than that of local hard X-ray-selected AGN, regardless of the merger stage. This implies that these objects have very obscured nuclear environments, with the N_H ≥ 10²³ cm⁻² gas almost completely covering the AGN in late mergers. CT AGNs tend to have systematically higher absorption-corrected X-ray luminosities than less obscured sources. This could either be due to an evolutionary effect, with more obscured sources accreting more rapidly because they have more gas available in their surroundings, or to a selection bias. The latter scenario would imply that we are still missing a large fraction of heavily obscured, lower luminosity (⁠L₂₋₁₀ ≲ 10⁴³ ergs⁻¹⁠) AGNs in U/LIRGs.

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