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

The merger of two or more galaxies can enhance the inflow material from galactic scales into close environments Active Galactic Nuclei (AGN), obscuring and feeding supermassive black hole (SMBH). Both recent simulations observations AGN in mergers have confirmed that are related to strong nuclear obscuration. However, it is still unclear how obscuration evolves last phases process. We study a sample 60 Luminous Ultra-luminous IR (U/LIRGs) GOALS observed by NuSTAR. find fraction Compton-thick (CT; $N_{\rm H}\geq 10^{24}\rm\,cm^{-2}$) peaks at $74_{-19}^{+14}\%$ late stage, prior coalescence, when nuclei projected separations $d_{\rm sep}\sim 0.4-6$ kpc. A similar peak also median H}$ [$(1.6\pm0.5)\times10^{24}\rm\,cm^{-2}$]. vast majority ($85^{+7}_{-9}\%$) final stages ($d_{\rm sep}\lesssim 10$ kpc) heavily obscured ($N_{\rm 10^{23}\rm\,cm^{-2}$), accreting SMBHs our systematically higher than local hard X-ray selected AGN, regardless stage. This implies these objects very environments, with 10^{23}\rm\,cm^{-2}$ gas almost completely covering mergers. CT tend absorption-corrected luminosities less sources. could either be due an evolutionary effect, sources rapidly because they available their surroundings, selection bias. latter scenario would imply we missing large obscured, lower luminosity ($L_{2-10}\lesssim 10^{43}\rm\,erg\,s^{-1}$) U/LIRGs.