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The merger of two or more galaxies can enhance the inflow of material fromgalactic scales into the close environments of Active Galactic Nuclei (AGN),obscuring and feeding the supermassive black hole (SMBH). Both recentsimulations and observations of AGN in mergers have confirmed that mergers arerelated to strong nuclear obscuration. However, it is still unclear how AGNobscuration evolves in the last phases of the merger process. We study a sampleof 60 Luminous and Ultra-luminous IR galaxies (U/LIRGs) from the GOALS sampleobserved by NuSTAR. We find that the fraction of AGN that are Compton-thick(CT; $N_{\rm H}\geq 10^{24}\rm\,cm^{-2}$) peaks at $74_{-19}^{+14}\%$ at a latemerger stage, prior to coalescence, when the nuclei have projected separationsof $d_{\rm sep}\sim 0.4-6$ kpc. A similar peak is also observed in the median$N_{\rm H}$ [$(1.6\pm0.5)\times10^{24}\rm\,cm^{-2}$]. The vast majority($85^{+7}_{-9}\%$) of the AGN in the final merger stages ($d_{\rm sep}\lesssim10$ kpc) are heavily obscured ($N_{\rm H}\geq 10^{23}\rm\,cm^{-2}$), and themedian $N_{\rm H}$ of the accreting SMBHs in our sample is systematicallyhigher than that of local hard X-ray selected AGN, regardless of the mergerstage. This implies that these objects have very obscured nuclear environments,with the $N_{\rm H}\geq 10^{23}\rm\,cm^{-2}$ gas almost completely covering theAGN in late mergers. CT AGN tend to have systematically higherabsorption-corrected X-ray luminosities than less obscured sources. This couldeither be due to an evolutionary effect, with more obscured sources accretingmore rapidly because they have more gas available in their surroundings, or toa selection bias. The latter scenario would imply that we are still missing alarge fraction of heavily obscured, lower luminosity ($L_{2-10}\lesssim10^{43}\rm\,erg\,s^{-1}$) AGN in U/LIRGs.

A hard X-ray view of Luminous and Ultra-luminous Infrared Galaxies in GOALS: I - AGN obscuration along the merger sequence