2015ApJ...800...69G

Herschel/PACS observations of 29 local (ultra)luminous infrared galaxies, including both starburst and active galactic nucleus (AGN) dominated sources as diagnosed in the mid-infrared/optical, show that the equivalent width of the absorbing OH 65 µm Π_3/2 J = 9/2-7/2 line (W_eq(OH65)) with lower level energy E_low~ 300 K, is anticorrelated with the [C II]158 µm line to far-infrared luminosity ratio, and correlated with the far-infrared luminosity per unit gas mass and with the 60-to-100 µm far-infrared color. While all sources are in the active L_IR/M_H2> 50L_☉/M_☉mode as derived from previous CO line studies, the OH65 absorption shows a bimodal distribution with a discontinuity at L_FIR/M_H2~ 100 L_☉/M_☉. In the most buried sources, OH65 probes material partially responsible for the silicate 9.7 µm absorption. Combined with observations of the OH 71 µm Π_1/2J = 7/2-5/2 doublet (E_low~ 415 K), radiative transfer models characterized by the equivalent dust temperature, T_dust, and the continuum optical depth at 100 µm, τ₁₀₀, indicate that strong [C II]158 µm deficits are associated with far-IR thick (τ₁₀₀ ≳ 0.7, N_H ≳ 10²⁴/cm2), warm (T_dust ≳ 60 K) structures where the OH 65 µm absorption is produced, most likely in circumnuclear disks/tori/cocoons. With their high L_FIR/M_H2 ratios and columns, the presence of these structures is expected to give rise to strong [C II] deficits. W_eq(OH65) probes the fraction of infrared luminosity arising from these compact/warm environments, which is ≳ 30%-50% in sources with high W_eq(OH65). Sources with high W_eq(OH65) have surface densities of both L_IRand M_H2higher than inferred from the half-light (CO or UV/optical) radius, tracing coherent structures that represent the most buried/active stage of (circum)nuclear starburst-AGN co-evolution.