High-lying oh absorption, [C II] deficits, and extreme LFIR /MH2 ratios in galaxies

Abstract

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 ∏3/2 J = 9/2-7/2 line (Weq(OH65)) with lower level energy Elow ≈ 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 LIR /MH2 > 50L⊙/M⊙ mode as derived from previous CO line studies, the OH65 absorption shows a bimodal distribution with a discontinuity at LFIR/MH2 ≈ 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 ∏1/2 J = 7/2-5/2 doublet (Elow ≈ 415 K), radiative transfer models characterized by the equivalent dust temperature, Tdust, 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, NH ≳ 1024 cm-2 ), warm (Tdust ≳ 60 K) structures where the OH 65 μm absorption is produced, most likely in circumnuclear disks/tori/cocoons. With their high LFIR /MH2 ratios and columns, the presence of these structures is expected to give rise to strong [C II] deficits. Weq (OH65) probes the fraction of infrared luminosity arising from these compact/warm environments, which is ≳30%-50% in sources with high Weq (OH65). Sources with high Weq (OH65) have surface densities of both LIR and MH2 higher 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.