High-lying OH Absorption, [C II] Deficits, and Extreme L FIR/M H2 Ratios in Galaxies
Lutz, D.; Genzel, R.; Poglitsch, A.; Sturm, E.; Veilleux, S.; Farrah, D.; Contursi, A.; Martín-Pintado, J.; Verma, A.; van der Werf, P.; Ashby, M. L. N.; García-Burillo, S.; González-Alfonso, E.; Graciá-Carpio, J.; Aalto, S.; Henkel, C.; Spaans, M.; Meijerink, R.; Fischer, J.; Smith, H. A.; Hailey-Dunsheath, S.; Spoon, H. W. W.; Falstad, N.; Meléndez, M.; Blasco, A.
Spain, United States, Germany, Sweden, Saudi Arabia, United Kingdom, Netherlands
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 Π3/2 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 Π1/2 J = 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 cm-2), 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 IR and M H2 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.