Astrophys. J., 800, 69 (2015/February-2)
High-lying OH absorption, [C II] deficits, and extreme LFIR/MH2 ratios in galaxies.
GONZALEZ-ALFONSO E., FISCHER J., STURM E., GRACIA-CARPIO J., VEILLEUX S., MELENDEZ M., LUTZ D., POGLITSCH A., AALTO S., FALSTAD N., SPOON H.W.W., FARRAH D., BLASCO A., HENKEL C., CONTURSI A., VERMA A., SPAANS M., SMITH H.A., ASHBY M.L.N., HAILEY-DUNSHEATH S., GARCIA-BURILLO S., MARTIN-PINTADO J., VAN DER WERF P., MEIJERINK R. and GENZEL R.
Abstract (from CDS):
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 (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 Π1/2J = 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/cm2), 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 LIRand MH2higher 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.
galaxies: evolution - galaxies: ISM - infrared: galaxies - line: formation
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