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Herschel observations of water vapour in Markarian 231

González-Alfonso, E. and Armus, L. and Lord, S. and Mazzarella, J. (2010) Herschel observations of water vapour in Markarian 231. Astronomy and Astrophysics, 518 . L43. ISSN 0004-6361.

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The Ultra luminous infrared galaxy (ULIRG) Mrk 231 reveals up to seven rotational lines of water (H_2O) in emission, including a very high-lying (E_(upper) = 640 K) line detected at a 4σ level, within the Herschel/SPIRE wavelength range (190 < λ (μm) < 640), whereas PACS observations show one H_2O line at 78 μm in absorption, as found for other H_2O lines previously detected by ISO. The absorption/emission dichotomy is caused by the pumping of the rotational levels by far-infrared radiation emitted by dust, and subsequent relaxation through lines at longer wavelengths, which allows us to estimate both the column density of H_2O and the general characteristics of the underlying far-infrared continuum source. Radiative transfer models including excitation through both absorption of far-infrared radiation emitted by dust and collisions are used to calculate the equilibrium level populations of H_2O and the corresponding line fluxes. The highest-lying H_2O lines detected in emission, with levels at 300–640 K above the ground state, indicate that the source of far-infrared radiation responsible for the pumping is compact (radius = 110–180 pc) and warm (Tdust = 85–95 K), accounting for at least 45% of the bolometric luminosity. The high column density, N(H_2O) ~ 5×10^(17) cm^(-2), found in this nuclear component, is most probably the consequence of shocks/cosmic rays, an XDR chemistry, and/or an “undepleted chemistry” where grain mantles are evaporated. A more extended region, presumably the inner region of the 1-kpc disk observed in other molecular species, could contribute to the flux observed in low-lying H2O lines through dense hot cores, and/or shocks. The H_2O 78 μm line observed with PACS shows hints of a blue-shifted wing seen in absorption, possibly indicating the occurrence of H_2O in the prominent outflow detected in OH (Fischer et al. 2010, A&A, 518, L41). Additional PACS/HIFI observations of H_2O lines are required to constrain the kinematics of the nuclear component, as well as the distribution of H_2O relative to the warm dust.

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Mazzarella, J.0000-0002-8204-8619
Additional Information:© 2010 ESO. Received 31 March 2010; Accepted 28 April 2010; Published online 16 July 2010. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. We thank the SHINING consortium for proving us with the spectrum of the H2O 423→312, E. Habart for providing us the SPIRE spectrum of the Orion Bar prior to its publication in this volume, and the SPIRE ICC FTS team for their great help in data reduction/analysis. E.G-A is a Research Associate at the Harvard-Smithsonian Center for Astrophysics. Dark Cosmology Centre is funded by DNRF.
Subject Keywords:ISM: molecules; galaxies: ISM; galaxies: individual: Mrk 231; line: formation; infrared: ISM; submillimeter: galaxies
Record Number:CaltechAUTHORS:20101116-100142083
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Official Citation:Herschel observations of water vapour in Markarian 231 E. González-Alfonso, J. Fischer, K. Isaak, A. Rykala, G. Savini, M. Spaans, P. van der Werf, R. Meijerink, F. P. Israel, A. F. Loenen, C. Vlahakis, H. A. Smith, V. Charmandaris, S. Aalto, C. Henkel, A. Weiß, F. Walter, T. R. Greve, J. Martín-Pintado, D. A. Naylor, L. Spinoglio, S. Veilleux, A. I. Harris, L. Armus, S. Lord, J. Mazzarella, E. M. Xilouris, D. B. Sanders, K. M. Dasyra, M. C. Wiedner, C. Kramer, P. P. Papadopoulos, G. J. Stacey, A. S. Evans and Y. Gao A&A 518 L43 (2010) DOI: 10.1051/0004-6361/201014664
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:20830
Deposited By: Jason Perez
Deposited On:17 Nov 2010 19:02
Last Modified:05 Oct 2017 21:23

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