2010A&A...518L..43G -
Astronomy and Astrophysics, volume 518, L43-43 (2010/7-2)
Herschel observations of water vapour in Markarian 231.
GONZALEZ-ALFONSO E., FISCHER J., ISAAK K., RYKALA A., SAVINI G., SPAANS M., VAN DER WERF P., MEIJERINK R., ISRAEL F.P., LOENEN A.F., VLAHAKIS C., SMITH H.A., CHARMANDARIS V., AALTO S., HENKEL C., WEISS A., WALTER F., GREVE T.R., MARTIN-PINTADO J., NAYLOR D.A., SPINOGLIO L., VEILLEUX S., HARRIS A.I., ARMUS L., LORD S., MAZZARELLA J., XILOURIS E.M., SANDERS D.B., DASYRA K.M., WIEDNER M.C., KRAMER C., PAPADOPOULOS P.P., STACEY G.J., EVANS A.S. and GAO Y.
Abstract (from CDS):
The Ultra luminous infrared galaxy (ULIRG) Mrk231 reveals up to seven rotational lines of water (H
2O) in emission, including a very high-lying (E
upper=640K) 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 78µm 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=110-180pc) and warm (T
dust=85-95K), accounting for at least 45% of the bolometric luminosity. The high column density, N(H
2O)∼5x10
17cm
–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 H
2O lines through dense hot cores, and/or shocks. The H
2O 78µm 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 (Fisher et al.,
2010A&A...518L..41F). 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.
Abstract Copyright:
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Journal keyword(s):
ISM: molecules - galaxies: ISM - galaxies: individual: Mrk 231 - line: formation - infrared: ISM - submillimeter: galaxies
Simbad objects:
3
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