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Astron. Astrophys. 351, 472-476 (1999)

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OH lines in the FIR spectra of the OH megamaser galaxies IRAS 20100-4156 and 3Zw35 *

W.H. Kegel 1, T. Hertenstein 1 and A. Quirrenbach 2

1 Institut für Theoretische Physik der Universität Frankfurt am Main, Postfach 111932, 60054 Frankfurt am Main, Germany
2 University of California, San Diego, Department of Physics, Center of Astrophysics and Space Sciences, 9500 Gilman Drive, La Jolla, CA 92093-0424, USA

Received 12 April 1999 / Accepted 16 August 1999


Using the SWS and LWS instruments on board of the ISO spacecraft, we searched for OH-lines in the FIR spectra of the two OH megamaser galaxies IRAS 20100-4156 and 3 Zw 35. The lines at 34 µ and 53 µ were detected in absorption with certainty in the spectrum of IRAS 20100-4156 and probably in the spectrum of 3 Zw 35. From the measured equivalent widths we derived column densitites of [FORMULA] [FORMULA] cm-2 and [FORMULA] [FORMULA] cm-2, respectively. The 119 µ line was tentatively detected in the spectrum of IRAS 20100-4156, but definitely is much weaker than would be expected from the strengths of the lines at 34 µ and 53 [FORMULA] In the spectrum of 3 Zw 35 the 119 µ line was not found. We take this as an indication that at 119 µ the continuum source is considerably larger than at 34 µ and 53 µ and is covered only partly by the absorbing OH cloud. - The finding that the OH column density is similar in both objects despite substantial differences in their IR and maser luminosities, is taken as support of the hypothesis that OH megamasers are radiatively pumped by the [FORMULA] and/or the [FORMULA] lines.

Key words: galaxies: active – galaxies: individual: IRAS 20100-4156 – galaxies: individual: 3Zw35 – masers – ISM: molecules – infrared: galaxies

* Based on observations made with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with participation of ISAS and NASA.

Send offprint requests to: W.H. Kegel (Kegel@astro.uni-frankfurt.de)

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999