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

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1. Introduction

In order to study the pump mechanism in OH megamaser galaxies we performed extensive radiative transfer calculations in an on-the-spot approximation. Our model accounts for radiative pumping by FIR radiation and for the effects of line overlap caused by a molecular outflow (or infall). One of the results is that under conditions leading to population inversion for the 1667 MHz line, we also find weak inversion for the [FORMULA] transition. Motivated by this tentative prediction of an iraser line at 115 µ, we searched for OH lines in the FIR spectra of two OH megamaser galaxies. The spectra were taken with the ISO Short-Wavelength and Long-Wavelength spectrometer, respectively. We searched for the putative iraser line at 115 µ as well as for the lines at 34 µ, 53 µ, and 119 [FORMULA] The latter three lines all connect to the rotational ground state and are predicted by our computations to occur in absorption.

As targets we chose IRAS 20100-4156 and 3 Zw 35. IRAS 20100-4156 is the second most luminous and the second most distant [FORMULA] OH megamaser galaxy known. The (isotropic) luminosity emitted in the 1667 MHz line is of the order of 10[FORMULA] (Staveley-Smith et al. 1989). It belongs to the class of ultraluminous galaxies, having an IR luminosity of about [FORMULA] According to Duc et al. 1997it appears to be a system of two disk galaxies in collision. The R-band image shows clearly two nuclei separated by [FORMULA] or 5.4 kpc.

3 Zw 35 is a more or less average OH megamaser galaxy at a redshift of [FORMULA] The (isotropic) luminosity of its OH maser emission is about 540 [FORMULA] and its IR luminosity is [FORMULA] (Chapman et al. 1990). Similar to IRAS 20100-4156, 3 Zw 35 is a double system. The OH maser emission is associated with the brighter northern component which has been classified by Chapman et al. 1990 either as a LINER or a Seyfert 2 galaxy.

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999