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

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 NOH ≈ (2.2 ± 0.4) ·1017 cm−2 and NOH ≈ (2.0 ± 0.6) ·1017 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 μ. 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 34μ and/or the 53μ lines.