## 3. ResultsOur targets are so faint that we had to use quite long integration
times. Nevertheless, the spectra are rather noisy. Therefore our
measurements are not very accurate. One major problem was to define
the continuum, which showed in some cases an unrealistic apparent
wavelength dependence (see in particular Fig. 1 and Fig. 4). We did
not subtract the off-source from the on-source spectra since the
off-source spectra do not correspond to white noise but show spurious
structures. This introduces an additional uncertainty in the
measurement of the continuum. In particular the equivalent width of
the 53 It appears that our measurements were at the limit of what could be achieved with the ISO spectrometers. To our knowledge this is the first detection of OH lines in LWS spectra of OH megamaser galaxies. ## 3.1. IRAS 20100-4156The lines at 34
For the redshift independent quantity we measured the values given in Table 2. In order to estimate the column density we consider the simple model of a cool OH cloud in front of a bright IR continuum source. For an isolated absorption line then the following relation holds: The equal sign applies in the case that the absorbing cloud fully covers the continuum source and in addition we neglect the spontaneous emission. We further have the relation where is the column density of the molecules in the lower level and of those in the upper level. is the Einstein coefficient for absorption. Eq. (4) gives a (possibly rather crude) estimate of the coulumn density of the molecules in the lower level, which in our case is the rotational ground state. This estimate is close to the true value only if the line is optically thin and if If one wants to account for optical thickness effects one has to measure at least two lines and to calculate curves of growth. For this one has to know the broadening mechanism. In our particular case it is important to note that we measure the absorption by unresolved multiplets rather than by single lines. In calculating a curve of growth one therefore has to account for the individual components caused by doubling and hyperfine splitting. We calculated curves of growth assuming that the individual components of the multiplets are broadened by a turbulent velocity field. We accounted for this in the microturbulent approximation. Furthermore we considered only the case of pure absorption , which is a reasonable assumption as long as the excitation temperature is well below the excitation energy of the upper level. Finally we assumed that the relative population of the individual levels of the rotational ground state corresponds to their statistical weight. This means, we calculated from (2) with and L is the number of components of a given multiplet (6 for the
34 where and
() are the thermal and the turbulent
velocities, respectively. The thermal velocity is negligible as
compared to the turbulent velocity derived from the measured data.
Since the number of components and the splitting is different for the
two lines, we calculated separate curves of growth for the
34 From the equivalent widths of the 34 The microturbulent velocity ( It is obvious that the tentatively measured equivalent width of the
119 As mentioned in the introduction, the primary aim of this project
was to search for an emission in the 115
## 3.2. 3 Zw 35The spectra we obtained for 3 Zw 35 are considerably more noisy
than those for IRAS 20100-4156. There seems to be some absorption at
the positions of the 34 and for the microturbulent velocity a value of
km s
© European Southern Observatory (ESO) 1999 Online publication: November 3, 1999 |