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

4. Discussion

We estimated for two OH megamaser galaxies the OH column densities. Despite the fact that these two galaxies differ in their 1667 MHz luminosity by about a factor of 20 and in their IR luminosity by about a factor of 7, the derived values for are about the same. This finding supports the hypothesis that OH megamasers are radiatively pumped, since the column density (more precisely the column density over the line width) is the most important parameter determining radiative transfer effects. If the maser is pumped by just one IR line, one would expect the maximal output in the maser line at a given intensity of the pump radiation when the optical depth in the pump line is about unity. If the optical depth is small compared to unity, most of the pump photons pass through the OH cloud without being absorbed. If, on the other hand, the optical depth is very large, the larger part of the cloud is not pumped at all. In the unpumped region the absorption coefficient for the 1667 MHz line is positive, implying that the maser radiation emitted at the backside of the cloud is reabsorbed when passing through the unpumped region. - In this context we note that, assuming a microturbulent velocity of 100 km s^-1, the central optical depth of the lines at 34 µ, 53 µ, 79 µ and 119 µ becomes unity (accounting for splitting, but not for hyperfine splitting) for , , and , respectively. Thus, the column densities derived for IRAS 20100-4156 and 3Zw35 are consistent with the general view that OH megamasers are pumped by the 34 µ and/or the 53 µ lines.

This is a very general argument not refering to any detail of the pumping mechanism. In this context we further note that Skinner et al. 1997observed the 34 µ line in the spectrum of Arp 220, another OH megamaser galaxy. From their spectrum we determined , implying a lower limit for the column density of . By a somewhat more detailed analysis the authors derive .

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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