Formaldehyde Absorption and Maser Emission in Galaxies

A sensitive search for the 1(10)-1(11)(6.2 cm) H2CO transition toward eight galaxies with prominent H I absorption and OH absorption or emission has resulted in the detection of three weak absorption lines (NGC 660, NGC 3079, and NGC 3628) and, in addition, of H2CO emission toward the OH megamaser galaxy IC 4553 (= Arp 220). This feature is the first extragalactic H2CO maser, and is at least 7 orders of magnitudes more powerful than the two known Galactic maser sources. The maser is likely due to amplification of the strong nonthermal radiation field from IC 4553 by foreground material with inverted level populations. In terms of a radiative transfer model, it is suggested that strong pumping between the J = 2 levels (at 14.5 GHz) causes, via the J = 2-1 millimeter transition, a population inversion in the ground-state doublet. Although the inversion processes are different for these molecules, the scenario for this maser emission is very similar to that proposed for the extragalactic OH and H2O megamasers.