The following document lists the file abstract/HSMITH_OHMEGA.abs
from catalogue VI/111.
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The most powerful spectral lines known in the cm wavelength regime are the `Megamaser' lines seen in radio spectra of a group of about 50 galaxies. Each is an Ultra-Luminous Infrared Galaxy with an active nucleus, and the individual OH maser lines can radiate as much as 10,000 solar luminosities. OH maser lines have been observed in the winds of galactic AGB stars and interstellar clouds for more than two decades, but the pumping mechanism for the masers has been the subject of much debate. The most popular models of OH maser sources currently involve radiative pumps, wherein far-IR OH transitions lead to population inversions and thus maser emission. However, it is not clear whether the mechanism is basically the same in all three types of OH maser source (AGB stars, galactic interstellar clouds, and AGNs), or whether the somewhat different conditions in these diffreent types of source generate different pump mechanisms in each. In the LWS Team program we are observing samples of AGB stars and IR-bright active galaxies, in order to directly observe the OH far-IR absorption lines and radiative cascade which are thought to pump the maser lines. In this program we have already observed the prototype Megamaser galaxy, Arp 220, finding that the 34.6 micron OH absorption is so strong that according to radiative pump models it could power the observed maser emission alone, thus strongly supporting such models. The results have also been used to show that a non-AGN (e.g. starburst) model of the active nucleus is strongly favored for this galaxy. However, we will be hampered by the small number of galaxies (three) which can be observed in that proposal. We are therefore proposing here to observe additional sources, in order to acquire a statistically useful sample of IR-bright galaxies with which to critically test the best radiative pump models. The results of this test will be directly compared with those from the associated AGB star OH maser proposals (both LWS Team time and Open Time), thus revealing whether a similar pump mechanism can be invoked for all OH maser sources.