THE OH ZONE IN OH MEGAMASER GALAXIES

The physical conditions of kinetic temperature T-K, dust temperature T -d, OH number density n(OH), H-2 number density n(H-2) velocity fields and microturbulence, in OH-containing clouds in OH megamaser galaxies , have been investigated using Accelerated Lambda Iteration (ALI). Com pact (3 pc) clouds, containing dust with T-d > T-K > 40 K to 50 K, wit h n(H-2) between a few times 10(3) and a few times 10(4) cm(-3) and n( OH) similar to 10(-4) cm(-3), are radiatively pumped to yield dominant 1667 MHz emission, with 1667/1665 flux ratios covering the full range of those observed. Larger clouds, between 25-50 pc and 300 pc, tend t o show weaker 18 cm emission or 18 cm absorption for the same OH colum n densities of similar to 10(15) cm(-2) These clouds yield 6 GHz absor ption, as observed in Arp 220. Velocity fields are influential in dete rmining the emission and absorption properties of clouds via effects o f FIR line overlap. Microturbulence does not however play an important role in determining OH megamaser characteristics. A failure of the pr esent work is the inability to model absorption at 4765 MHz, observed in a number of megamaser galaxies. Using the results of similar to 200 0 models, it is possible to estimate physical conditions in several zo nes in Arp 220, IRAS 17208-0014 and III Zw 35, using the observed 18 c m line ratios, with data for the satellite lines. Models also predict that OH-absorption galaxies, showing absorption in 18 cm and 5 cm main lines, may frequently be accompanied by weak 1720 MHz emission. 1612 MHz emission may be diagnostic of the presence of larger clouds (50-10 0 pc). These and other predictions may be directly investigated by obs ervation. Multi-frequency, phase referenced interferometric data, incl uding continuum maps, are essential for significant progress in this f ield. Our results are consistent with a model in which OH megamaser ga laxies may evolve into OH absorption galaxies, through OH-containing c louds coalescing, becoming depleted in OH and suffering a reduction in dust temperature to equal or below a kinetic temperature of 40 K to 4 5 K.