Mega-masers and galaxies
Abstract
▪ Abstract In the Galaxy, microwave radiation can be amplified in the interstellar medium in the immediate neighborhood of young stellar objects, or circumstellar envelopes around evolved stars, resulting in cosmic maser emission. Cosmic masers exist because, in contrast to terrestrial conditions, the interstellar gas density is very low so that level population in molecules is typically not in thermal equilibrium, and sometimes inverted. In the nuclear regions of external galaxies, there exist very powerful OH (λ18 cm) and H 2 O (λ1.35 cm) cosmic masers with line luminosities of ∼ 10 2 −10 4 L ⊙ , ≥ 10 6 times more luminous than typical Galactic maser sources. These are the “mega-masers,” found in high-density molecular gas located within parsecs of active galactic nuclei in the case of H 2 O mega-masers, or within the central 100 pc of nuclear star-burst regions in the case of OH mega-masers. H 2 O mega-masers are most frequently found in galactic nuclei with Seyfert2 or LINER spectral characteristics, in spiral and some elliptical galaxies. OH mega-masers are found in ultra-luminous IR galaxies (ULIRG) with the warmest IR colors, and importantly, the OH luminosity is observed to increase with the IR luminosity: L OH ∝ L 1.2 IR . Because of the extremely high-surface brightness, H 2 O mega-maser emission can be mapped at sub-milli-arc-second resolution by Very Long Baseline Interferometry (VLBI), providing a powerful tool to probe spatial and kinematic distributions of…
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