The nature of megamasers and megamaser galaxies is briefly reviewed. OH megamasers have luminosities in the OH 18cm lines which are a million times greater than those of the strongest OH masers in our Galaxy. The first were discovered in searches of AGN, but searches of AGN did not achieve much success. It was soon realised that infrared emission is the key: OH megamaser galaxies are powerful infrared sources with distinctive infrared colours. Searches based on IRAS criteria were immediately successful (success rates of 25% or more) and increased the number of known OH megamasers to 60. The OH megamaser galaxies are gas-rich systems with disturbed optical structures which suggest interactions or mergers. It has frequently been suggested that the gas might fuel an obscured active nucleus. Indeed a few OH megamaser galaxies are Seyfert2s. However ISO measurements of the prototype Arp220 provide no evidence for an AGN, but suggest a distributed starburst as the source of the infrared luminosity in this source. ISO data also provide direct evidence for infrared pumping of the megamaser. New ISO results on other megamasers will be presented. VLBI measurements of the structure and kinematics of OH megamasers will be reviewed. In contrast the nature of the other megamaser species is still unclear. Powerful H2O masers have been detected in the nuclei of 16 galaxies but the criteria for maser emission are blurred and the success rates of the surveys are still low (only a few percent). Strong beaming of the H2O maser emission is one possible explanation. Other megamasers such as formaldehyde are even rarer. Most surprisingly there are no methanol megamasers, although methanol is the second most widespread maser species in the Galaxy.
IAU Joint Discussion
- Pub Date: