Megamaser - History

History

In 1965, twelve years after the first maser was built in a laboratory, a hydroxyl (OH) maser was discovered in the plane of the Milky Way. Masers of other molecules were discovered in the Milky Way in the following years, including water (H₂O), silicon monoxide (SiO), and methanol (CH₃OH). The typical isotropic luminosity for these galactic masers is 10−6–10−3 L_☉. The first evidence for extragalactic masing was detection of the hydroxyl molecule in NGC 253 in 1973, and was roughly ten times more luminous than galactic masers.

In 1982, the first megamaser was discovered in the ultraluminous infrared galaxy Arp 220. The luminosity of the source, assuming it emits isotropically, is roughly 103 L_☉. This luminosity is roughly one hundred million times stronger than the typical maser found in the Milky Way, and so the maser source in Arp 220 was called a megamaser. At this time, extragalactic water (H₂O) masers were already known. In 1984, water maser emission was discovered in NGC 4258 and NGC 1068 that was of comparable strength to the hydroxyl maser in Arp 220, and are as such considered water megamasers.

Over the next decade, megamasers were also discovered for formaldehyde (H₂CO) and methine (CH). Galactic formaldehyde masers are relatively rare, and more formaldehyde megamasers are known than are galactic formaldehyde masers. Methine masers, on the other hand, are quite common in the Milky Way. Both types of megamaser were found in galaxies in which hydroxyl had been detected. Methine is seen in galaxies with hydroxyl absorption, while formaldehyde is found in galaxies with hydroxyl absorption as well as those with hydroxyl megamaser emission.

As of 2007, 109 hydroxyl megamaser sources were known, up to a redshift of . Over 100 extragalactic water masers are known, and of these, 65 are bright enough to be considered megamasers.