Megamaser
A megamaser is a type of astrophysical maser, which is a naturally occurring source of stimulated spectral line emission.
The first hydroxyl megamaser was found in 1982 in Arp 220, which is the nearest ultraluminous infrared galaxy to the Milky Way.
The population inversion in hydroxyl molecules is produced by far infrared radiation that results from absorption and re-emission of light from forming stars by surrounding interstellar dust.
Observations of water megamasers have been used to make accurate measurements of distances to galaxies in order to provide constraints on the Hubble constant.
The maser is a predecessor to lasers, which operate at optical wavelengths, and is named by the replacement of "microwave" with "light".
The repetition of this process is what leads to amplification, and since all of the photons are the same energy, the light produced is monochromatic.
Masers in laboratories have systems with high densities, which limits the transitions that may be used for masing, and requires using a resonant cavity in order to bounce light back and forth many times.
Long path lengths provide photons traveling through the medium many opportunities to stimulate emission, and produce amplification of a background source of radiation.
These factors accumulate to "make interstellar space a natural environment for maser operation.
[6] Masers of other molecules were discovered in the Milky Way in the following years, including water (H2O), silicon monoxide (SiO), and methanol (CH3OH).
[8] 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.
[citation needed] The masing molecule must have a pumping mechanism to create the population inversion, and sufficient density and path length for significant amplification to take place.
Arp 220 hosts the first megamaser discovered, is the nearest ultraluminous infrared galaxy, and has been studied in great detail at many wavelengths.
The starlight in turn heats dust, which re-radiates in the far infrared and produces the high LFIR observed in hydroxyl megamaser hosts.
[29] As more hydroxyl megamasers were discovered, and care was taken to account for the Malmquist bias, this observed relationship was found to be flatter, with LOH
[31] Recent infrared observations using the Spitzer Space Telescope are, however, able to distinguish hydroxyl megamaser hosts galaxies from non-masing LIRGs, as 10–25% of hydroxyl megamaser hosts show evidence for an active galactic nucleus, compared to 50–95% for non-masing LIRGs.
[33] The emission of hydroxyl megamasers occurs predominantly in the so-called "main lines" at 1665 and 1667 MHz.
This continuum is primarily composed of synchrotron radiation produced by Type II supernovae.
[38] A few hydroxyl megamasers, including Arp 220, have been observed with very long baseline interferometry (VLBI), which allows sources to be studied at higher angular resolution.
VLBI observations indicate that hydroxyl megamaser emission is composed of two components, one diffuse and one compact.
The diffuse component displays gains of less than a factor of one and linewidths of order hundreds of kilometers per second.
These characteristics are similar to those seen with single dish observations of hydroxyl megamasers that are unable to resolve individual masing components.
There are, however, some regions of extended galactic maser emission from other molecules that resemble the diffuse component of hydroxyl megamasers.
[45] Recent observations with the Spitzer Space Telescope confirm this basic picture, but there are still some discrepancies between details of the model and observations of hydroxyl megamaser host galaxies such as the required dust opacity for megamaser emission.
[32] Hydroxyl megamasers occur in the nuclear regions of LIRGs, and appear to be a marker in the stage of the formation of galaxies.
[47] Detection of hydroxyl megamasers in such galaxies would allow precise determination of the redshift, and aid understanding of star formation in these objects.
By then comparing the physical radius to the angular diameter measured on the sky, the distance to the maser may be determined.
This method is effective with water megamasers because they occur in a small region around an AGN, and have narrow linewidths.
The method is limited, however, by the small number of water megamasers known at distances within the Hubble flow.
