Megamaser

A megamaser is a naturally occurring cosmic maser—a source that amplifies microwave radiation through stimulated emission, much like a laser but at microwave frequencies. What sets megamasers apart from other astrophysical masers is their enormous isotropic luminosity, typically around 1,000 times that of the Sun. This makes them about 100 million times brighter than typical masers found in the Milky Way, which is why they are called "mega." For context, masers outside the Milky Way are also classified by luminosity: kilomasers are thousands of times brighter than average Milky Way masers, gigamasers are billions of times brighter, and the term "extragalactic maser" covers all masers found beyond our galaxy. Most known extragalactic masers are megamasers, and the majority of these are hydroxyl (OH) megamasers, meaning the amplified spectral line comes from a transition in the hydroxyl molecule. Megamasers have also been detected for three other molecules: water (H₂O), formaldehyde (H₂CO), and methine (CH).

Water megamasers were the first type discovered, in 1979, in the galaxy NGC 4945. The first hydroxyl megamaser was found in 1982 in Arp 220, the nearest ultraluminous infrared galaxy to the Milky Way. All later OH megamasers have been found in luminous infrared galaxies, and a few OH kilomasers exist in galaxies with lower infrared brightness. Most luminous infrared galaxies have recently merged or interacted with another galaxy and are undergoing bursts of star formation. Hydroxyl megamaser emission differs from Milky Way hydroxyl masers in several ways, including the amplification of background radiation and the relative strength of hydroxyl lines at different frequencies. The population inversion in hydroxyl molecules is driven by far-infrared radiation, which comes from starlight absorbed and re-emitted by interstellar dust. Zeeman splitting of hydroxyl megamaser lines can be used to measure magnetic fields in the masing regions, and this was the first detection of Zeeman splitting in a galaxy beyond the Milky Way.

Water megamasers and kilomasers are mainly found near active galactic nuclei, while galactic and weaker extragalactic water masers occur in star-forming regions. Despite these different settings, the conditions that produce extragalactic water masers appear similar to those for galactic water masers. Observations of water megamasers have been used to make precise distance measurements to galaxies, helping to constrain the Hubble constant.

The word "maser" stands for "Microwave Amplification by Stimulated Emission of Radiation." It is the microwave predecessor to the laser, which operates at optical wavelengths. In a system of atoms or molecules with different energy levels, a photon can be absorbed to raise an atom to a higher energy state, or it can stimulate the emission of another identical photon, causing a drop to a lower state. For a maser to work, a population inversion is needed—more atoms or molecules in a higher energy level than a lower one. This is not a thermal equilibrium state, so it requires an external energy source to "pump" the system. Once inversion is achieved, a photon of the right energy triggers stimulated emission, producing a cascade of identical photons, all the same energy, making the output monochromatic.

Astrophysical masers and laboratory masers both need population inversion, but the conditions differ greatly. Laboratory masers operate at high densities, limiting usable transitions, and require a resonant cavity to amplify light. In space, densities are low, making it easier to stay out of thermal equilibrium (since collisions are rare), and the long path lengths give photons many chances to stimulate emission, amplifying background radiation. As a result, interstellar space is a natural environment for masers. Astrophysical masers can be pumped radiatively, where higher-energy infrared photons preferentially excite molecules to the upper maser state, or collisionally, where collisions raise molecules to even higher levels, which then decay to the upper maser state.