Megamaser

A megamaser is a type of astrophysical maser, which is a naturally occurring source of stimulated spectral line emission. Megamasers are distinguished from other astrophysical masers by their large isotropic luminosity. Megamasers have typical luminosities of 10³ solar luminosities, which is 100 million times brighter than masers in the Milky Way, hence the prefix mega. Likewise, the term kilomaser is used to describe masers outside the Milky Way that have luminosities of order, or thousands of times stronger than the average maser in the Milky Way, gigamaser is used to describe masers billions of times stronger than the average maser in the Milky Way, and extragalactic maser encompasses all masers found outside the Milky Way. Most known extragalactic masers are megamasers, and the majority of megamasers are hydroxyl megamasers, meaning the spectral line being amplified is one due to a transition in the hydroxyl molecule. There are known megamasers for three other molecules: water, formaldehyde, and methine.
Water megamasers were the first type of megamaser discovered. The first water megamaser was found in 1979 in NGC 4945, a galaxy in the nearby Centaurus A/M83 Group. The first hydroxyl megamaser was found in 1982 in Arp 220, which is the nearest ultraluminous infrared galaxy to the Milky Way. All subsequent OH megamasers that have been discovered are also in luminous infrared galaxies, and there are a small number of OH kilomasers hosted in galaxies with lower infrared luminosities. Most luminous infrared galaxies have recently merged or interacted with another galaxy, and are undergoing a burst of star formation. Many of the characteristics of the emission in hydroxyl megamasers are distinct from that of hydroxyl masers within the Milky Way, including the amplification of background radiation and the ratio of hydroxyl lines at different frequencies. 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. Zeeman splitting of hydroxyl megamaser lines may be used to measure magnetic fields in the masing regions, and this application represents the first detection of Zeeman splitting in a galaxy other than the Milky Way.
Water megamasers and kilomasers are found primarily associated with active galactic nuclei, while galactic and weaker extragalactic water masers are found in star forming regions. Despite different environments, the circumstances that produce extragalactic water masers do not seem to be very different from those that produce galactic water masers. Observations of water megamasers have been used to make accurate measurements of distances to galaxies in order to provide constraints on the Hubble constant.

Background

Masers

The word maser derives from the acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation". The maser is a predecessor to lasers, which operate at optical wavelengths, and is named by the replacement of "microwave" with "light". Given a system of atoms or molecules, each with different energy states, an atom or molecule may absorb a photon and move to a higher energy level, or the photon may stimulate emission of another photon of the same energy and cause a transition to a lower energy level. Producing a maser requires population inversion, which is when a system has more members in a higher energy level relative to a lower energy level. In such a situation, more photons will be produced by stimulated emission than will be absorbed. Such a system is not in thermal equilibrium, and as such requires special conditions to occur. Specifically, it must have some energy source that can pump the atoms or molecules to the excited state. Once population inversion occurs, a photon with a photon energy corresponding to the energy difference between two states can then produce stimulated emission of another photon of the same energy. The atom or molecule will drop to the lower energy level, and there will be two photons of the same energy, where before there was only one. 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.

Astrophysical masers

Masers and lasers built on Earth and masers that occur in space both require population inversion in order to operate, but the conditions under which population inversion occurs are very different in the two cases. 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. Astrophysical masers are at low densities, and naturally have very long path lengths. At low densities, being out of thermal equilibrium is more easily achieved because thermal equilibrium is maintained by collisions, meaning population inversion can occur. 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." Astrophysical masers may be pumped either radiatively or collisionally. In radiative pumping, infrared photons with higher energies than the maser transition photons preferentially excite atoms and molecules to the upper state in the maser in order to produce population inversion. In collisional pumping, this population inversion is instead produced by collisions that excite molecules to energy levels above that of the upper maser level, and then the molecule decays to the upper maser level by emitting photons.

History

In 1965, twelve years after the first maser was built in a laboratory, a hydroxyl 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, silicon monoxide, and methanol. The typical isotropic luminosity for these galactic masers is. 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. 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 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 and methine. 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.

General requirements

Regardless of the masing molecule, there are a few requirements that must be met for a strong maser source to exist. One requirement is a radio continuum background source to provide the radiation amplified by the maser, as all maser transitions take place at radio wavelengths. 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. These combine to constrain when and where megamaser emission for a given molecule will take place. The specific conditions for each molecule known to produce megamasers are different, as exemplified by the fact that there is no known galaxy that hosts both of the two most common megamaser species, hydroxyl and water. As such, the different molecules with known megamasers will be addressed individually.

Hydroxyl megamasers

hosts the first megamaser discovered, is the nearest ultraluminous infrared galaxy, and has been studied in great detail at many wavelengths. For this reason, it is the prototype of hydroxyl megamaser host galaxies, and is often used as a guide for interpreting other hydroxyl megamasers and their hosts.

Hosts and environment

Hydroxyl megamasers are found in the nuclear region of a class of galaxies called luminous infrared galaxies, with far-infrared luminosities in excess of one hundred billion solar luminosities, or L_FIR >, and ultra-luminous infrared galaxies, with L_FIR > are favored. These infrared luminosities are very large, but in many cases LIRGs are not particularly luminous in visible light. For instance, the ratio of infrared luminosity to luminosity in blue light is roughly 80 for Arp 220, the first source in which a megamaser was observed.
The majority of the LIRGs show evidence of interaction with other galaxies or having recently experienced a galaxy merger, and the same holds true for the LIRGs that host hydroxyl megamasers. Megamaser hosts are rich in molecular gas compared to spiral galaxies, with molecular hydrogen masses in excess of one billion solar masses, or H₂ >. Mergers help funnel molecular gas to the nuclear region of the LIRG, producing high molecular densities and stimulating high star formation rates characteristic of LIRGs. The starlight in turn heats dust, which re-radiates in the far infrared and produces the high L_FIR observed in hydroxyl megamaser hosts. The dust temperatures derived from far-infrared fluxes are warm relative to spirals, ranging from 40–90 K.
The far-infrared luminosity and dust temperature of a LIRG both affect the likelihood of hosting a hydroxyl megamaser, through correlations between the dust temperature and far-infrared luminosity, so it is unclear from observations alone what the role of each is in producing hydroxyl megamasers. LIRGs with warmer dust are more likely to host hydroxyl megamasers, as are ULIRGs, with L_FIR >. At least one out of three ULIRGs hosts a hydroxyl megamaser, as compared with roughly one out of six LIRGs. Early observations of hydroxyl megamasers indicated a correlation between the isotropic hydroxyl luminosity and far-infrared luminosity, with L_OH L_FIR². 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 L_OH L_FIR^1.20.1.
Early spectral classification of the nuclei of the LIRGs that host hydroxyl megamasers indicated that the properties of LIRGs that host hydroxyl megamasers cannot be distinguished from the overall population of LIRGs. Roughly one third of megamaser hosts are classified as starburst galaxies, one quarter are classified as Seyfert 2 galaxies, and the remainder are classified as low-ionization nuclear emission-line regions, or LINERs. The optical properties of hydroxyl megamaser hosts and non-hosts are not significantly different. 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.
The LIRGs that host hydroxyl megamasers may be distinguished from the general population of LIRGs by their molecular gas content. The majority of molecular gas is molecular hydrogen, and typical hydroxyl megamaser hosts have molecular gas densities greater than 1000 cm⁻³. These densities are among the highest mean densities of molecular gas among LIRGs. The LIRGs that host hydroxyl megamasers also have high fractions of dense gas relative to typical LIRGs. The dense gas fraction is measured by the ratio of the luminosity produced by hydrogen cyanide relative to the luminosity of carbon monoxide.