810:. 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. The compact components have high gains, ranging from tens to hundreds, high ratios of flux at 1667 MHz to flux at 1665 MHz, and linewidths are of order a few kilometers per second. These general features have been explained by a narrow circumnuclear ring of material from which the diffuse emission arises, and individual masing clouds with sizes of order one
481:
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.
461:
415:(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.
824:
megamasers, as they required a very high fraction of infrared photons to be absorbed by hydroxyl and lead to a maser photon being emitted, making collisional excitation a more plausible pumping mechanism. However, a model of maser emission with a clumpy masing medium appear to be able to reproduce the observed properties of compact and diffuse hydroxyl emission. A recent detailed treatment finds that photons with a wavelength of 53
799:. Amplification of this background is low, with amplification factors, or gains, ranging from a few percent to a few hundred percent, and sources with larger hyperfine ratios typically exhibiting larger gains. Sources with higher gains typically have narrower emission lines. This is expected if the pre-gain linewidths are all roughly the same, as line centers are amplified more than the wings, leading to line narrowing.
196:
763:. The hydroxyl molecule also has two "satellite lines" that emit at 1612 and 1720 MHz, but few hydroxyl megamasers have had satellite lines detected. Emission in all known hydroxyl megamasers is stronger in the 1667 MHz line; typical ratios of the flux in the 1667 MHz line to the 1665 MHz line, called the hyperfine ratio, range from a minimum of 2 to greater than 20. For hydroxyl emitting in
172:, 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
737:
22:
502:
303:
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,
307:
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
814:
that give rise to the compact emission. The hydroxyl masers observed in the Milky Way more closely resemble the compact hydroxyl megamaser components. There are, however, some regions of extended galactic maser emission from other molecules that resemble the diffuse component of hydroxyl megamasers.
297:
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
948:
in the water molecule. The upper state is at an energy corresponding to 643 kelvins about the ground state, and populating this upper maser level requires number densities of molecular hydrogen of order 10 cm or greater and temperatures of at least 300 kelvins. The water molecule comes into thermal
783:
is typically many hundreds of kilometers per second, and individual features that make up the total emission profile have widths ranging from tens to hundreds of kilometers per second. These may also be compared with galactic hydroxyl masers, which typically have linewidths of order a kilometer per
140:
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,
898:
Whereas hydroxyl megamasers seem to be fundamentally distinct in some ways from galactic hydroxyl masers, water megamasers do not seem to require conditions too dissimilar from galactic water masers. Water masers stronger than galactic water masers, some of which are strong enough to be classified
480:
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
302:
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
865:
of z ~ 2, there are LIRG-like galaxies more luminous than the ones in the nearby universe. The observed relationship between the hydroxyl luminosity and far-infrared luminosity suggests that hydroxyl megamasers in such galaxies may be tens to hundreds of times more luminous than observed hydroxyl
610:
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.
492:
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
274:
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
823:
The observed relationship between the luminosity of the hydroxyl line and the far infrared suggests that hydroxyl megamasers are radiatively pumped. Initial VLBI measurements of nearby hydroxyl megamasers seemed to present a problem with this model for compact emission components of hydroxyl
719:
greater than 1000 cm. 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
2401:
Kuo, C. Y.; Braatz, J. A.; Condon, J. J.; Impellizzeri, C. M. V.; Lo, K. Y.; Zaw, I.; Schenker, M.; Henkel, C.; Reid, M. J.; Greene, J. E. (2011). "The
Megamaser Cosmology Project. III. Accurate Masses of Seven Supermassive Black Holes in Active Galaxies with Circumnuclear Megamaser Disks".
981:
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. This method of measuring distances is being used to provide an independent measure of the
683:
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
828:
are the primary pump for main line maser emission, and applies to all hydroxyl masers. In order to provide enough photons at this wavelength, the interstellar dust that reprocesses stellar radiation to infrared wavelengths must have a temperature of at least 45
622:. 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
889:
magnetic field strength. Zeeman splitting has been detected in five hydroxyl megamasers, and the typical strength of a detected field is of order a few milligauss, similar to the field strengths measured in galactic hydroxyl masers.
949:
equilibrium at molecular hydrogen number densities of roughly 10 cm, so this places an upper limit on the number density in a water masing region. Water masers emission has been successfully modelled by masers occurring behind
2351:
Herrnstein, J. R.; Moran, J. M.; Greenhill, L. J.; Diamond, P. J.; Inoue, M.; Nakai, N.; Miyoshi, M.; Henkel, C.; Riess#, A. (1999). "A geometric distance to the galaxy NGC4258 from orbital motions in a nuclear gas disk".
998:. Black hole mass measurements using water megamasers is the most accurate method of mass determination for black holes in galaxies other than the Milky Way. The black hole masses that are measured are consistent with the
262:, 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
957:. These shocks produce the high number densities and temperatures (relative to typical conditions in the interstellar medium) required for maser emission, and are successful in explaining observed masers.
266:, 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
903:
as galactic water masers. Some extragalactic water masers occur in star forming regions, like galactic water masers, while stronger water masers are found in the circumnuclear regions around
2692:
Reid, M. J.; Braatz, J. A.; Condon, J. J.; Greenhill, L. J.; Henkel, C.; Lo, K. Y. (2009). "The megamaser cosmology project. I. Very long baseline interferometric observations of UGC 3789".
2829:
Willett, K.; Darling, J.; Spoon, H.; Charmandaris, V.; Armus, L. (2011). "Mid-infrared properties of OH megamaser host galaxies. II: Analysis and modeling of the maser environment".
779:, in which the 1612 MHz line is often strongest, and of the main lines, 1667 MHz emission is frequently stronger than 1612 MHz. The total width of emission at a given
837:
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
711:
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
446:
866:
megamasers. Detection of hydroxyl megamasers in such galaxies would allow precise determination of the redshift, and aid understanding of star formation in these objects.
1942:
2891:
674:
643:
2790:
Weaver, H.; Williams, D. R. W.; Dieter, N. H.; Lum, W. T. (1965). "Observations of a Strong
Unidentified Microwave Line and of Emission from the OH Molecule".
234:". 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
693:
704:
are, however, able to distinguish hydroxyl megamaser hosts galaxies from non-masing LIRGs, as 10β25% of hydroxyl megamaser hosts show evidence for an
153:
in hydroxyl molecules is produced by far infrared radiation that results from absorption and re-emission of light from forming stars by surrounding
771:, so line ratios greater than 2 are indicative of a population out of thermal equilibrium. This may be compared with galactic hydroxyl masers in
607:
observed in hydroxyl megamaser hosts. The dust temperatures derived from far-infrared fluxes are warm relative to spirals, ranging from 40β90 K.
243:
2577:
2316:
2282:
1891:
2886:
93:
encompasses all masers found outside the Milky Way. Most known extragalactic masers are megamasers, and the majority of megamasers are
165:
in the masing regions, and this application represents the first detection of Zeeman splitting in a galaxy other than the Milky Way.
803:
2589:
Parra, R.; Conway, J. E.; Elitzur, M.; PihlstrΓΆm, Y. M. (2005). "A compact starburst ring traced by clumpy OH megamaser emission".
595:. Mergers help funnel molecular gas to the nuclear region of the LIRG, producing high molecular densities and stimulating high
977:
of water maser spots yields the physical diameter subtended by the maser spots. By then comparing the physical radius to the
2483:
101:
being amplified is one due to a transition in the hydroxyl molecule. There are known megamasers for three other molecules:
882:
363:
145:. Many of the characteristics of the emission in hydroxyl megamasers are distinct from that of hydroxyl masers within the
2119:"Optical Spectral Classification of Major Mergers: OH Megamaser Hosts versus Nonmasing (Ultra)Luminous Infrared Galaxies"
2638:
Randell, J.; Field, D.; Jones, K. N.; Yates, J. A.; Gray, M. D. (August 1995). "The OH zone in OH megamaser galaxies".
861:
in its host LIRG, hydroxyl masers may be useful probes of the conditions where star formation in LIRGs takes place. At
149:, including the amplification of background radiation and the ratio of hydroxyl lines at different frequencies. The
994:. This distance measurement also provides a measurement of the mass of the central object, which in this case is a
553:
are favored. These infrared luminosities are very large, but in many cases LIRGs are not particularly luminous in
886:
764:
1062:
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in another galaxy was made through observations of hydroxyl megamasers. The Zeeman effect is the splitting of a
308:
above that of the upper maser level, and then the molecule decays to the upper maser level by emitting photons.
275:
process is what leads to amplification, and since all of the photons are the same energy, the light produced is
258:
of another photon of the same energy and cause a transition to a lower energy level. Producing a maser requires
2896:
970:
697:
524:
400:
that was of comparable strength to the hydroxyl maser in Arp 220, and are as such considered water megamasers.
137:
1798:
Baan, W. A.; Haschick, A. D. (1984). "The peculiar galaxy IC 4553 β VLA-A observations of the OH megamaser".
124:
Water megamasers were the first type of megamaser discovered. The first water megamaser was found in 1979 in
995:
945:
834:
705:
701:
564:
The majority of the LIRGs show evidence of interaction with other galaxies or having recently experienced a
449:
2049:
Darling, J.; Giovanelli, R. (2002). "A Search for OH Megamasers at z > 0.1. III. The
Complete Survey".
904:
854:
511:
169:
965:
Water megamasers may be used to provide accurate distance determinations to distant galaxies. Assuming a
849:
Hydroxyl megamasers occur in the nuclear regions of LIRGs, and appear to be a marker in the stage of the
990:. The method is limited, however, by the small number of water megamasers known at distances within the
862:
792:
759:
The emission of hydroxyl megamasers occurs predominantly in the so-called "main lines" at 1665 and 1667
419:
210:
129:
999:
1744:
Andreasian, N.; Alloin, D. (October 1994). "More ultraluminous IRAS galaxies as interacting systems".
2848:
2799:
2760:
2711:
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2647:
2608:
2544:
2505:
2460:
2421:
2371:
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2216:
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2103:
2068:
2017:
1974:
1951:
1916:
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1842:
1807:
1776:
1753:
259:
150:
472:(lower) β the microwave emissions from MCG+01-38-005 were used to calculate a refined value for the
380:. This luminosity is roughly one hundred million times stronger than the typical maser found in the
2741:
Robishaw, T.; Quataert, E.; Heiles, C. (2008). "Extragalactic Zeeman
Detections in OH Megamasers".
954:
900:
751:
568:, and the same holds true for the LIRGs that host hydroxyl megamasers. Megamaser hosts are rich in
425:
288:
263:
255:
217:
89:
is used to describe masers billions of times stronger than the average maser in the Milky Way, and
42:
38:
2205:"Global VLBI Observations of the Compact OH Megamaser Emission from III Zw 35 and IRAS 17208β0014"
2864:
2838:
2815:
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2727:
2701:
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2521:
2495:
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2361:
2300:
2189:
2163:
2084:
2058:
2035:
1990:
1940:
Burdiuzha, V. V.; Vikulov, K. A. (May 1990). "The excitation and physical nature of megamasers".
807:
558:
460:
2573:
2312:
2278:
1887:
1003:
858:
796:
784:
second or narrower, and are spread over a velocity of a few to tens of kilometers per second.
776:
659:
628:
600:
154:
29:
that beams out microwave emission rather than visible light (hence the βmβ replacing the βlβ).
2856:
2807:
2768:
2719:
2678:
2674:
2651:
2616:
2612:
2552:
2513:
2468:
2464:
2429:
2379:
2341:
2257:
2224:
2181:
2138:
2076:
2025:
1982:
1978:
1924:
1920:
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1850:
1815:
1784:
1757:
1208:
978:
850:
838:
721:
685:
528:
337:
158:
56:
2472:
2240:
Elitzur, M.; Hollenbach, D. J.; McKee, C. F. (1989). "H2O masers in star-forming regions".
1767:
Baan, W. A.; Wood, P. A. D.; Haschick, A. D. (1982). "Broad hydroxyl emission in IC 4553".
2566:
Moran, James (1976). "Radio
Observations of Galactic Masers". In Avrett, Eugene H. (ed.).
987:
983:
966:
788:
725:
569:
473:
299:
173:
2535:
Mirabel, I. F.; Sanders, D. B. (1987). "OH megamasers in high-luminosity IRAS galaxies".
775:
regions, where the 1665 MHz line is typically strongest, and hydroxyl masers around
611:
LIRGs with warmer dust are more likely to host hydroxyl megamasers, as are ULIRGs, with L
384:, and so the maser source in Arp 220 was called a megamaser. At this time, extragalactic
2852:
2803:
2764:
2715:
2682:
2548:
2509:
2425:
2375:
2253:
2220:
2177:
2134:
2107:
2072:
2021:
1955:
1875:
1846:
1811:
1780:
469:
465:
452:
water masers are known, and of these, 65 are bright enough to be considered megamasers.
355:. The first evidence for extragalactic masing was detection of the hydroxyl molecule in
2305:
2102:. Vol. 340. San Francisco: Astronomical Society of the Pacific. pp. 216β223.
2098:
Darling, Jeremy (2005). "OH Megamasers: Discoveries, Insights, and Future
Directions".
1059:
944:
Water maser emission is observed primarily at 22 GHz, due to a transition between
878:
772:
689:
650:
596:
162:
142:
2723:
907:(AGN). The isotropic luminosities of these span a range of order one to a few hundred
696:, or LINERs. The optical properties of hydroxyl megamaser hosts and non-hosts are not
48:
emission. Megamasers are distinguished from other astrophysical masers by their large
2880:
2860:
2731:
2433:
2391:
2039:
1994:
874:
870:
768:
649:. As more hydroxyl megamasers were discovered, and care was taken to account for the
573:
565:
554:
276:
271:
98:
45:
2868:
2780:
2628:
2525:
2441:
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2193:
2088:
1247:
1022:
404:
251:
195:
110:
736:
21:
2620:
2567:
2272:
1929:
1904:
78:
is used to describe masers outside the Milky Way that have luminosities of order
2100:
Future
Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA
1965:
Burdyuzha, V. V.; Komberg, B. V. (1990). "Powerful masers at the early epochs".
991:
561:
is roughly 80 for Arp 220, the first source in which a megamaser was observed.
1883:
950:
915:
825:
581:
392:
O) masers were already known. In 1984, water maser emission was discovered in
348:
OH). The typical isotropic luminosity for these galactic masers is 10β10
321:
52:
780:
760:
501:
381:
370:
325:
228:
204:
146:
67:
49:
332:
were discovered in the Milky Way in the following years, including water (H
85:, or thousands of times stronger than the average maser in the Milky Way,
2603:
2366:
2063:
974:
926:
745:
712:
577:
527:(LIRGs), with far-infrared luminosities in excess of one hundred billion
397:
393:
341:
329:
304:
239:
125:
94:
2203:
Diamond, P. J.; Lonsdale, C. J.; Lonsdale, C. J.; Smith, H. E. (1999).
1986:
741:
716:
505:
489:
412:
366:
359:
in 1973, and was roughly ten times more luminous than galactic masers.
356:
133:
118:
802:
A few hydroxyl megamasers, including Arp 220, have been observed with
2811:
830:
811:
520:
267:
247:
168:
Water megamasers and kilomasers are found primarily associated with
2772:
2556:
2517:
2328:
Henkel, C.; Wilson, T. L. (March 1990). "OH megamasers explained".
2261:
2229:
2204:
2185:
2143:
2118:
2080:
2030:
2005:
1855:
1830:
1819:
1788:
2843:
2755:
2706:
2500:
2416:
2383:
2168:
2006:"Photometric Study of Galaxies with OH Megamasers in the Infrared"
1002:, an empirical correlation between stellar velocity dispersion in
735:
557:. For instance, the ratio of infrared luminosity to luminosity in
519:
Hydroxyl megamasers are found in the nuclear region of a class of
500:
459:
385:
317:
294:
194:
190:
102:
71:
26:
2484:"The Effect of 53 ΞΌm IR Radiation on 18 cm OH Megamaser Emission"
203:
The word maser derives from the acronym MASER, which stands for "
235:
16:
Astrophysical maser, source of stimulated spectral line emission
791:
continuum of its host. This continuum is primarily composed of
603:, which re-radiates in the far infrared and produces the high L
418:
As of 2007, 109 hydroxyl megamaser sources were known, up to a
2154:
Darling, J. (2007). "A Dense Gas
Trigger for OH Megamasers".
242:, each with different energy states, an atom or molecule may
1866:
Baan, W. A. (1993). "Molecular megamasers after ten years".
653:, this observed relationship was found to be flatter, with L
599:
rates characteristic of LIRGs. The starlight in turn heats
324:, a hydroxyl (OH) maser was discovered in the plane of the
767:, this ratio will range from 1.8 to 1, depending upon the
715:, and typical hydroxyl megamaser hosts have molecular gas
403:
Over the next decade, megamasers were also discovered for
66:), which is 100 million times brighter than masers in the
1870:. Lecture Notes in Physics. Vol. 412. p. 73.
542:, and ultra-luminous infrared galaxies (ULIRGs), with L
1465:
1463:
1006:
and the mass of the central supermassive black hole.
806:(VLBI), which allows sources to be studied at higher
662:
631:
428:
1341:
1339:
132:. The first hydroxyl megamaser was found in 1982 in
362:In 1982, the first megamaser was discovered in the
161:of hydroxyl megamaser lines may be used to measure
2304:
1230:
1228:
787:The radiation amplified by hydroxyl masers is the
700:different. Recent infrared observations using the
668:
637:
508:, the prototypical hydroxyl megamaser host galaxy
440:
369:. The luminosity of the source, assuming it emits
199:Diagram showing the process of stimulated emission
1943:Monthly Notices of the Royal Astronomical Society
1480:
1478:
1440:
1438:
2451:Lo, K. Y. (2005). "Mega-Masers and Galaxies".
2004:Chen, P. S.; Shan, H. G.; Gao, Y. F. (2007).
55:. Megamasers have typical luminosities of 10
8:
2661:Reid, M. J.; Moran, J. M. (1981). "Masers".
1403:
1401:
899:"mega" masers, may be described by the same
881:, and the size of the splitting is linearly
694:low-ionization nuclear emission-line regions
2663:Annual Review of Astronomy and Astrophysics
2453:Annual Review of Astronomy and Astrophysics
708:, compared to 50β95% for non-masing LIRGs.
41:, which is a naturally occurring source of
1905:"Radio properties of FIR-megamaser nuclei"
940:Line characteristics and pumping mechanism
2842:
2754:
2705:
2602:
2499:
2415:
2365:
2228:
2167:
2142:
2062:
2029:
1928:
1854:
953:propagating through dense regions in the
853:. As hydroxyl emission is not subject to
661:
630:
427:
914:, and are found in nearby galaxies like
20:
2892:Astronomical objects discovered in 1982
1696:Elitzur, Hollenbach, & McKee (1989)
1014:
2473:10.1146/annurev.astro.41.011802.094927
1903:Baan, W. A.; KlΓΆckner, H. -R. (2006).
1063:"Charles H. Townes 1964 Nobel Lecture"
692:, and the remainder are classified as
316:In 1965, twelve years after the first
1746:Astronomy and Astrophysics Supplement
1661:Robishaw, Quataert, and Heiles (2008)
1649:Robishaw, Quataert, and Heiles (2008)
740:The 1665 and 1667 MHz maser lines in
7:
2117:Darling, J.; Giovanelli, R. (2006).
1831:"Infrared properties of OH galaxies"
1671:
1307:
1283:
1209:"A Catalog of Galaxies Detected in H
1172:
1138:
1081:
724:(HCN) relative to the luminosity of
25:A megamaser acts as an astronomical
2683:10.1146/annurev.aa.19.090181.001311
1706:
1625:
1345:
1319:
14:
2482:Lockett, P.; Elitzur, M. (2008).
1660:
1648:
1577:
1532:
925:) and more distant galaxies like
804:very long baseline interferometry
1695:
1613:
1601:
1589:
1543:
1469:
1455:
1444:
1430:
1418:
1392:
1380:
1369:
1357:
1330:
1295:
688:, one quarter are classified as
580:masses in excess of one billion
2307:Introduction to Electrodynamics
1485:Baan, Wood, and Haschick (1982)
1248:"From microwaves to megamasers"
1151:Baan, Wood, and Haschick (1982)
1105:
986:that does not rely upon use of
833:. Recent observations with the
1967:Astrophysics and Space Science
1565:
1554:
1127:
1046:
141:and are undergoing a burst of
1:
1717:
1520:
1508:
1496:
1484:
1431:Darling and Giovanelli (2006)
1419:Darling and Giovanelli (2002)
1407:
1393:Darling and Giovanelli (2002)
1358:Darling and Giovanelli (2002)
1331:Darling and Giovanelli (2002)
1296:Darling and Giovanelli (2002)
1271:
1195:
1184:
1161:
1150:
1116:
441:{\displaystyle z\approx 0.27}
364:ultraluminous infrared galaxy
138:ultraluminous infrared galaxy
97:(OH) megamasers, meaning the
2572:. Harvard University Press.
1728:
1626:Burdyuzha and Komberg (1990)
1346:Burdyuzha and Vikulov (1990)
1320:Andreasian and Alloin (1994)
2724:10.1088/0004-637X/695/1/287
1683:
1636:
1234:
1207:Braatz, Jim (May 4, 2010).
1093:
869:The first detection of the
293:Masers and lasers built on
2913:
2887:Astronomical radio sources
2861:10.1088/0004-637X/730/1/56
2640:Astronomy and Astrophysics
2621:10.1051/0004-6361:20052971
2591:Astronomy and Astrophysics
2434:10.1088/0004-637X/727/1/20
2330:Astronomy and Astrophysics
1930:10.1051/0004-6361:20042331
1909:Astronomy and Astrophysics
1602:Lockett and Elitzur (2008)
1590:Lockett and Elitzur (2008)
1381:Lockett and Elitzur (2008)
1370:Mirabel and Sanders (1987)
1196:Chen, Shan, and Gao (2007)
525:luminous infrared galaxies
286:
188:
2831:The Astrophysical Journal
2743:The Astrophysical Journal
2694:The Astrophysical Journal
2569:Frontiers of Astrophysics
2537:The Astrophysical Journal
2488:The Astrophysical Journal
2404:The Astrophysical Journal
2242:The Astrophysical Journal
2209:The Astrophysical Journal
2156:The Astrophysical Journal
1884:10.1007/3-540-56343-1_216
1835:The Astrophysical Journal
1800:The Astrophysical Journal
1769:The Astrophysical Journal
877:due to the presence of a
765:thermodynamic equilibrium
128:, a galaxy in the nearby
2123:The Astronomical Journal
2051:The Astronomical Journal
2010:The Astronomical Journal
1707:Herrnstein et al. (1999)
1509:Baan and Klockner (2006)
1162:Baan and Haschick (1984)
971:centripetal acceleration
946:rotational energy levels
841:for megamaser emission.
669:{\displaystyle \propto }
638:{\displaystyle \propto }
2675:1981ARA&A..19..231R
2652:1995A&A...300..659R
2613:2005A&A...443..383P
2465:2005ARA&A..43..625L
2342:1990A&A...229..431H
2271:Elitzur, Moshe (1992).
1979:1990Ap&SS.171..125B
1921:2006A&A...449..559B
1758:1994A&AS..107...23A
996:supermassive black hole
835:Spitzer Space Telescope
706:active galactic nucleus
702:Spitzer Space Telescope
136:, which is the nearest
1578:Lonsdale et al. (1998)
1533:Lonsdale et al. (1998)
1252:www.spacetelescope.org
1027:www.spacetelescope.org
905:active galactic nuclei
756:
670:
639:
516:
512:Hubble Space Telescope
477:
442:
200:
170:active galactic nuclei
30:
1544:Diamond et al. (1999)
1497:Reid and Moran (1981)
1470:Randell et al. (1995)
1445:Willett et al. (2011)
1117:Reid and Moran (1981)
851:formation of galaxies
793:synchrotron radiation
748:to lower frequencies
739:
671:
640:
504:
497:Hosts and environment
463:
443:
373:, is roughly 10
250:and move to a higher
198:
130:Centaurus A/M83 Group
74:. Likewise, the term
24:
1868:Astrophysical Masers
1829:Baan, W. A. (1989).
1106:Weaver et al. (1965)
1023:"A cosmic megamaser"
732:Line characteristics
660:
629:
456:General requirements
426:
283:Astrophysical masers
260:population inversion
254:, or the photon may
151:population inversion
2853:2011ApJ...730...56W
2804:1965Natur.208...29W
2765:2008ApJ...680..981R
2716:2009ApJ...695..287R
2549:1987ApJ...322..688M
2510:2008ApJ...677..985L
2426:2011ApJ...727...20K
2376:1999Natur.400..539H
2274:Astronomical Masers
2254:1989ApJ...346..983E
2221:1999ApJ...511..178D
2178:2007ApJ...669L...9D
2135:2006AJ....132.2596D
2108:2005ASPC..340..216D
2073:2002AJ....124..100D
2022:2007AJ....133..496C
1956:1990MNRAS.244...86B
1876:1993LNP...412...73B
1847:1989ApJ...338..804B
1812:1984ApJ...279..541B
1781:1982ApJ...260L..49B
1566:Parra et al. (2005)
1555:Parra et al. (2005)
955:interstellar medium
901:luminosity function
752:Arecibo Observatory
485:Hydroxyl megamasers
289:astrophysical maser
264:thermal equilibrium
91:extragalactic maser
70:, hence the prefix
39:astrophysical maser
1987:10.1007/BF00646831
1718:Reid et al. (2009)
1060:Townes, Charles H.
808:angular resolution
797:Type II supernovae
757:
744:, which have been
690:Seyfert 2 galaxies
686:starburst galaxies
666:
635:
529:solar luminosities
517:
478:
438:
328:. Masers of other
256:stimulate emission
201:
57:solar luminosities
31:
2579:978-0-674-32659-0
2318:978-0-13-805326-0
2311:. Prentice Hall.
2284:978-0-7923-1216-1
1893:978-3-540-56343-3
1729:Kuo et al. (2011)
1213:O Maser Emission"
859:interstellar dust
819:Pumping mechanism
576:, with molecular
298:requires using a
155:interstellar dust
2904:
2873:
2872:
2846:
2824:
2823:
2812:10.1038/208029a0
2785:
2784:
2758:
2736:
2735:
2709:
2687:
2686:
2656:
2655:
2633:
2632:
2606:
2604:astro-ph/0507436
2584:
2583:
2561:
2560:
2530:
2529:
2503:
2477:
2476:
2446:
2445:
2419:
2396:
2395:
2369:
2367:astro-ph/9907013
2346:
2345:
2323:
2322:
2310:
2301:Griffiths, David
2295:
2294:
2292:
2291:
2266:
2265:
2235:
2234:
2232:
2198:
2197:
2171:
2149:
2148:
2146:
2112:
2111:
2093:
2092:
2066:
2064:astro-ph/0205185
2044:
2043:
2033:
1999:
1998:
1960:
1959:
1935:
1934:
1932:
1898:
1897:
1861:
1860:
1858:
1824:
1823:
1793:
1792:
1762:
1761:
1731:
1726:
1720:
1715:
1709:
1704:
1698:
1693:
1687:
1681:
1675:
1669:
1663:
1658:
1652:
1646:
1640:
1634:
1628:
1623:
1617:
1611:
1605:
1599:
1593:
1587:
1581:
1575:
1569:
1563:
1557:
1552:
1546:
1541:
1535:
1530:
1524:
1518:
1512:
1506:
1500:
1494:
1488:
1482:
1473:
1467:
1458:
1453:
1447:
1442:
1433:
1428:
1422:
1416:
1410:
1405:
1396:
1390:
1384:
1378:
1372:
1367:
1361:
1355:
1349:
1343:
1334:
1328:
1322:
1317:
1311:
1305:
1299:
1293:
1287:
1281:
1275:
1269:
1263:
1262:
1260:
1258:
1244:
1238:
1232:
1223:
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1220:
1219:
1204:
1198:
1193:
1187:
1182:
1176:
1170:
1164:
1159:
1153:
1148:
1142:
1136:
1130:
1125:
1119:
1114:
1108:
1103:
1097:
1091:
1085:
1079:
1073:
1072:
1070:
1069:
1056:
1050:
1047:Griffiths (2005)
1044:
1038:
1037:
1035:
1033:
1019:
1000:Mβsigma relation
988:standard candles
979:angular diameter
969:, measuring the
894:Water megamasers
755:
722:hydrogen cyanide
675:
673:
672:
667:
644:
642:
641:
636:
515:
447:
445:
444:
439:
338:silicon monoxide
159:Zeeman splitting
2912:
2911:
2907:
2906:
2905:
2903:
2902:
2901:
2897:Radio astronomy
2877:
2876:
2828:
2827:
2789:
2788:
2740:
2739:
2691:
2690:
2660:
2659:
2637:
2636:
2588:
2587:
2580:
2565:
2564:
2534:
2533:
2481:
2480:
2450:
2449:
2400:
2399:
2350:
2349:
2327:
2326:
2319:
2299:
2298:
2289:
2287:
2285:
2270:
2269:
2239:
2238:
2202:
2201:
2153:
2152:
2116:
2115:
2097:
2096:
2048:
2047:
2003:
2002:
1964:
1963:
1939:
1938:
1902:
1901:
1894:
1865:
1864:
1828:
1827:
1797:
1796:
1766:
1765:
1743:
1742:
1739:
1734:
1727:
1723:
1716:
1712:
1705:
1701:
1694:
1690:
1682:
1678:
1670:
1666:
1659:
1655:
1647:
1643:
1635:
1631:
1624:
1620:
1612:
1608:
1600:
1596:
1588:
1584:
1576:
1572:
1564:
1560:
1553:
1549:
1542:
1538:
1531:
1527:
1519:
1515:
1507:
1503:
1495:
1491:
1483:
1476:
1468:
1461:
1454:
1450:
1443:
1436:
1429:
1425:
1417:
1413:
1406:
1399:
1391:
1387:
1379:
1375:
1368:
1364:
1356:
1352:
1344:
1337:
1329:
1325:
1318:
1314:
1306:
1302:
1294:
1290:
1282:
1278:
1270:
1266:
1256:
1254:
1246:
1245:
1241:
1233:
1226:
1217:
1215:
1212:
1206:
1205:
1201:
1194:
1190:
1183:
1179:
1171:
1167:
1160:
1156:
1149:
1145:
1137:
1133:
1126:
1122:
1115:
1111:
1104:
1100:
1092:
1088:
1080:
1076:
1067:
1065:
1058:
1057:
1053:
1045:
1041:
1031:
1029:
1021:
1020:
1016:
1012:
1004:galactic bulges
984:Hubble constant
967:Keplerian orbit
963:
942:
935:
932:
924:
921:
913:
910:
896:
847:
821:
749:
734:
726:carbon monoxide
679:
658:
657:
656:
648:
627:
626:
625:
621:
618:
614:
606:
594:
591:
587:
574:spiral galaxies
552:
549:
545:
541:
538:
534:
509:
499:
487:
474:Hubble constant
458:
424:
423:
410:
391:
379:
376:
354:
351:
347:
335:
320:was built in a
314:
300:resonant cavity
291:
285:
193:
187:
182:
174:Hubble constant
163:magnetic fields
116:
108:
84:
81:
65:
62:
17:
12:
11:
5:
2910:
2908:
2900:
2899:
2894:
2889:
2879:
2878:
2875:
2874:
2825:
2786:
2773:10.1086/588031
2737:
2700:(1): 287β291.
2688:
2657:
2634:
2585:
2578:
2562:
2557:10.1086/165764
2531:
2518:10.1086/533429
2478:
2459:(1): 625β676.
2447:
2397:
2347:
2336:(2): 431β440.
2324:
2317:
2296:
2283:
2267:
2262:10.1086/168080
2236:
2230:10.1086/306681
2199:
2186:10.1086/523756
2150:
2144:10.1086/508513
2113:
2094:
2081:10.1086/341166
2045:
2031:10.1086/510130
2000:
1961:
1936:
1899:
1892:
1862:
1856:10.1086/167237
1825:
1820:10.1086/161918
1794:
1789:10.1086/183868
1763:
1738:
1735:
1733:
1732:
1721:
1710:
1699:
1688:
1686:, pp. 629β630.
1676:
1674:, pp. 314β316.
1672:Elitzur (1992)
1664:
1653:
1641:
1639:, pp. 656β657.
1629:
1618:
1614:Darling (2005)
1606:
1594:
1582:
1580:, pp. L15βL16.
1570:
1558:
1547:
1536:
1525:
1513:
1501:
1499:, pp. 247β251.
1489:
1474:
1459:
1456:Darling (2007)
1448:
1434:
1423:
1421:, pp. 118β120.
1411:
1397:
1395:, pp. 117β118.
1385:
1373:
1362:
1350:
1335:
1333:, pp. 115β116.
1323:
1312:
1308:Elitzur (1992)
1300:
1288:
1286:, pp. 308β310.
1284:Elitzur (1992)
1276:
1264:
1239:
1224:
1210:
1199:
1188:
1177:
1173:Elitzur (1992)
1165:
1154:
1143:
1139:Elitzur (1992)
1131:
1120:
1109:
1098:
1096:, pp. 628β629.
1086:
1082:Elitzur (1992)
1074:
1051:
1049:, pp. 350β351.
1039:
1013:
1011:
1008:
962:
959:
941:
938:
933:
930:
922:
919:
911:
908:
895:
892:
879:magnetic field
846:
843:
820:
817:
733:
730:
677:
665:
654:
651:Malmquist bias
646:
634:
623:
619:
616:
612:
604:
597:star formation
592:
589:
585:
550:
547:
543:
539:
536:
532:
498:
495:
486:
483:
457:
454:
437:
434:
431:
408:
389:
377:
374:
352:
349:
345:
333:
313:
310:
287:Main article:
284:
281:
189:Main article:
186:
183:
181:
178:
143:star formation
114:
106:
82:
79:
63:
60:
15:
13:
10:
9:
6:
4:
3:
2:
2909:
2898:
2895:
2893:
2890:
2888:
2885:
2884:
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2866:
2862:
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2845:
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2836:
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2801:
2797:
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2729:
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2575:
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2423:
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2409:
2405:
2398:
2393:
2389:
2385:
2384:10.1038/22972
2381:
2377:
2373:
2368:
2363:
2360:(6744): 539.
2359:
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2015:
2011:
2007:
2001:
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928:
917:
906:
902:
893:
891:
888:
887:line-of-sight
884:
880:
876:
875:spectral line
872:
871:Zeeman effect
867:
864:
860:
856:
852:
844:
842:
840:
836:
832:
827:
818:
816:
813:
809:
805:
800:
798:
794:
790:
785:
782:
778:
777:evolved stars
774:
770:
769:optical depth
766:
762:
753:
747:
743:
738:
731:
729:
727:
723:
718:
714:
709:
707:
703:
699:
698:significantly
695:
691:
687:
681:
663:
652:
632:
615:> 10
608:
602:
598:
588:> 10
583:
579:
575:
571:
570:molecular gas
567:
566:galaxy merger
562:
560:
556:
555:visible light
546:> 10
535:> 10
530:
526:
522:
513:
507:
503:
496:
494:
491:
484:
482:
475:
471:
470:MCG+01-38-005
467:
466:MCG+01-38-004
462:
455:
453:
451:
450:extragalactic
435:
432:
429:
421:
416:
414:
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387:
383:
372:
371:isotropically
368:
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343:
339:
331:
327:
323:
319:
311:
309:
306:
301:
296:
290:
282:
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277:monochromatic
273:
272:photon energy
269:
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100:
99:spectral line
96:
92:
88:
77:
73:
69:
58:
54:
51:
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46:spectral line
44:
40:
37:is a type of
36:
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2795:
2791:
2746:
2742:
2697:
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2662:
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2568:
2540:
2536:
2491:
2487:
2456:
2452:
2407:
2403:
2357:
2353:
2333:
2329:
2306:
2288:. Retrieved
2277:. Springer.
2273:
2245:
2241:
2212:
2208:
2159:
2155:
2126:
2122:
2099:
2054:
2050:
2013:
2009:
1973:(1β2): 125.
1970:
1966:
1947:
1941:
1912:
1908:
1867:
1838:
1834:
1803:
1799:
1772:
1768:
1749:
1745:
1724:
1713:
1702:
1691:
1679:
1667:
1656:
1644:
1632:
1621:
1609:
1597:
1585:
1573:
1561:
1550:
1539:
1528:
1523:, pp. 74β76.
1516:
1504:
1492:
1451:
1426:
1414:
1388:
1376:
1365:
1353:
1326:
1315:
1303:
1291:
1279:
1274:, pp. 80β81.
1267:
1255:. Retrieved
1251:
1242:
1216:. Retrieved
1202:
1191:
1180:
1168:
1157:
1146:
1134:
1128:Moran (1976)
1123:
1112:
1101:
1089:
1084:, pp. 56β58.
1077:
1066:. Retrieved
1054:
1042:
1030:. Retrieved
1026:
1017:
964:
961:Applications
943:
897:
883:proportional
868:
848:
845:Applications
822:
801:
795:produced by
786:
773:star-forming
758:
710:
682:
609:
582:solar masses
572:compared to
563:
518:
488:
479:
468:(upper) and
417:
405:formaldehyde
402:
361:
315:
292:
252:energy level
229:
222:
218:
214:mplification
211:
205:
202:
167:
123:
111:formaldehyde
90:
86:
75:
34:
32:
18:
2646:: 659β674.
2129:(6): 2596.
1521:Baan (1993)
1408:Baan (1989)
1272:Baan (1993)
1185:Baan (1993)
1032:26 December
992:Hubble flow
951:shock waves
826:micrometres
448:. Over 100
340:(SiO), and
2881:Categories
2749:(2): 981.
2597:(2): 383.
2494:(2): 985.
2290:2010-12-24
2215:(1): 178.
2057:(1): 100.
2016:(2): 496.
1915:(2): 559.
1737:References
1218:2010-08-20
1068:2010-12-25
929:(120
918:(0.8
916:Messier 51
855:extinction
746:redshifted
559:blue light
322:laboratory
221:timulated
180:Background
53:luminosity
43:stimulated
2844:1101.4946
2837:(1): 56.
2756:0803.1832
2732:119205037
2707:0811.4345
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2392:204995005
2169:0710.1080
2162:(1): L9.
2040:122101526
1995:121736761
1950:: 86β92.
1752:: 23β28.
1684:Lo (2005)
1651:, p. 981.
1637:Lo (2005)
1616:, p. 217.
1604:, p. 991.
1592:, p. 985.
1568:, p. 394.
1511:, p. 559.
1487:, p. L51.
1472:, p. 660.
1383:, p. 986.
1310:, p. 309.
1257:28 August
1237:, p. 668.
1235:Lo (2005)
1175:, p. 315.
1141:, p. 308.
1094:Lo (2005)
863:redshifts
781:frequency
717:densities
664:∝
633:∝
464:Galaxies
433:≈
382:Milky Way
330:molecules
326:Milky Way
240:molecules
147:Milky Way
117:CO), and
87:gigamaser
76:kilomaser
68:Milky Way
50:isotropic
35:megamaser
2869:51362028
2781:13875219
2629:17406397
2526:10181212
2442:43300756
2303:(1999).
1360:, p. 116
1348:, p. 86.
1298:, p. 115
975:velocity
934:☉
927:NGC 4258
923:☉
912:☉
713:hydrogen
620:☉
593:☉
578:hydrogen
551:☉
540:☉
521:galaxies
420:redshift
411:CO) and
398:NGC 1068
394:NGC 4258
378:☉
353:☉
342:methanol
305:infrared
232:adiation
208:icrowave
126:NGC 4945
95:hydroxyl
83:☉
64:☉
2849:Bibcode
2820:4293176
2800:Bibcode
2761:Bibcode
2712:Bibcode
2671:Bibcode
2669:: 231.
2648:Bibcode
2609:Bibcode
2545:Bibcode
2543:: 688.
2506:Bibcode
2461:Bibcode
2422:Bibcode
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2174:Bibcode
2131:Bibcode
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2018:Bibcode
1975:Bibcode
1952:Bibcode
1917:Bibcode
1872:Bibcode
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1808:Bibcode
1806:: 541.
1777:Bibcode
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1754:Bibcode
885:to the
839:opacity
831:kelvins
742:Arp 220
523:called
506:Arp 220
493:hosts.
490:Arp 220
413:methine
367:Arp 220
357:NGC 253
312:History
270:with a
225:mission
134:Arp 220
119:methine
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2792:Nature
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2576:
2524:
2440:
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2354:Nature
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2281:
2192:
2087:
2038:
1993:
1890:
812:parsec
728:(CO).
584:, or H
531:, or L
268:photon
248:photon
244:absorb
185:Masers
121:(CH).
2865:S2CID
2839:arXiv
2816:S2CID
2777:S2CID
2751:arXiv
2728:S2CID
2702:arXiv
2625:S2CID
2599:arXiv
2522:S2CID
2496:arXiv
2438:S2CID
2412:arXiv
2388:S2CID
2362:arXiv
2190:S2CID
2164:arXiv
2085:S2CID
2059:arXiv
2036:S2CID
1991:S2CID
1010:Notes
789:radio
754:data)
386:water
318:maser
295:Earth
236:atoms
191:maser
103:water
27:laser
2574:ISBN
2313:ISBN
2279:ISBN
1888:ISBN
1259:2017
1034:2016
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601:dust
436:0.27
396:and
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109:O),
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2857:doi
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2808:doi
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