June 27, 2001 PASADENA, Calif. -- When galaxies collide, they leave clues in the wake of their primordial history: radio beacons from their tell-tale hearts.
Thanks to an upgrade of the radio telescope at Arecibo Observatory in Puerto Rico, these radio beacons -- 50 peculiar extragalactic objects called OH megamasers -- have been revealed. Astronomers from Cornell University in Ithaca, N.Y., said today (June 5) at the national meeting of the American Astronomical Society (AAS) that these newly discovered OH (oxygen-hydrogen) megamasers could yield major clues in understanding the physics behind the formation of galaxies.
When galaxies collide, they send out a strong radio signal that can now be picked up by Arecibo, the world's most sensitive radio telescope, says Jeremy Darling, a Cornell doctoral student in astronomy. In that process there is a strong amplification of radio waves in the frigid interstellar clouds close to the galactic centers, which is known to astronomers as microwave amplification of stimulated emission of radiation, or maser. (While light is amplified in a laser, it is radio or microwave radiation that is amplified in a maser.)
Astronomical masers were first observed in 1965 in our galaxy, the Milky Way, where they are associated with stars shrouded in molecular gas. In colliding galaxies, astronomical masers can be a million times stronger than regular masers, thus earning them the name megamasers. The first known megamaser was discovered in 1982 by Willem Baan, then a staff scientist at the Arecibo Observatory.
OH megamaser radiation comes from compact clouds rich in oxygen and hydrogen that orbit galactic nuclei, bound by the nuclei's strong gravitational fields. The collision of two galaxies and the merger of their nuclei lead to a high compression of the gas, a star formation rate increase and an environment that favors the maser process, Darling says. The megamasers observed at Arecibo shine of frequencies around 1600 megahertz.
Astronomers can use the OH megamaser detection rate to measure the frequency of galaxy mergers throughout the history of the universe, Darling says. "The luminous -- in radio terms -- megamasers allow us to witness the merging process at work."
Arecibo Observatory, where the survey of distant galaxies and megamasers was carried out, is a national center managed and operated by Cornell for the National Science Foundation. In 1997, an extensive upgrade, which added a Gregorian reflector to Arecibo, was completed, increasing the telescope's sensitivity and radio spectrum coverage. This made it possible to search for extragalactic megamasers that were undetectable before the upgrade, says Darling.
One limiting factor in future developments of this research will be the deteriorating electromagnetic environment, Darling says. Relatively nearby OH megamasers are observable at frequencies close to 1600 megahertz. The detection of OH megamasers will require observations in parts of the electromagnetic spectrum that are increasingly obscured by interference from man-made radio sources.
Riccardo Giovanelli, Cornell professor of astronomy who is Darling's academic adviser and the co-author of the AAS paper, says: "The electromagnetic spectrum is our window to the cosmos, and human activity is cluttering it with the cacophony of our own sounds. It's as if we rented a room with a lovely view and proceeded to block the window with heavy drapes and inward-facing mirrors."
Darling and Giovanelli's AAS paper is titled "OH Megamasers: Luminous Radio Beacons of Merging Galaxies."
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