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

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.”