Water really is everywhere. Two teams of astronomers, each led by
scientists at the California Institute of Technology (Caltech),
have discovered the largest and farthest reservoir of water ever
detected in the universe. Looking from a distance of 30 billion
trillion miles away into a quasar—one of the brightest and most
violent objects in the cosmos—the researchers have found a mass of
water vapor that's at least 140 trillion times that of all the
water in the world's oceans combined, and 100,000 times more
massive than the sun.
Because the quasar is so far away, its light has taken 12 billion
years to reach Earth. The observations therefore reveal a time when
the universe was just 1.6 billion years old. "The environment
around this quasar is unique in that it's producing this huge mass
of water," says Matt Bradford, a scientist at NASA's Jet Propulsion
Laboratory (JPL), and a visiting associate at Caltech. "It's
another demonstration that water is pervasive throughout the
universe, even at the very earliest times." Bradford leads one of
two international teams of astronomers that have described their
quasar findings in separate papers that have been accepted for
publication in the Astrophysical
Journal Letters.
A quasar is powered by an enormous black hole that is steadily
consuming a surrounding disk of gas and dust; as it eats, the
quasar spews out huge amounts of energy. Both groups of astronomers
studied a particular quasar called APM 08279+5255, which harbors a
black hole 20 billion times more massive than the sun and produces
as much energy as a thousand trillion suns.
Since astronomers expected water vapor to be present even in the
early universe, the discovery of water is not itself a surprise,
Bradford says. There's water vapor in the Milky Way, although the
total amount is 4,000 times less massive than in the quasar, as
most of the Milky Way's water is frozen in the form of ice.
Nevertheless, water vapor is an important trace gas that reveals
the nature of the quasar.
In this particular quasar, the water vapor is distributed around
the black hole in a gaseous region spanning hundreds of light-years
(a light-year is about six trillion miles), and its presence
indicates that the gas is unusually warm and dense by astronomical
standards. Although the gas is a chilly ? degrees Celsius (?
degrees Fahrenheit) and is 300 trillion times less dense than
Earth's atmosphere, it's still five times hotter and 10 to 100
times denser than what's typical in galaxies like the Milky
Way.
The water vapor is just one of many kinds of gas that surround the
quasar, and its presence indicates that the quasar is bathing the
gas in both X-rays and infrared radiation. The interaction between
the radiation and water vapor reveals properties of the gas and how
the quasar influences it. For example, analyzing the water vapor
shows how the radiation heats the rest of the gas. Furthermore,
measurements of the water vapor and of other molecules, such as
carbon monoxide, suggest that there is enough gas to feed the black
hole until it grows to about six times its size. Whether this will
happen is not clear, the astronomers say, since some of the gas may
end up condensing into stars or may be ejected from the quasar.
Bradford's team made their observations starting in 2008, using an
instrument called Z-Spec at the Caltech Submillimeter Observatory
(CSO), a 10-meter telescope near the summit of Mauna Kea in Hawaii.
Z-Spec is an extremely sensitive spectrograph, requiring
temperatures cooled to within 0.06 degrees Celsius above absolute
zero. The instrument measures light in a region of the
electromagnetic spectrum called the millimeter band, which lies
between infrared and microwave wavelengths. The researchers'
discovery of water was possible only because Z-Spec's spectral
coverage is 10 times larger than that of previous spectrometers
operating at these wavelengths. The astronomers made follow-up
observations with the Combined Array for Research in
Millimeter-Wave Astronomy (CARMA), an array of radio dishes in the
Inyo Mountains of Southern California.
This discovery highlights the benefits of observing in the
millimeter and submillimeter wavelengths, the astronomers say. The
field has developed rapidly over the last two to three decades, and
to reach the full potential of this line of research, the
astronomers—including the study authors—are now designing CCAT, a
25-meter telescope to be built in the Atacama Desert in Chile. CCAT
will allow astronomers to discover some of the earliest galaxies in
the universe. By measuring the presence of water and other
important trace gases, astronomers can study the composition of
these primordial galaxies.
The second group, led by Dariusz Lis, senior research associate in
physics at Caltech and deputy director of the CSO, used the Plateau
de Bure Interferometer in the French Alps to find water. In 2010,
Lis's team was looking for traces of hydrogen fluoride in the
spectrum of APM 08279+5255, but serendipitously detected a signal
in the quasar's spectrum that indicated the presence of water. The
signal was at a frequency corresponding to radiation that is
emitted when water transitions from a higher energy state to a
lower one. While Lis's team found just one signal at a single
frequency, the wide bandwidth of Z-Spec enabled Bradford and his
colleagues to discover water emission at many frequencies. These
multiple water transitions allowed Bradford's team to determine the
physical characteristics of the quasar's gas and the water's
mass.
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California Institute of Technology