This artist’s concept illustrates a quasar, or feeding black hole, similar to APM 08279+5255, where astronomers discovered huge amounts of water vapor. Gas and dust likely form a torus around the black hole, with clouds of charged gas above and below. X-rays emerge from the center, while dust throughout the torus emits infrared radiation. While this figure shows the quasar’s torus approximately edge-on, the torus around APM 08279+5255 is likely positioned face-on from our point of view. Image: NASA/ESA
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, 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.
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.
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.
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, and its presence indicates
that the gas is unusually warm and dense by astronomical standards. Although
the gas is a chilly –53 C (–63 F) 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.
made their observations starting in 2008, using an instrument called Z-Spec at
the Caltech Submillimeter Observatory (CSO), a 10 m telescope near the summit
of Mauna Kea in Hawaii.
Z-Spec is an extremely sensitive spectrograph, requiring temperatures cooled to
within 0.06 C 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
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.