The image above, taken with NASA's Spitzer
infrared space telescope, shows the mysterious galactic cloud, seen as
the black object on the left. The galactic center is the bright spot on
the right. It's the mystery of the curiously dense cloud near the
crowded galactic center, where billowing clouds of gas and dust cloak a
supermassive black hole three million times as massive as the sun—a
black hole whose gravity is strong enough to grip stars that are
whipping around it at thousands of kilometers per second.
The baffling object, dubbed G0.253+0.016, defies the rules of star
formation. In infrared images of the galactic center, the cloud—which is
30 light-years long—appears as a bean-shaped silhouette against a
bright backdrop of dust and gas glowing in infrared light. The cloud's
darkness means it is dense enough to block light. According to
conventional wisdom, clouds of gas that are this dense should clump up
to create pockets of even denser material that collapse due to their own
gravity and eventually form stars. One such gaseous region famed for
its prodigious star formation is the Orion Nebula. And yet, although the
galactic-center cloud is 25 times denser than Orion, only a few stars
are being born there—and even then, they are small.
In fact, Caltech astronomers say, its star-formation rate is 45 times
lower than what astronomers might expect from such a dense cloud. "It's
a very dense cloud and it doesn't form any massive stars—which is very
weird," says Jens Kauffmann, a senior postdoctoral scholar at Caltech.
In a series of new observations, Kauffmann, along with Caltech postdoctoral scholar Thushara Pillai and Qizhou Zhang of the Harvard-Smithsonian Center for Astrophysics,
have discovered why: not only does it lack the necessary clumps of
denser gas, but the cloud itself is swirling so fast that it can't
settle down to collapse into stars.
The Spitzer image below of the cloud (left). The SMA image (center)
shows the relative lack of dense cores of gas that are thought to form
stars. The CARMA image (right) shows the presence of silicon monoxide,
which suggests the cloud might be the result of two colliding clouds.
The results, which show that star formation may be more complex than
previously thought and that the presence of dense gas does not
automatically imply a region where such formation occurs, may help
astronomers better understand the process. The team presented their
findings—which have been recently accepted for publication in the Astrophysical Journal Letters—at the 221st meeting of the American Astronomical Society in Long Beach, California.
To determine whether the cloud contained clumps of denser gas, called dense cores, the team used the Submillimeter Array (SMA),
a collection of eight radio telescopes on top of Mauna Kea in Hawaii.
In one possible scenario, the cloud does contain these dense cores,
which are roughly 10 times denser than the rest of the cloud, but strong
magnetic fields or turbulence in the cloud disturbs them, thus
preventing them from turning into full-fledged stars. However, by
observing the dust mixed into the cloud's gas and measuring N2H+—an ion
that can only exist in regions of high density and is therefore a marker
of very dense gas—the astronomers found hardly any dense cores.
"That was very surprising," Pillai says. "We expected to see a lot
more dense gas." Next, the astronomers wanted to see if the cloud is
being held together by its own gravity—or if it is swirling so fast that
it is on the verge of flying apart. If it is churning too fast, it
can't form stars. Using the Combined Array for Research in Millimeter-wave Astronomy (CARMA)—a
collection of 23 radio telescopes in eastern California run by a
consortium of institutions, of which Caltech is a member—the astronomers
measured the velocities of the gas in the cloud and found that it is up
to 10 times faster than is normally seen in similar clouds.
This particular cloud, the astronomers found, was barely held
together by its own gravity. In fact, it may soon fly apart. The CARMA
data revealed yet another surprise: the cloud is full of silicon
monoxide (SiO),
which is only present in clouds where streaming gas collides with and
smashes apart dust grains, releasing the molecule. Typically, clouds
only contain a smattering of the compound. It is usually observed when
gas flowing out from young stars plows back into the cloud from which
the stars were born. But the extensive amount of SiO in the
galactic-center cloud suggests that it may consist of two colliding
clouds, whose impact sends shockwaves throughout the galactic-center
cloud.
"To see such shocks on such large scales is very surprising," Pillai
says. G0.253+0.016 may eventually be able to make stars, but to do so,
the researchers say, it will need to settle down so that it can build
dense cores, a process that could take several hundred thousand years.
But during that time, the cloud will have traveled a great distance
around the galactic center, and it may crash into other clouds or be
yanked apart by the gravitational pull of the galactic center. In such a
disruptive environment, the cloud may never give birth to stars.
The findings also further muddle another mystery of the galactic center: the presence of young star clusters.
The Arches Cluster,
for example, contains about 150 bright, massive, young stars, which
only live for a few million years. Because that is too short an amount
of time for the stars to have formed elsewhere and migrated to the
galactic center, they must have formed at their current location.
Astronomers thought this occurred in dense clouds like G0.253+0.016. If
not there, then where do the clusters come from?
The astronomers' next step is to study similarly dense clouds around
the galactic center. The team has just completed a new survey with the
SMA and is continuing another with CARMA. This year, they will also use
the Atacama Large Millimeter Array
(ALMA) in Chile's Atacama Desert—the largest and most advanced
millimeter telescope in the world—to continue their research program,
which the ALMA proposal committee has rated a top priority for 2013.
The title of the Astrophysical Journal Letters paper is, "The
galactic center cloud G0.253+0.016: a massive dense cloud with low star
formation potential." This research was supported by the National
Science Foundation. Journal reference: Astrophysical Journal Letters
Source: The Daily Galaxy via California Institute of Technology
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