MessageToEagle.com - It's something curious with a dense cloud near the Milky Way's center.
The cloud, dubbed G0.253+0.016, defies the rules of star formation.
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—one particular cloud has baffled astronomers.
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
Click on image to enlarge
At the Center of the Milky Way
At the center of our Milky Way Galaxy lies a supermassive black hole. Once a controversial claim, this conclusion is
now solidly based on 16 years of observations that map the orbits of 28 stars very near the galactic center. Using European
Southern Observatory telescopes and sophisticated near infrared cameras, astronomers patiently measured the positions of the
stars over time, following one star, designated S2, through a complete orbit as it came within about 1 light-day of the center
of the Milky Way. Their results convincingly show that S2 is moving under the influence of the enormous gravity of a compact,
unseen object -- a black hole with 4 million times the mass of the Sun. Their ability to track stars so close to the galactic
center accurately measures the black hole's mass and also determines the distance to the center to be 27,000 light-years.
Credit: ESO, Stefan Gillessen (MPE), F. Eisenhauer, S. Trippe, T. Alexander, R. Genzel, F. Martins, T. Ott
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, the 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 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.
Click on image to enlarge
The Spitzer image 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. Credit: Caltech/Kauffmann, Pillai, Zhang
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.
Click on image to enlarge
This image, 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. Credit: NASA/Spitzer/
Benjamin et al., Churchwell et al.
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.
Astronomers are also puzzled over the presence of young star clusters in the galactic center.
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 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."