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This Hubble Space
Telescope image of galaxy NGC 1275 reveals the fine, thread-like
filamentary structures in the gas surrounding the galaxy. The red
filaments are composed of cool gas being suspended by a magnetic field,
and are surrounded by the 100-million-degree Fahrenheit gas in the
center of the Perseus galaxy cluster. The filaments are dramatic markers
of the feedback process through which energy is transferred from the
central massive black hole to the surrounding gas.
Credit: Courtesy of NASA (edited by Jose-Luis Olivares/MIT)
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A handful of
new stars are born each year in the Milky Way, while many more blink on
across the universe. But astronomers have observed that galaxies should
be churning out millions more stars, based on the amount of interstellar
gas available.
Now researchers from MIT and Michigan State University have pieced
together a theory describing how clusters of galaxies may regulate star
formation. They describe their framework this week in the journal Nature.
When intracluster gas cools rapidly, it condenses, then collapses to
form new stars. Scientists have long thought that something must be
keeping the gas from cooling enough to generate more stars -- but
exactly what has remained a mystery.
For some galaxy clusters, the researchers say, the intracluster gas
may simply be too hot -- on the order of hundreds of millions of degrees
Celsius. Even if one region experiences some cooling, the intensity of
the surrounding heat would keep that region from cooling further -- an
effect known as conduction.
"It would be like putting an ice cube in a boiling pot of water --
the average temperature is pretty much still boiling," says Michael
McDonald, a Hubble Fellow in MIT's Kavli Institute for Astrophysics and
Space Research. "At super-high temperatures, conduction smooths out the
temperature distribution so you don't get any of these cold clouds that
should form stars."
For so-called "cool core" galaxy clusters, the gas near the center
may be cool enough to form some stars. However, a portion of this cooled
gas may rain down into a central black hole, which then spews out hot
material that serves to reheat the surroundings, preventing many stars
from forming -- an effect the team terms "precipitation-driven
feedback."
"Some stars will form, but before it gets too out of hand, the black
hole will heat everything back up -- it's like a thermostat for the
cluster," McDonald says. "The combination of conduction and
precipitation-driven feedback provides a simple, clear picture of how
star formation is governed in galaxy clusters."
Crossing a galactic threshold
Throughout the universe, there exist two main classes of galaxy
clusters: cool core clusters -- those that are rapidly cooling and
forming stars -- and non-cool core clusters -- those have not had
sufficient time to cool.
The Coma cluster, a non-cool cluster, is filled with gas at a
scorching 100 million degrees Celsius. To form any stars, this gas would
have to cool for several billion years. In contrast, the nearby Perseus
cluster is a cool core cluster whose intracluster gas is a relatively
mild several million degrees Celsius. New stars occasionally emerge from
the cooling of this gas in the Perseus cluster, though not as many as
scientists would predict.
"The amount of fuel for star formation outpaces the amount of stars
10 times, so these clusters should be really star-rich," McDonald says.
"You really need some mechanism to prevent gas from cooling, otherwise
the universe would have 10 times as many stars."
McDonald and his colleagues worked out a theoretical framework that relies on two anti-cooling mechanisms.
The group calculated the behavior of intracluster gas based on a
galaxy cluster's radius, mass, density, and temperature. The researchers
found that there is a critical temperature threshold below which the
cooling of gas accelerates significantly, causing gas to cool rapidly
enough to form stars.
According to the group's theory, two different mechanisms regulate
star formation, depending on whether a galaxy cluster is above or below
the temperature threshold. For clusters that are significantly above the
threshold, conduction puts a damper on star formation: The surrounding
hot gas overwhelms any pockets of cold gas that may form, keeping
everything in the cluster at high temperatures.
"For these hotter clusters, they're stuck in this hot state, and will
never cool and form stars," McDonald says. "Once you get into this very
high-temperature regime, cooling is really inefficient, and they're
stuck there forever."
For gas at temperatures closer to the lower threshold, it's much
easier to cool to form stars. However, in these clusters,
precipitation-driven feedback starts to kick in to regulate star
formation: While cooling gas can quickly condense into clouds of
droplets that can form stars, these droplets can also rain down into a
central black hole -- in which case the black hole may emit hot jets of
material back into the cluster, heating the surrounding gas back up to
prevent further stars from forming.
"In the Perseus cluster, we see these jets acting on hot gas, with
all these bubbles and ripples and shockwaves," McDonald says. "Now we
have a good sense of what triggered those jets, which was precipitating
gas falling onto the black hole."
On track
McDonald and his colleagues compared their theoretical framework to
observations of distant galaxy clusters, and found that their theory
matched the observed differences between clusters. The team collected
data from the Chandra X-ray Observatory and the South Pole Telescope --
an observatory in Antarctica that searches for far-off massive galaxy
clusters.
The researchers compared their theoretical framework with the gas
cooling times of every known galaxy cluster, and found that clusters
filtered into two populations -- very slowly cooling clusters, and
clusters that are cooling rapidly, closer to the rate predicted by the
group as a critical threshold.
By using the theoretical framework, McDonald says researchers may be
able to predict the evolution of galaxy clusters, and the stars they
produce.
"We've built a track that clusters follow," McDonald says. "The nice,
simple thing about this framework is that you're stuck in one of two
modes, for a very long time, until something very catastrophic bumps you
out, like a head-on collision with another cluster."
The researchers hope to look deeper into the theory to see whether
the mechanisms regulating star formation in clusters also apply to
individual galaxies. Preliminary evidence, he says, suggests that is the
case.
"If we can use all this information to understand why or why not
stars form around us, then we've made a big step forward," McDonald
says.
Story Source:
The above story is based on
materials provided by
Massachusetts Institute of Technology. The original article was written by Jennifer Chu.
Note: Materials may be edited for content and length.
