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| Credit: NASA/ESA/Hubble Heritage Team |
Without carbon, life on Earth would not be possible. However, without stars, carbon itself could not exist. With the exception of hydrogen and helium, almost every element in the universe, including carbon, oxygen, and iron, was created in a stellar furnace and then released into space when its star perished. Whether it's the iron in Earth's core, the oxygen in its atmosphere, or the carbon in Earthlings' bodies, these star-built atoms are incorporated into the composition of planets like ours in a final act of galactic recycling.
The fact that carbon and other star-formed atoms are
not merely aimlessly floating through space until they are dragooned for new
purposes was recently proven by a team of scientists from the United States and
Canada. These atoms travel in a roundabout way in galaxies like ours, which are
still actively producing new stars. They travel on enormous currents that reach
into intergalactic space to orbit their home galaxy.
By pushing material out and pulling it back into the galactic interior, these
currents—also referred to as the circumgalactic medium—resemble enormous
conveyer belts. There, gravity and other factors can assemble these raw
materials into planets, moons, asteroids, comets, and even new stars.
"Think of the circumgalactic medium as a giant
train station: It is constantly pushing material out and pulling it back
in," researcher Samantha Garza, a doctorate candidate at the University of
Washington, said. "The heavy elements that stars make get pushed out of
their host galaxy and into the circumgalactic medium through their explosive
supernovae deaths, where they can eventually get pulled back in and continue
the cycle of star and planet formation."
Garza is the lead author of a study that was published in The Astrophysical
Journal Letters on December 27th, detailing these discoveries.
"The implications for galaxy evolution, and for the nature of the reservoir of carbon available to galaxies for forming new stars, are exciting," UW professor and Department of Astronomy chair Jessica Werk, who co-authored the study, said. "The same carbon in our bodies most likely spent a significant amount of time outside of the galaxy."
The long-held belief that star-forming galaxies like
ours are encircled by a circumgalactic medium—and that this vast, swirling
cloud of material includes hot gases high in oxygen—was first validated by a
group of scientists in 2011. Garza, Werk, and their colleagues have found that
lower-temperature materials like carbon are also circulated in the
circumgalactic medium of star-forming galaxies.
"We can now confirm that the circumgalactic
medium acts like a giant reservoir for both carbon and oxygen," Garza
stated. "And, at least in star-forming galaxies, we suggest that this
material then falls back onto the galaxy to continue the recycling
process."
Scientists may be able to better grasp how this recycling process slows down in
the circumgalactic medium, which will eventually occur in all galaxies,
including our own. One explanation for the long-term drop in a galaxy's star
populations is that the circumgalactic medium's participation to the recycling
process may be slowed down or broken down.
"If you can keep the cycle going—pushing material out and pulling it back in—then theoretically you have enough fuel to keep star formation going," Garza stated.
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| Credit: NASA/ESA/A. Field |
The Cosmic Origins Spectrograph on the Hubble Space Telescope was utilized by the researchers for this investigation. The spectrograph studied the effects of the circumgalactic medium of 11 star-forming galaxies on light from nine distant quasars, which are extremely intense sources of light in the universe.
According to the Hubble measurements, a particular
element in the circumgalactic medium—carbon—was absorbing a portion of the
light from the quasars. In certain instances, they found carbon reaching into
intergalactic space from about 400,000 light years away, which is four times
the diameter of our own galaxy.
Future studies are required to measure the complete breadth of the other components of the circumgalactic medium and to compare the differences in composition between galaxies that are still producing a lot of stars and those that have mostly stopped star formation. These answers may help us understand not only when but also why galaxies like ours become star deserts.
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