In this artistic rendering, light from a distant quasar passes through the halo-like circumgalactic medium of a galaxy on its way to Earth, where it is measured by Hubble’s Cosmic Origins Spectrograph to determine the composition of the halo. Credit: NASA/ESA/A
Life on Earth depends on carbon, but carbon itself owes its existence to stars. Nearly all elements beyond hydrogen and helium — such as carbon, oxygen, and iron — were forged in the intense furnaces of stars and dispersed into the cosmos when those stars perished. Through a grand process of galactic recycling, planets like Earth formed by integrating these star-created atoms into their composition, from the iron in Earth’s core to the oxygen in its atmosphere and the carbon within living beings.
A team of scientists based in the U.S. and Canada recently confirmed that carbon and other star-formed atoms don’t just drift idly through space until they are dragooned for new uses. For galaxies like ours, which are still actively forming new stars, these atoms take a circuitous journey. They circle their galaxy of origin on giant currents that extend into intergalactic space. These currents — known as the circumgalactic medium — resemble giant conveyer belts that push material out and draw it back into the galactic interior, where gravity and other forces 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,” said team member Samantha Garza, a University of Washington doctoral candidate. “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.”
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Garza is lead author on a paper describing these findings that was published Dec. 27 in the Astrophysical Journal Letters.
“The implications for galaxy evolution, and for the nature of the reservoir of carbon available to galaxies for forming new stars, are exciting,” said co-author Jessica Werk, UW professor and chair of the Department of Astronomy. “The same carbon in our bodies most likely spent a significant amount of time outside of the galaxy!”
In 2011, a team of scientists for the first time confirmed the long-held theory that star-forming galaxies like ours are surrounded by a circumgalactic medium — and that this large, circulating cloud of material includes hot gases enriched in oxygen. Garza, Werk and their colleagues have discovered that the circumgalactic medium of star-forming galaxies also circulates lower-temperature material like carbon.
“We can now confirm that the circumgalactic medium acts like a giant reservoir for both carbon and oxygen,” said Garza. “And, at least in star-forming galaxies, we suggest that this material then falls back onto the galaxy to continue the recycling process.”
