Life on Earth couldn’t exist without carbon. But coal itself couldn’t exist without stars. Almost all elements except hydrogen and helium – including carbon, oxygen and iron – exist only because they were forged in stellar furnaces and later thrown into space when their stars died. In the ultimate act of galactic recycling, planets like ours are created by incorporating atoms made out of stars, whether iron in the Earth’s core, oxygen in its atmosphere, or carbon in the bodies of Earthlings.
A team of scientists from the US and Canada recently confirmed that carbon and other atoms created in stars don’t just float idle in space until they’re repurposed for brand spanking new uses. For galaxies like ours which can be still actively forming recent stars, the atoms make a circuitous journey. They orbit their galaxy of origin on gigantic currents that stretch into intergalactic space. These currents – often called the circumgalactic medium – resemble giant conveyor belts that push material out and pull it back into the galaxy’s interior, where gravity and other forces can fuse these materials into planets, moons, asteroids, comets and even recent stars.
“Think of the circumgalactic medium as a giant train station: it’s constantly pushing out and pulling in material,” said team member Samantha Garza, a doctoral student at the University of Washington. “Heavy elements produced by stars are pushed out of the host galaxy into the circumgalactic medium through the explosive death of supernovae, where they can eventually be pulled back in and continue the cycle of star and planet formation.”
Garza is the lead writer of a paper describing these findings, published Dec. 27 in the journal
“The implications for the evolution of galaxies and the nature of the carbon resources available in galaxies to form new stars are exciting,” said co-author Jessica Werk, a UW professor and chair of the Department of Astronomy. “The same carbon in our bodies has most likely spent a significant amount of time outside the galaxy!”
In 2011, a team of scientists confirmed for the first time the long-held theory that star-forming galaxies like ours are surrounded by a circumgalactic medium and that this huge, circulating cloud of fabric comprises hot oxygen-enriched gases. Garza, Werk and their colleagues found that lower-temperature material, equivalent to carbon, also circulates in the circumgalactic medium of star-forming galaxies.
“We can now confirm that the circumgalactic medium acts as a giant reservoir of both carbon and oxygen,” Garza said. “And at least in star-forming galaxies, we suggest that this material then falls back into the galaxy to continue the recycling process.”
Studying the circumgalactic medium could help scientists understand how the recycling process peters out, which is able to eventually occur to all galaxies – even our own. One theory is that a slowdown or collapse in the contribution of the circumgalactic medium to the recycling process may explain why the galaxy’s star population declines over long periods of time.
“If you can keep the cycle going – pushing material out and pulling it back in – then theoretically you have enough fuel to continue forming stars,” Garza said.
For this study, scientists used the Cosmic Origins Spectrograph on the Hubble Space Telescope. The spectrograph measured how the light from nine distant quasars – ultra-bright light sources in space – is influenced by the circumgalactic environment of 11 star-forming galaxies. Hubble’s readings showed that a few of the quasar light was absorbed by a specific component of the circumgalactic medium: carbon, and in large quantities. In some cases, they found carbon extending almost 400,000 light-years – or 4 times the diameter of our own galaxy – into intergalactic space.
Future research is required to quantify the full range of other elements that make up the circumgalactic medium and to more precisely compare the differences in their composition between galaxies that also produce large amounts of stars and galaxies which have largely ceased star formation. These answers could explain not only when galaxies like ours transition to stellar deserts, but additionally why.
The paper’s co-authors are Trystyn Berg, a research associate at the Herzberg Center for Research in Astronomy and Astrophysics in British Columbia; Jakow Faerman, a researcher at the University of Warsaw in the field of astronomy; Benjamin Oppenheimer, research associate at the University of Colorado at Boulder; Rongmon Bordoloi, assistant professor of physics at North Carolina State University; and Sara Ellison, professor of physics and astronomy at the University of Victoria. The research was funded by NASA and the National Science Foundation.