Star-forming galaxies, including ours, are rich in elements — and secrets. Stars come to life fastest in the early histories of such galaxies, but this process doesn’t last at such a high rate forever. The star-formation rate (SFR) eventually slows.
Thanks to a recent study published Dec. 27, 2024, in The Astrophysical Journal Letters, we may now know why a galaxy’s SFR declines over billions of years: massive, conveyor belt-like currents, traveling in and out of their host galaxies, forming a structure known as the circumgalactic medium.
Peering through the fog
A team of researchers led by Samantha Garza at the University of Washington in Seattle, analyzed the light of nine distant quasars, as seen through the circumgalactic medium of 11 star-forming galaxies, to see how their light was affected. To do so, they used the Cosmic Origins Spectrograph on the Hubble Space Telescope. Their observations confirmed that these regions act as giant reservoirs for their home galaxy, containing hot gases rich in oxygen and carbon.
In some cases, the team discovered carbon atoms commuting up to 400,000 light-years outside their host galaxy. By comparison, the Milky Way’s bright disk is only 100,000 light-years across.
“The same carbon in our bodies most likely spent a significant amount of time outside of the galaxy,” said study co-author, Jessica Werk, in a press release.
Hop on the galactic express
Garza likened the circumgalactic medium to a large train station where material is constantly being pushed out and pulled back in. “The heavy elements that stars make get pushed out of their host galaxy and into the circumgalactic medium through their explosive supernovae deaths, where [the elements] can eventually get pulled back in and continue the cycle of star and planet formation,” she explained.
A previous study, published in 2011 in Science, confirmed that star-forming galaxies are surrounded by a circumgalactic medium ubiquitously filled will ionized oxygen, as opposed to galaxies with little or no star formation, which contain significantly less ionized oxygen. And the circumgalactic medium acts as a far better gas reservoir than the host galaxy itself.
The new study took this investigation a step further. The authors realized that the circumgalactic medium doesn’t just contain hot gas, but also low-temperature material like carbon atoms. And these currents contribute to star-forming galaxies’ recycling process of forming new stars from old ones by gathering elements and taking them for joy rides throughout the galaxy, until gravity forces them to find a new home.
Clocking the slowdown
By better understanding this recycling process, astronomers could also finally understand what causes galaxies to eventually slow and stop forming new stars. For example, a breakdown in the circumgalactic medium’s regular circulation patterns could be responsible for ultimately starving a galaxy of the material it needs to build new stars and planets.
Additional studies are needed to further examine and quantify the chemical elements traveling in and out of the circumgalactic medium, as well as determine the differences in these currents between galaxies that are and aren’t forming stars. But it’s clear that a correlation exists between the SFR of a galaxy and the extent of the circumgalactic medium’s reservoir surrounding it.
These two studies — along with many more — are helping us to clarify our picture of galaxy evolution, and peer back at the journey the materials that make up Earth and its inhabitants have taken before we got here.
