The circumgalactic medium is an invisible cloak thatcontrols how galaxies live and die
By Lisa Grossman
A GALAXY’S ECOSYSTEM
Whirls of cold and hot
gas billow in this simulation of a circumgalactic
a galaxy. With new
tools and simulations,
learned that the CGM
helps a galaxy recycle
There’s more to a galaxy than meets the ye. Galaxies’ bright stars seem to spiral serenely against the dark backdrop of space. But a more careful look reveals a
whole lot of mayhem.
“Galaxies are just like you and me,” Jessica
Werk, an astronomer at the University of
Washington in Seattle, said in January at a meet-
ing of the American Astronomical Society. “They
Much of that turmoil takes place in a huge,
complicated setting called the circumgalactic
medium, or CGM. This vast, roiling cloud of dust
and gas is a galaxy’s fuel source, waste dump and
recycling center all in one. Astronomers think
the answers to some of the most pressing galactic
mysteries — how galaxies keep forming new stars
for billions of years, why star formation abruptly
stops — are hidden in a galaxy’s enveloping CGM.
“To understand the galaxies, you have to
understand the ecosystem that they’re in,” says
astronomer Molly Peeples of the Space Telescope
Science Institute in Baltimore.
Yet this galactic atmosphere is so diffuse that
it’s invisible — a liter of CGM contains just a single
atom. It has taken almost 60 years and an upgrade
to the Hubble Space Telescope just to begin probing
distant CGMs and figuring out how their constant
churning can make or break galaxies.
“Only recently have we been able to really, truly,
observationally characterize the relationship
between this gaseous cycle and the properties of
the galaxy itself,” Werk says.