Weather Forecast in Galaxy Clusters: Little Chance for Starmaking

The recipe for making stars out in space is about as uncomplicated as cooking up a batch of ramen noodles here on Earth. Simply take some gas, mostly hydrogen and helium, add in a pinch of heavier elements if you like, cool it all down, and then let gravity go to work. As the gas clumps together under gravity's attraction, it grows denser and warmer. The center of the congealing gas-knot eventually grows hot enough for the gas atoms to collide and fuse, releasing energy and light that counteract the gravitational collapse. Presto, you've got a star—a stable, glowing orb that will blaze forth (in most cases) for billions of years.

Seeing how straightforward it is to make stars, you'd be correct in assuming the universe produces loads of them. Indeed—galaxies, which themselves number in the hundreds of billions, usually minimally contain millions of stars, and in some instances up to a trillion.

Yet a relatively gas-rich place where stars seemingly ought to form, in the regions between galaxies that populate galaxy clusters, is pretty much a starless wasteland. New research by Kavli Institute for Cosmology, Cambridge (KICC) scientists is helping to solve this mystery, with tie-ins to how galaxies overall assume their distinctive shapes.

The gas outside of galaxies that is associated with the galaxies' resident cluster is known as the intracluster medium, or ICM for short. In general, it exists as a hot, energized gas called a plasma, which is what stars are made out of, but is the opposite of the cool gas needed to make stars in the first place. Still, one would expect the ICM to chill out over time, especially toward the denser environment of a galaxy cluster's core. Yet any starmaking that happens in galaxy clusters is confined to the galaxies themselves.

Something, then, is keeping the ICM hot and bothered. That something is thought to be supermassive black holes, found at the centers of most galaxies. As these powerful objects devour matter, they send out intense jets of energy that run clear out of the galaxy and into the ICM. Hot lobes of gas form at the ends of these jets. Yet in ways heretofore unclear, the heat in those lobes must distribute throughout the ICM to keep the whole of it unsuitably toasty for crafting stars.

"What people have been trying to figure out is how exactly this heating process works," says Martin Bourne, lead author of a recent study and a postdoctoral research associate at the Institute of Astronomy (IoA) at the University of Cambridge and a member of KICC.

Along with senior paper author Debora Sijacki, an astrophysicist and cosmologist at KICC and IoA, as well as a former KICC colleague, Bourne ran high-powered computer simulations modeling the black hole jets' lobes. The model simulated the sort of observable X-ray signatures the lobes would be expected to produce as they interact with the ICM. The researchers knew they'd gotten it right when the simulations matched up well with real observations of galaxy clusters.

From there, the team put together a plausible explanation for how the heat in the lobes spreads out amongst the ICM. It turns out that terrestrial weather offers a good analogy. The galaxies in a cluster are always in motion about each other. The ICM itself, being a turbulent gas, also does not stay still. Together, these motions stir the hot lobes, mixing their heated material in with the otherwise-cooling regions of the ICM. As a result, the ambient ICM gas remains too warm to settle down into stars. The process is akin to the generation of wind, and in the case of the cluster environment, gales of material blow about at over 1000 kilometers per second.

"The key thing that we have found is that it is not just about injecting a lot of energy," says Bourne. "We also need the cluster to help release and redistribute this energy, which is where the cluster weather comes in."

Notably, this extragalactic result fits squarely with the increasingly well-developed paradigm of how supermassive black holes' energetic outpourings quench star formation inside galaxies as well. A basic dividing line between galaxies is that separating young, starforming, more-bluish-in-color types from old, quiescent, more-reddish-in-color types. (Regions of fresh star formation produce bright blue stars, while older stars are generally reddish in color.) The activity of a galaxy's central black hole is a major determinant of whether a galaxy stays young or transitions into old age.

"Feedback from supermassive black holes is believed to be one of the key ingredients to understand cosmic structure formation and more specifically the evolution of galaxies," says Sijacki.

Researchers are keen to nail down the apparently colossal impact black holes have on their host galaxies, their cluster environments, and extending from there, to the whole of cosmic structure and organization. Accordingly, expect much more research in this vein, Sijacki says.

"Over the last four years of intense research, we have developed new computational methods to simulate supermassive black hole jets with unprecedented resolution and fidelity," she says. "Our goal is to push cosmic structure simulation to the next level of realism and re-assess the current paradigm of how galaxies form and evolve."

When it comes to comprehending galactic phenomena, Sijacki offers another analogy to frame the advances recently made in the field of supercomputing: "It's a bit like transitioning from a Beetle to a Ferrari in our race to understand the physics behind this all."