Astronomers have caught a galaxy cluster in the prime of its life — perhaps just before it transitions to retirement.

Galaxy cluster CL J1001+0220
The massive galaxy cluster known as CL J1001+0220 exists in a universe just 2.6 billion years old. An X-ray-emitting halo of hot gas (marked purple) is characteristic of galaxy clusters. The halo surrounds the dots of infrared (red, green, and blue) and radio (green) that mark sites of furious star formation.
X-ray: NASA / CXC / Paris Diderot University / T.Wang et al.; Infrared: ESO / UltraVISTA; Radio: ESO / NAOJ / NRAO / ALMA

Galaxy clusters are the largest gravitationally bound objects in the universe. They contain dozens, hundreds, or even thousands of galaxies swirling around each other. The most massive galaxies sit in their cores — for example in the nearby Coma and Virgo clusters, where ancient ellipticals that haven’t seen fresh star formation in billions of years squat in the centers.

But that isn’t always the case. Look back in time to when the universe was half its current age, and galaxy clusters Benjamin Button their way back to youth. Galaxies are still falling into the busy cores of these so-called protoclusters, and they’re alight with newborn stars.

Now astronomers have caught one galaxy cluster that might be in the act of transitioning from youthful frenzy to settled maturity. Named CL J1001+0220, it may provide insight into questions about how a cluster’s core forms, and when the galaxies in that core lose their gas. Does the very act of “falling in” to the core strip galaxies of their gas, or do they lose it later, by interacting with other core galaxies?

Tao Wang (Paris Diderot University and Nanjing University, China) and colleagues published the discovery of CL J1001+0220 in the September 1st Astrophysical Journal. just Just 2.6 billion years after the Big Bang, eleven massive galaxies live in the cluster’s core, seven of them churning out stars at a rate thousands of times that of our own sedate galaxy.

At that rate, the cluster’s gas reservoir, the raw material needed to form stars, is shrinking fast: the galaxies will burn through it in just 200 million years — a blink of an astronomical eye.

“It appears that we have captured this galaxy cluster at a critical stage just as it has shifted from a loose collection of galaxies into a young, but fully formed galaxy cluster,” says coauthor David Elbaz (Paris Diderot University). That suggests that star formation survives the galaxies’ fall into the center — quenching must happen later on.

Yet protoclusters are so hard to find, it’s hard to say where exactly this one fits in the larger evolutionary picture. In fact, even the dividing line between protocluster and cluster isn’t always clear. If CL J1001+0220 really is in transition, though, it provides a unique view into what might be a fleeting process.

“We think we’re going to learn a lot about the formation of clusters and the galaxies they contain by studying this object,” says coauthor Alexis Finoguenov (University of Helsinki, Finland), “and we’re going to be searching hard for other examples.”


Tao Wang et al. "Discovery of a galaxy cluster with a violently starbursting core at z = 2.506." Accepted for publication in The Astrophysical Journal. Full text.


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