CFHT and W.M. Keck Observatories help NASA space telescopes find Galaxy Cluster with a Vibrant Heart

September 10 2015






Astronomers have discovered a rare beast of a galaxy cluster whose heart is bursting with new stars. The unexpected find, made with NASA's Spitzer and Hubble space telescopes, MOSFIRE on Keck Observatory and MegaCam on CFHT, suggests that behemoth galaxies at the cores of these massive clusters can grow significantly by feeding off gas stolen from another galaxy.

"Usually, the stars at the centers of galaxies clusters are old and dead, essentially fossils," said Tracy Webb of McGill University, Montreal, Canada, lead author of a new paper on the findings accepted for publication in the Astrophysical Journal. "But we think the giant galaxy at the center of this cluster is furiously making new stars after merging with a smaller galaxy."

Galaxy clusters are vast families of galaxies bound by the ties of gravity. Our own Milky Way resides in a small galaxy group, called the Local Group, which itself is on the periphery of the vast Laniakea supercluster of 100,000 galaxies (Laniakea is Hawaiian for "immeasurable heaven.")

The cluster in the new study, referred to by astronomers as SpARCS1049+56, has at least 27 galaxy members, probably more, and a combined mass equal to nearly 400 trillion suns. It is located 4 billion light-years away in the Ursa Major constellation. The object was initially discovered using Spitzer and the Canada-France-Hawaii telescope, located on Maunakea in Hawaii, and confirmed using MOSFIRE on Keck Observatory.

What makes this cluster unique is its luminous heart of new stars. At the core of most massive galaxy clusters lies one hulking galaxy. The galaxy dominating the cluster SpARCS1049+56 is rapidly spitting out an enormous number of stars -- about 860 new ones a year. For reference, our Milky Way makes only about one to two stars per year.

"With Spitzer's infrared camera, we can actually see the ferocious heat from all these hot young stars," said co-author Jason Surace from NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, California. Spitzer detects infrared light, so it can see the warm glow of hidden, dusty regions where stars form.

Follow-up studies with Hubble in visible-light helped confirm the source of the fuel, or gas, for the new stars. A smaller galaxy seems to have recently merged with the monster galaxy in the middle of the cluster, lending its gas to the larger galaxy and igniting a fury of new stars.

"Hubble found a train wreck of a merger at the center of this galaxy," said Webb.

Hubble specifically detected features called beads on a string, which are pockets of gas that condense where new stars are forming. Beads on a string are telltale signs of collisions between gas-rich galaxies, a phenomenon known to astronomers as wet mergers, where "wet" refers to the presence of gas. In these smash-ups, the gas is quickly converted to new stars. Dry mergers, by contrast, occur when galaxies with little gas collide and no new stars are formed.

Typically, galaxy clusters grow in mass either through dry mergers at their core, or by siphoning gas into their centers, as is the case with the megatropolis of a galaxy cluster known as the Phoenix cluster.

The new discovery is one of the first known cases of a wet merger at the core of a distant galaxy cluster. Hubble previously discovered another closer galaxy cluster containing a wet merger, but it wasn't forming stars as vigorously.

The researchers are planning more studies to find out how common galaxy clusters like SpARCS1049+56 are. The cluster may be an outlier -- or it may represent an early time in our universe when gobbling up gas-rich galaxies was the norm.




Keck Observatory release
Hubblesite release
Spitzer Space Telecope release
Link to science paper.

Additional information

Media Information:
Mary Beth Laychak
Outreach Manager
Canada-France-Hawaii Telescope

Science Information:
Tracy Webb
Department of Physics
McGill University, Canada
Tel: +514 398 7226

Adam Muzzin
Institute of Astronomy
University of Cambridge, United Kingdom
Tel: +44 1223 337526

Mathias Jäger
ESA/Hubble, Public Information Officer
Garching, Germany
Cell: +49 176 62397500