APEX reveals hidden star formation in protocluster

The study showed that star formation in the Spiderweb Cluster is concentrated mostly in a single region, not on the large filaments connecting galaxies as expected.
By | Published: October 15, 2014 | Last updated on May 18, 2023
Spiderweb Galaxy Cluster illustration
This artist’s impression depicts the formation of a galaxy cluster in the early universe. The galaxies are vigorously forming new stars and interacting with each other. Such a scene closely resembles the Spiderweb Galaxy (formally known as MRC 1138-262) and its surroundings, which is one of the best-studied protoclusters.
ESO/M. Kornmesser
Astronomers have used the APEX telescope to probe a huge galaxy cluster that is forming in the early universe and revealed that much of the star formation taking place is not only hidden by dust, but also occurring in unexpected places. This is the first time that a full census of the star formation in such an object has been possible.

Galaxy clusters are the largest objects in the universe held together by gravity, but their formation is not well understood. The Spiderweb Galaxy (formally known as MRC 1138-262) and its surroundings have been studied for 20 years using the European Southern Observatory (ESO) and other telescopes and is thought to be one of the best examples of a protocluster in the process of assembly, more than 10 billion years ago.

But Helmut Dannerbauer of the University of Vienna, Austria, and his team strongly suspected that the story was far from complete. They wanted to probe the dark side of star formation and find out how much of the star formation taking place in the Spiderweb Galaxy Cluster was hidden from view behind dust.

Spiderweb Galaxy Cluster captured by APEX telescope
This image shows the APEX view in submillimeter light of the region around the Spiderweb Galaxy — a protocluster of galaxies in the early universe surrounding a radio galaxy containing a supermassive black hole. Some of the blobs in this image correspond to dusty star-forming galaxies in the protocluster that cannot be seen in visible light due to absorption by dust. The fainter features here are artifacts of the difficult APEX image processing.
The team used the LABOCA camera on the APEX telescope in Chile to make 40 hours of observations of the Spiderweb Cluster at millimeter wavelengths — wavelengths of light that are long enough to peer right through most of the thick dust clouds. LABOCA has a wide field and is the perfect instrument for this survey.

“This is one of the deepest observations ever made with APEX and pushes the technology to its limits — as well as the endurance of the staff working at the high-altitude APEX site, 5,050 meters [16,570 feet] above sea level,” said Carlos De Breuck, APEX project scientist at ESO and a co-author of the new study.

The APEX observations revealed that there were about four times as many sources detected in the area of the Spiderweb compared to the surrounding sky. And by carefully comparing the new data with complementary observations made at different wavelengths, the scientists were able to confirm that many of these sources were at the same distance as the galaxy cluster itself and must be parts of the forming cluster.

“The new APEX observations add the final piece needed to create a complete census of all inhabitants of this mega star city,” Dannerbauer said. “These galaxies are in the process of formation, so, rather like a construction site on Earth, they are very dusty.”

But a surprise awaited the team when they looked at where the newly detected star formation was taking place. They were expecting to find this star formation region on the large filaments connecting galaxies. Instead, they found it concentrated mostly in a single region, and that region is not even centered on the central Spiderweb Galaxy in the protocluster.

“We aimed to find the hidden star formation in the Spiderweb Cluster — and succeeded — but we unearthed a new mystery in the process,” Dannerbauer said. “It was not where we expected! The mega city is developing asymmetrically.”

To continue the story, further observations are needed, and the Atacama Large Millimeter/submillimeter Array will be the perfect instrument to take the next steps and study these dusty regions in far greater detail.