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.

The Spiderweb Galaxy (formally known as MRC 1138-262) and its surroundings have been studied for twenty years, using ESO and other telescopes, and is thought to be one of the best examples of a protocluster in the process of assembly, more than ten billion years ago. This is the first time that a full census of the star formation in such an object has been possible. "To obtain a full census requires a lot of observing time at a broad range of telescopes using different techniques.

As the observing time is very expensive and hard to obtain, this can only be done in a few very carefully selected regions, such as the proto-cluster surrounding the Spiderweb Galaxy," Carlos De Breuck, APEX project scientist at ESO, and a co-author of the new study told

Galaxy clusters are the largest objects in the Universe held together by gravity but their formation is not well understood.

Helmut Dannerbauer of the University of Vienna, Austria, and his team 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.

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

The team used the LABOCA camera on the APEX telescope in Chile to make 40 hours of observations of the Spiderweb Cluster at millimetre 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.

The cluster was discovered almost 15 years ago, during the first observations with the ESO VLT. "We have now extended this study by obtaining a very deep image (40 hours of observing time) with the LABOCA instrument on the APEX 12m telescope at Cerro Chajnantor. LABOCA has the advantage that it has a 11 arcminute field of view, so we can see all companion galaxies in this wide area at once. As such, LABOCA is very complimentary to ALMA, which is more sensitive, but has a much smaller field of view. So ALMA would have to spend of order 100 individual pointings to cover the full field of view," De Breuck revealed.

“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, 5050 metres above sea level,” he added.

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 they 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.

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 centred on the central Spiderweb Galaxy in the protocluster.

"This is because the host galaxy of the powerful active supermassive black hole is also one of the most massive galaxies known early in the Universe. It has almost 10^12 solar masses of stars," De Breuck explained. "Such massive galaxies are often found at the crossing of filamentary structures which form the proto-cluster. This is what the artist impression [image on top of the page] shows."

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

This image shows the APEX view in sub-millimetre 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. Credit: ESO

De Breuck, asked about why this 'mega city' is developing asymmetrically, admitted that: "This is one of the open questions from our findings. At this point, we do not have a clear answer for this puzzle."

The researchers hope to continue their study with observations using another instrument at APEX, Artemis. This will study the dust emission at a shorter wavelength of 350 µm instead of 870 µm with LABOCA, improve the positional accuracy of the candidate dusty companion galaxies, and allow to determine their temperature. But ultimately, they want to observe these galaxies in further detail with ALMA, which is more sensitive and can observe with much better spatial resolution than APEX.

This research was presented in a paper, “An excess of dusty starbursts related to the Spiderweb galaxy”, by Dannerbauer, Kurk, De Breuck et al., to appear online in the journal Astronomy&Astrophysics on 15 October 2014.

The important part allowing this research is the fact that the scientists have been able to combine the data from many different telescopes such as APEX, ESO VLT, the Herschel space telescope, and the NRAO JVLA. Without this combination, they wouldn't have been able to obtain these exciting new results.

APEX is a collaboration between the Max Planck Institute for Radio Astronomy (MPIfR), the Onsala Space Observatory (OSO) and ESO. Operation of APEX at Chajnantor is entrusted to ESO.