Imagine trying to study the foundations of an ancient city that is still under construction. A deafening noise echoes, dust flies everywhere, and you can barely see through the fog. This is much the same challenge astronomers face when trying to understand how a vast city of hundreds of galaxies first formed. New discoveries give us our best look yet.
Astronomers discovered the remarkable object, called J0846, using two Very Large Array radio telescopes in New Mexico and ALMA high in the Atacama Desert in Chile. It’s a primordial galaxy cluster, an early stage cluster of galaxies where gravity is busy assembling what will eventually become one of the most massive structures in the universe. But this one has a very special trick hidden in it.
*Galaxy cluster lens J0846 in optical light (bottom right), an ALMA view strongly lensed into a bright arc (top right), and a composite image (left) that reveals at least 11 dusty galaxies within the core of a compact protocluster more than 11 billion light-years away, magnified by the foreground star cluster (Credit: NSF/AUI/NSF NRAO/B. Saxton; NSF/NOIRLab) Gravity. Credit: NSF/AUI/NSF NRAO/B. Saxton. NSF/NOIRLab*
Sitting almost perfectly between us and J0846 is another, closer galaxy cluster that acts as a gravitational lens. The huge mass of the foreground galaxy cluster bends and amplifies the light from the more distant protocluster behind it, making it appear much brighter and larger than it would otherwise be. The universe is doing the work for us by providing zoom lenses that human engineers cannot create.
The result was the first powerfully lensed protocluster core ever discovered, and when they examined it through a magnifying glass, they discovered something surprising. Old telescope data revealed that what previously looked like a single bright speck of light turned out to be at least 11 separate galaxies, all packed into a region of space smaller than the distance between our Milky Way and neighboring Andromeda galaxy.
Very large array in New Mexico (Credit: Hajor)
What makes these galaxies so dramatic is what they do. Each star is undergoing a starburst, a period of intense, almost frenetic star formation, pumping out new stars at such a rate that our galaxy seems completely lazy. They are also completely covered in dust, which absorbs visible light and makes them completely invisible to normal optical telescopes. ALMA’s ability to detect faint glows of cold dust and gas cuts straight through this cosmic fog and reveals the raw materials being consumed to create all these stars.
Lead researcher Nicholas Hu, a graduate student at Arizona State University, compared the whole scenario to an archaeologist digging through the layers of history. The cluster in the foreground is a mature modern city, and the primitive cluster behind it is the ancient settlement that grew out of it. The deeper we look into the universe, the further back in time we find ourselves. In this case, astronomers went back more than 11 billion years to capture the first chapter of the galaxy cluster. What was once just a smudge of ancient survey data has turned out to be one of the most unusual objects in the early universe.
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