Astronomers have spotted an energetic starburst galaxy 12.5 billion light-years away, nuzzled between a triplet of smaller, less active galaxies. Called AzTEC-3, the cosmic powerhouse at the center produces stars nearly a thousand times faster than the Milky Way. And together, this cohort of galaxies is a good representation of the first steps toward forming a cluster in the early universe.
“AzTEC-3 is a very compact, highly disturbed galaxy that is bursting with new stars at close to its theoretically predicted maximum limit and is surrounded by a population of more normal, but also actively star-forming galaxies," Cornell’s Dominik Riechers explains in a news release. "This particular grouping of galaxies represents an important milestone in the evolution of our universe: the formation of a galaxy cluster and the early assemblage of large, mature galaxies."
When the universe was still young, starburst galaxies made new stars at a crazy fast pace by devouring star-forming material and merging with other galaxies. These mergers continued over billions of years and eventually produced the large galaxies and galaxy clusters we see today.
Evidence for this process of "hierarchical merging” has been piling up, and now Riechers and colleagues were able to clearly detect this system using data from the Atacama Large Millimeter/submillimeter Array (ALMA). AzTEC-3 and the three smaller galaxies are considered a protocluster because, while they all seem to be part of the same system, they don’t appear to be gravitationally bound in a clearly defined cluster, yet.
The three smaller galaxies (labeled in the image above) are producing stars from their gas at a comparatively calm, steady rate. AzTEC-3, on the other hand, forms more new stars each day than the Milky Way does in an entire year. Additionally, the fact that there’s very little rotation in the dust and gas of AzTEC-3 further suggests that it recently merged with another adolescent galaxy. "AzTEC-3 is currently undergoing an extreme, but short-lived event," Riechers adds. "This is perhaps the most violent phase in its evolution, leading to a star formation activity level that is very rare at its cosmic epoch."
AzTEC-3 is in the direction of the constellation Sextans, and it’s what’s called a submillimeter galaxy—these shine brightly in that portion of the spectrum but not at optical and infrared wavelengths. That’s because its starlight has become stretched due to the universe’s expansion, and by the time it arrives at Earth, the far-infrared light has shifted to the submillimeter/millimeter portion of the spectrum.
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