Herschel discovers galaxy merger in the very early Universe

13 November 2017What seemed at first like a rare instance of a huge, ancient galaxy revealed itself to be an even rarer pair of extremely massive galaxies, seen on the brink of merging when the Universe was only a billion years old.



This outstanding source was first identified by ESA’s Herschel space observatory, and later exposed in greater detail with the ground-based Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

The earliest galaxies in the 13.8 billion-year history of our cosmos started to take shape a few hundred million years after the Big Bang, as matter flowed towards increasingly denser spots and as the first stars were igniting into life.

Primordial galaxies, however, were not like the ones we are familiar with today. They started out much smaller than our Milky Way galaxy, which is now home to hundreds of billions of stars, and slowly grew more massive over billions of years.

In this scenario, astronomers could not easily explain the observation of massive galaxies populated by ageing stars at relatively early epochs in cosmic history, when the Universe was only a few billion years old.

How did such massive galaxies assemble in this relatively short time?

The recent discovery of possible progenitors to such galaxies – massive galaxies observed at even earlier cosmic times – has brought new clarity to these investigations.


“Scrutinising several hundred thousand galaxies observed by Herschel, we identified a very rare instance of a massive object in the very early Universe,” says Dominik Riechers from Cornell University, USA, who is the lead author of the new study.

The wide range of infrared and submillimetre wavelengths probed by Herschel, which scanned the sky between 2009 and 2013, was key to finding this source.

Galaxies that are forming stars shine brightly in the infrared. This is due to cosmic dust present in the dense clouds where stars are born, which absorbs part of the starlight and radiates it again at longer wavelengths.

In addition, because the Universe is expanding, the wavelength of light emitted by distant galaxies is stretched even longer along its cosmic journey, up to the submillimetre in the case of the most distant galaxies, whose light was released in the first few billion years of cosmic history.

It was 2012 when Dominik and colleagues singled out this source as potentially interesting and worthy of follow-up observations. After they looked at it at even longer submillimetre wavelengths with Europe’s Atacama Pathfinder Experiment (APEX), also located in Chile, it stood out as the ‘reddest’ in this galaxy sample, appearing consistently brighter at increasingly longer wavelengths, up to 870 micron.

This suggested that the galaxy is ripe with star formation and located at a great distance from us, in the depth of the very early Universe.