Professor Hai Fu led an international team of astronomers in the discovery of a rare encounter between two massive and gas-rich galaxies in a survey from the Herschel telescope. The event took place when the Universe was only about three billion years old and involved two galaxies forming stars with exceptional efficiency whilst in the process of merging. This galactic collision would go on to form a very massive elliptical galaxy with hardly any star formation activity. The discovery suggests a viable mechanism for the origin of the puzzling 'red and dead' galaxies that are seen in the young Universe.

Each of the two galaxies has a stellar mass of about 100 billion solar masses, and they each contain roughly the same amount of gas. The astronomers estimated that the merging process would take at most 200 million years to complete, resulting in a massive elliptical galaxy of about 400 billion solar masses.

"We see these two galaxies forming stars at a phenomenal rate – about 2000 stars like the Sun per year – and the conversion of gas into stars is more efficient than in normal galaxies by an order of magnitude," says Fu.

Such a high star-formation rate, spurred by the merger process itself, is not sustainable and would not take long to exhaust the gas reservoir of both progenitor galaxies and quench star formation. The astronomers believe that such a merger must have produced a descendant galaxy with passive star-forming activity and a stellar population of old and red stars.

Ref: Hai Fu, et al., "The rapid assembly of an elliptical galaxy of 400 billion solar masses at a redshift of 2.3", 2013, Nature. DOI: 10.1038/nature12184