According to an international team of astronomers working with ESA’s Herschel Space Telescope, two gas-rich, actively star-forming galaxies that collided 11 billion years ago are rapidly forming a new elliptical mega-galaxy, labeled HXMM01.

Most massive galaxies in the Universe fall into two major categories: gas-rich spiral galaxies and gas-poor elliptical galaxies. Scientists have been investigating for decades how these two different types of galaxies came into being and whether spirals may be the progenitors of elliptical galaxies via galactic mergers.

A puzzling discovery made in the past decade was that of so-called ‘red and dead’ galaxies in the young Universe: very massive and passive elliptical galaxies found at redshifts between 2 and 1.5, corresponding to epochs in cosmic history when the Universe was 5 to 3 billion years old. Somehow, on a short cosmological timescale, these galaxies managed to exhaust their gas reservoir and consequently stopped forming stars: this left them with stellar populations that had already aged enough to appear red and old.

The new mega-galaxy HXMM01 is about 10 times the size of the Milky Way. It is the brightest, most luminous and most gas-rich submillimeter-bright galaxy merger known.

“Capturing the creation of this type of large, short-lived star body is extremely rare – the equivalent of discovering a missing link between winged dinosaurs and early birds,” said the scientists, who have reported their discovery in the journal Nature (arXiv.org version).

HXMM01 is fading away as fast as it forms, a victim of its own cataclysmic birth. As the two parent galaxies smashed together, they gobbled up huge amounts of hydrogen, emptying that corner of the Universe of the star-making gas.

Study lead author Dr Hai Fu from the University of California Irvine said: “these galaxies entered a feeding frenzy that would quickly exhaust the food supply in the following hundreds of million years and lead to the new galaxy’s slow starvation for the rest of its life.”

“Finding this type of galaxy is as important as the discovery of the archaeopteryx was in understanding dinosaurs’ evolution into birds, because they were both caught at a critical transitional phase.”

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Bibliographic information: Hai Fu et al. The rapid assembly of an elliptical galaxy of 400 billion solar masses at a redshift of 2.3. Nature, published online May 22, 2013; doi: 10.1038/nature12184