Mathematical & Physical Sciences


Rare merger reveals secrets of galaxy evolution

22 May 2013

Two hungry young galaxies spotted in the act of colliding, billions of light years away, are rapidly forming a massive galaxy about 10 times the size of the Milky Way, according to new research published in the journal Nature. The research, carried out by an international team in including Mat Page (UCL Space and Climate Physics), answers a long-standing question of how giant elliptical galaxies formed in the early Universe, and why they stopped forming stars soon after.

Capturing one of these giant galaxies as it is forming is extremely rare as it is a fleeting period in a galaxy’s evolution – the equivalent of discovering a missing link between winged dinosaurs and early birds. There are likely to be only a few hundred of these in the entire sky, and their extreme distance makes them small, faint and difficult to find. The scientists used the European Space Agency’s Herschel Space Observatory and other telescopes around the world to find the elusive galaxy.

Massive galaxy merger caught in the act (Herschel + ground based)
Left: The Herschel Space Observatory's view, showing HXMM01 as a bright dot in the centre. Right: detailed follow-up observations from a range of radiotelescopes, plus data from Hubble and the WM Keck Observatory. Image credit: ESA/NASA/JPL-Caltech/UC Irvine/STScI/Keck/NRAO/SAO

The new galaxy, called HXMM01, is fading away as fast as it forms, a victim of its own cataclysmic birth. As the two parent galaxies smashed together, they used up their star-making clouds of hydrogen gas, creating a burst of bright new stars, which then fade and redden as they age. Once the gas is used up, new star formation stops, which is why elliptical galaxies in the local Universe, the descendants of galaxies like HXMM01, are dominated by old, red stars.

The light of HXMM01 has taken around 11 billion years to reach us, meaning we see it as it was in the early years of the Universe. As galaxies form and grow gradually, seeing such a large galaxy in the process of developing so early in cosmic history opens a window into the evolution of galaxies in the Universe’s infant years.

“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,” said the lead author Hai Fu, who is based at the University of California, Irvine.

The discovery solves a riddle in understanding how giant elliptical galaxies developed quickly in the early universe and why they stopped producing stars soon after. Other astronomers have theorised that processes driven by giant black holes in the heart of the galaxies blew strong winds that expelled the gas. But the team found definitive proof that cosmic mergers and the resulting highly efficient consumption of gas for star formation are causing the quick burnout.

The team, led by researchers from the University of California, Irvine included Mat Page, a member of UCL’s Department of Space and Climate Physics. Page was a member of the team that built the camera onboard the Herschel Space Observatory which discovered the galaxy, showing it as a bright blob among a sea of fainter galaxies.

“The quantity of stars being formed in this galaxy is truly staggering, but they are surrounded by so much dust that we wouldn’t have found this object without Herschel” said Page.

The Herschel Space Observatory, which ran out of coolant in April, was the biggest space telescope ever launched. It specialised in making far-infrared observations, which are useful for observing cold dust and gas which are invisible at optical wavelengths, as well as for seeing distant, highly redshifted objects.


The research appears in a papers in the 23 May 2013 issue of the journal Nature: "The rapid assembly of an elliptical galaxy of 400 billion solar masses at a redshift of 2.3"

Redshift is a measure of how much the waves of light coming from an object have been stretched, and hence how fast it is moving away from us or how fast the space the light is travelling through is being stretched by the expansion of the cosmos. For very faraway objects, the expansion of the cosmos makes redshift in effect a measure of distance, with the most distant galaxies ever observed lying at redshifts of around 10. This is equivalent to a light travel time of around 13 billion years, placing these objects within the first billion years of the Universe's existence. HXMM01, at a redshift of 2.3, is seen as it was around 2.2 billion years after the Big Bang, and its light has taken around 11.5 billion years to reach us.

Related links

High-resolution images

Herschel and composite images of HXMM01

Herschel image of HXMM01

Composite image of HXMM01

These images can be reproduced freely providing the source is credited

Researcher profiles

Science contact

Mat Page
UCL Department of Space and Climate Physics (Mullard Space Science Laboratory)
01483 204283

Media contact

Oli Usher
UCL Faculty of Mathematical and Physical Sciences
020 7679 7964