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First evidence of galaxy metamorphosis detected

Galaxies nearly colliding

A giant elliptical galaxy called Messier 60 and a smaller spiral galaxy named NGC 4647(Source: NASA/ESA/Hubble)

Shape shifters Many galaxies in the universe have undergone a dramatic transformation from flat discs into spherical shapes over the past 8 billion years or so, say researchers.

An international team looked at data on some 10,000 of the billions of galaxies in the observable universe today, and then used the Hubble and Herschel telescopes to peer back in time.

They found that 83 per cent of stars formed since the Big Bang about 13.8 billion years ago, were initially grouped in flat, rotating, disc-shaped galaxies, called spirals.

But only 49 per cent of stars that exist in the universe today are located in these disc-shaped galaxies, the remainder are located in spherical-shaped galaxies called elliptical galaxies, they report in The Monthly Notices of the Royal Astronomical Society..

"This is very clear evidence that stars form in disc-type galaxies, spiral galaxies, but they end up in elliptical galaxies, so there has to be a major transformation of spirals into ellipticals after the stars have formed," says one of the study's authors Professor Andrew Hopkins of the Australian Astronomical Observatory.

The bulk of stars in the universe are thought to have been formed between 12 billion to eight billion years ago.

Such galactic shape-shifting had been theorised before, says the study's lead author Steve Eales of Cardiff University.

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According to the most widely accepted hypothesis, transformation of galaxies is caused by many cosmic catastrophes, in which two galaxies, stray too close to each other, are forced by gravity to merge into a single galaxy, with the merger destroying any discs and producing a huge pileup of stars.

"By combining Herschel and Hubble, we have for the first time been able to accurately measure the extent of this transformation," he says.

The researchers used measurements of infrared emissions from dust in galaxies taken by the Herschel telescope and combined this with data from the Galaxy and Mass Assembly spectroscopic study to determine how far away these systems are.

"This allowed us to determine where the distribution of mass in the universe is, in terms of whether the mass in stars lives predominately in the ellipticals or the spirals and how that's changed from very early times to much later times," says Hopkins.

Tags: astronomy-space, galaxies