Giant star factory found in early galaxy

by Maggie McKee

The heart of the nearby galaxy Arp 220, shown in this Hubble image, is bursting with star birth. But a much larger starburst region has been found in a galaxy in the early universe (Image: NASA/ESA/C Wilson/McMaster University)

A stellar factory millions of times larger than anything comparable in the Milky Way has been identified in a galaxy in the very early universe. The work bolsters the case that massive galaxies formed very quickly - in spectacular bursts of star formation - soon after the big bang.

Regions of intense star formation, called starbursts, span a few light years at most in the Milky Way, and less than a few hundred light years in nearby, bright galaxies such as Arp 220 (pictured). But it has not been clear how large the stellar nurseries were in the early universe.

To find out, researchers led by Fabian Walter of the Max Planck Institute for Astronomy in Heidelberg, Germany, carefully scrutinised a distant galaxy whose light has taken so long to reach Earth that it appears as it was just 870 million years after the big bang.

Warm dust

It is visible at such distances because it hosts a beacon-like quasar, a bright region created by superheated gas falling towards a colossal black hole at the galaxy's core.

The quasar, called J114816.64+525150.3, is so bright that it overwhelms the surrounding galaxy's light at visible and near-infrared wavelengths. But the galaxy's gas and warm dust can be detected at radio and far-infrared wavelengths.

Using an array of telescopes in the French Alps, the team measured the galaxy's ionised carbon, which emits a strong signal at far-infrared wavelengths. Far-infrared radiation is thought to be a signature of dust that has been heated up by nearby star formation.

Maximum rate

The ionised carbon spanned a region at the heart of the galaxy about 5000 light years across. Based on the galaxy's brightness at far-infrared wavelengths, this starburst region is thought to produce an astounding 1000 Sun-like stars every year.

That is "about 1000 times higher than the star-formation rate of the Milky Way", says team member Chris Carilli, chief scientist at the National Radio Astronomy Observatory in Socorro, New Mexico.

"It's forming stars at the maximal rate allowed . . . on scales that are 10⁶ or 10⁸ times larger in volume" than similar regions in the Milky Way, he continues. "That's remarkable."

Merging galaxies?

The immense scale of the stellar factory is probably due to the fact that there was a lot more gas around in the early universe, Carilli says. Matter in the universe was indeed much denser soon after the big bang, since space itself has expanded over time.

But researchers don't know what ignited the star birth in the first place. Mergers between galaxies can trigger gas clouds to collapse into stars (and cause matter to fall into a galaxy's central black hole, turning on a quasar). However, it's not clear from the observations whether or not a merger was involved in this case.

An alternative theory, put forward recently by a team led by Avishai Dekel of Hebrew University, suggests that cold gas flowing into galaxies - in either smooth streams or clumps - may trigger starbursts. "This may be an example of this phenomenon," Dekel told New Scientist.

Sudden and dramatic

Carilli says gas may have fallen onto the galaxy over a long period of time, but some gravitational disturbance - perhaps a merger - must have suddenly kick-started its star birth. "Empirically, what we're seeing is a pretty dramatic event," he says.

The discovery suggests massive galaxies like this one, which is a blob-shaped 'elliptical' rather than a spiral, "form fairly quickly, relatively early in the universe", says Carilli.

Indeed, a similar process appears to have occurred in the Milky Way. The stars that form a bulge around its centre - essentially a miniature elliptical galaxy - all seem to have formed around the same time, in a starburst billions of years ago.

'Pathological objects'

But Carilli says it's too soon to say whether all galaxies formed their stars so quickly, or whether some eked out stars slowly over time.

That's because only the heaviest, brightest objects can be seen at such distances. "All we can study are 'pathological' objects - rare, extreme luminosity objects," he says. "They don't really tell us about normal galaxy formation; they tell us about massive galaxy formation."

A project called ALMA (Atacama Large Millimeter/submillimeter Array), which will be the world's largest array of millimetre-wave telescopes when it is completed in 2012, will shed light on more typical galaxies. It will be able to study objects 100 times less massive than can be detected with current telescopes.

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