7 January 2009

Which came first: galaxies or black holes?

Cosmos Online
Supermassive black holes at the centre of galaxies may form before the galaxies themselves, a new study has found.
VLA dish

Cosmic history: Part of the Very Large Array radio observatory in New Mexico, used to make the discovery. Credit: Image courtesy of NRAO/AUI and Photo: Matthew L. Abbondanzio

BRISBANE: Supermassive black holes at the centre of galaxies may form before the galaxies themselves, a new study has found.

The central bulge of a galaxy usually has a thousand times more mass than the black hole at its centre, but that this isn’t the case in very young galaxies, an international team reported yesterday to a meeting of the American Astronomical Society in Long Beach, California.

Efficient growth

Instead, the black holes are relatively much larger in baby galaxies, hinting that the holes came first. That changes the way astronomers will think about the growth and evolution of galaxies, according to team member Dominik Riechers.

“Our findings show that… a simple regulating process that allows simultaneous growth cannot be the only reason for the relationship [between black holes and bulges],” said Riechers, an astronomer at California Institute of Technology in Pasadena, USA.

“Even if we look at the most extreme, rapid star-forming events in the universe, those are less [fast and] efficient than the growth of massive black holes,” he said.

To make the discovery, Reicher’s team studied conditions during the first billion years of the universe using the Very Large Array radio observatory in New Mexico, and the Interferometer at Plateau de Bure in France.

Time travel

These telescopes are able to see galaxies that are so far away that they appear as they did during the first billion years of the universe’s existence.

It appears that all ‘spheroidal’ galaxies have a supermassive black hole at the centre, Riechers said, and until now, astronomers had observed that the black hole’s mass was always about one thousandth of the mass of the galaxy’s dense central bulge of stars.

This correlation suggested to scientists that the same physical process must regulate the growth of both black holes and the galaxies that surround them, but the new findings show that other forces must be at work as well.

It also helps solve a galactic chicken-and-egg problem: do galaxies form around black holes, or do black holes grow up inside existing galaxies?

“We see that some of the most massive black holes in today’s galaxies already have formed more than 12 billion years ago,” Riechers said, long before the galaxies themselves.

Riechers said that his team will now try to figure out how the black hole and the bulge affect each other’s growth and why they come to have the standard 1:1000 mass ratio.

The experts plan to use new telescopes coming online in the next few years, such as the Atacama Large Millimetre/submillimetre Array (ALMA), which will have the sensitivity and resolving power to see the formation of some of the universe’s earliest galaxies.

“A real milestone”

Showing that the ratio of mass of supermassive black holes and the central bulge of galaxies changes over time is the first step in understanding how these galaxies form, commented John Dickey, an astronomer at the University of Tasmania in Hobart, Australia.

“This is a real milestone,” Dickey said. “If this result can be [replicated] by other researchers, it will certainly point to black holes forming first. That’s rather different from what many people would have said a few years ago.”


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