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A New Spin on Star-Forming Galaxies

By Dave Reneke

Astronomers calculate that black holes at the heart of galaxies could swell to 50 billion times the mass of the Sun, and determine why some galaxies are “clumpy” rather than spiral in shape.

Australian researchers have discovered why some galaxies are “clumpy” rather than spiral in shape, and it appears low spin is to blame. The finding challenges an earlier theory that high levels of gas cause clumpy galaxies, and sheds light on the conditions that brought about the birth of most of the stars in the universe.

Ten billion years ago the universe was full of clumpy galaxies, but these developed more regularity as they evolved. Dr Danail Obreschkow of The University of Western Australia node of the International Centre for Radio Astronomy Research (ICRAR) said the majority of stars in the sky today, including our five-billion-year-old Sun, were probably born inside these clumpy formations. “The clumpy galaxies produce stars at phenomenal rates,” Obreschkow said. “A new star pops up about once a week, whereas spiral galaxies like our Milky Way only form about one new star a year.”

ICRAR and Swinburne University astronomers focused on a few rare galaxies, known as the DYNAMO galaxies, that still look clumpy even though they’re observed “a mere” 500 million years in the past. Obreschkow said this was like looking at a passport photo taken only last year whereas “the galaxies that are 10 billion light years away, that’s comparable to a picture from when you were 3 or 4 years old. That’s very different.”

The team used the Keck and Gemini observatories in Hawaii to measure the spin of the galaxies, along with millimetre and radio telescopes to measure the amount of gas they contained.

Obreschkow said the DYNAMO galaxies had a low spin and this was the dominant cause of their clumpiness rather than their high gas content, as previously thought. “While the Milky Way appears to have a lot of spin, the galaxies we studied here have a low spin, about three times lower,” he said.

It seems that the spin of the initial cloud of gas plays an important part in galaxy formation, suggesting that spin explains why early galaxies are gas-rich and lumpy while modern galaxies display beautiful symmetric patterns.

Black Holes Could Grow as Large as 50 Billion Suns
Black holes at the heart of galaxies could swell to 50 billion times the mass of the Sun before losing the discs of gas they rely on to sustain themselves.

A landmark study by Prof Andrew King of the University of Leicester has explored supermassive black holes at the centre of galaxies, around which are regions of space where gas settles into an orbiting disc. This gas can lose energy and fall inwards, feeding the black hole, but these discs are also unstable and prone to crumbling into stars. King calculated how big a black hole would have to be for its outer edge to keep a disc from forming, and arrived at the figure of 50 billion solar masses!

The study suggests that the black hole would stop growing without a disc, meaning 50 billion Suns would roughly be the upper limit. The only way it could get larger is if a star happened to fall straight in or another black hole merged with it. With the recent news confirming the detection of gravitational waves, this scenario is very compelling.

“The significance of this discovery is that astronomers have found black holes of almost the maximum mass, by observing the huge amount of radiation given off by the gas disc as it falls in,” King said. “The mass limit means that this procedure should not turn up any masses much bigger than those we know, because there would not be a luminous disc.”

King suggests that bigger black hole masses are possible. For example, a hole near the maximum mass could merge with another black hole, and the result would be bigger still. But no light would be produced in this merger, and the bigger merged black hole could not have a disc of gas that would make light.

“One might nevertheless detect it in other ways, for example as it bent light rays passing very close to it as in gravitational lensing, or perhaps in future from the aforementioned gravitational waves that Einstein’s general theory of relativity predicted would be emitted as it merged.”

Sagittarius A, a black hole measuring 4.3 million solar masses at the centre of the Milky Way, is the only black hole whose mass has been measured directly by analysing the full orbit of a circling star.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com