Low-Spin Galaxies Form More Stars

The Whirlpool Galaxy (left) forms stars far more slowly than a clumpy galaxy (right). Red areas have the most recently formed stars. Dr. Danail Obreschkow, ICRAR. Image uses data from the Hubble Space Telescope

Astronomers have attributed the rapid star-forming behavior of a handful of galaxies to a lack of spin. Answering this one question, however, has stimulated many others, including how it is that these few galaxies have failed to gain as much angular momentum as the Milky Way.

Early galaxies formed stars at astonishing rates, something we can observe billions of light-years away, since we are seeing them as they were long ago. However, this process has slowed. In the case of the Milky Way, the rate of star formation is roughly one new star a year.

Like many other astronomers, Dr. Danail Obreschkow of the University of Western Australia node of the International Centre for Radio Astronomy (ICRAR) is keen to learn more about this rapid star formation, and frustrated by the fact that distance makes most such galaxies difficult to observe in detail. So Obreschkow turned his attention to a rare class of objects known as DYNAMO galaxies

"A new star pops up about once a week [in DYNAMOs], whereas spiral galaxies like our Milky Way only form about one new star a year,” Obreschkow said in a statement. Yet they are only around 500 million years in the past, unfathomably distant for most of us, but close enough to study.

The mere existence of such rare outliers is intriguing, but Obreschkow is particularly keen to discover how much DYNAMO galaxies resemble earlier eras, and therefore whether they can be used to study what the universe was once like.

Part of the reason DYNAMOs are able to form so many stars is that their gas is concentrated in clumps dense enough to collapse under its own gravity.

In The Astrophysical Journal, Obreschkow reports that the cause of these clumps is not primarily an excess of gas, as previously proposed, but the fact that they have only about a third of the spin of regular galaxies. "The maths is different, but it is not a bad analogy to think of the way a solid spinning disk is more stable than one with less spin," Obreschkow told IFLScience.

One outstanding question is why DYNAMO galaxies have so little spin. Obreschkow told IFLScience that galaxies spin each other up, with one gaining angular momentum in one direction while the other gains it in the opposite. Lack of spin, he said, could be an indication that a galaxy never had an encounter that got it started, or that multiple brushes largely canceled each other out.

Another mystery is why DYNAMOs don't run out of fuel. Obreschkow explained that even the Milky Way's stately pace of star formation would exhaust its gas within a few hundred million years if it was not being replenished from outside the galaxy. "Clumpy galaxies need a stronger inflow of gas to keep forming stars," Obreschkow told IFLScience.

When the universe was smaller, there was plenty of gas nearby to feed galaxies, but Obreschkow admits he does not know whether DYNAMOs possess an extra-strong external gas supply for the modern universe or if we are seeing them just before they run out.

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