Astronomers Discover When Galaxies Got Their Spirals

Spiral galaxies didn’t exist in the early universe. Now astrophysicists think they know when spirals emerged

Perhaps the most closely studied image in all of astronomy is a picture taken by the Hubble Space Telescope over a four-month period in late 2003 and early 2004. This picture is so famous it even has its own name—the Hubble Ultra Deep Field.

It shows 10,000 galaxies of various different ages, some almost as old as universe itself. This image is important because it helps astronomers tackle one of the biggest outstanding questions in astrophysics: how galaxies evolved.

The basic technique is to sort the galaxies by age and then examine how they must have evolved.

But here’s the thing. While spiral galaxies are common in today’s universe, there were none in the early universe. So an important question is how galaxies got their spirals and when this happened.

Today, we get an answer thanks to the work of Debra Meloy Elmegreen at Vassar College in Poughkeepsie and Bruce Elmegreen at IBM’s T.J. Watson Research Center in Yorktown Heights. These guys have meticulously catalogued the evolution of galaxies in the Hubble Ultra Deep Field and say spiral structures first appeared when the universe was about 3.7 billion years old.

The study of galaxy evolution is a complex business, not least because spirals come in various different types. The Elmegreens examined 269 spirals in the Hubble Ultra Deep Field and discarded all but 41 because of factors such as an inability to discern a clear spiral structure or the lack of redshift data which gives a galaxy’s age.

They divided these 41 spiral galaxies into five different types, based on features such as the number and clarity of arms, whether well-defined or clumpy and so on. They then catalogued the properties of each of these different types, such as their age, brightness at various different frequencies and the size of any star-forming clumps they contain.

The results show a clear evolution in the shape of galaxies as they get older. They start off as a disk containing a number of bright clumps in which stars are forming but with little star formation elsewhere in the disk.

As a galaxy ages, a central bright bulge of star formation emerges. Then the clumps elsewhere in the glalxy begin to smear out forming woolly, indistinct arms.

Only later, when the universe was almost 4 billion years old, did these arms become more distinct and take on the familiar spiral shape.

At first, these spiral galaxies have only two arms, type known as grand design structures. More complex structures follow later. “Normal multiple arm galaxies are found only closer than z = 0.6 [when the universe was 8 billion years old],” say the Elmegreens.

That’s useful work which is consistent with some of the numerical simulations of galaxy formation that astronomers have carried out in recent years.

Of course, astronomers’ understanding of galaxy formation is by no means complete. One important outstanding question is why some galaxies evolve in to spirals and yet others remain as featureless elliptical galaxies, even to this day.

Another is the role of supermassive black holes which are found at the centre of most large galaxies and may even be an essential part of their structure and evolution.

Then there is the mysterious dark matter that fills the universe and seems to play an important role in holding galaxies together. What is the nature of this elusive stuff?

With the question of the origin of spiral structures now under their belts, astrophysicists can now focus on these other questions. Clearly, it’s an interesting time to be a galactic specialist.

Ref: arxiv.org/abs/1312.2215 : The Onset Of Spiral Structure In The Universe