Old galaxies get fat

Astronomers have discovered that galaxies suffer from middle-age spread, too: the older they get, the rounder they become.

Galaxies are thought to begin their lives as a spiral-shaped disc, shining blue from firecracker-like bursts of new star formation. Over time, some remain undisturbed and grow steadily redder as their stars age, while others collide and change their structure.

Now a new study, led by astrophysicist Jesse van de Sande , from the University of Sydney in Australia, has linked the age of galaxies to their three-dimensional shape. The paper is published in Nature Astronomy and has important implications for how galaxies form and evolve.

It’s tricky to measure the 3D shapes of galaxies because we only see them as two-dimensional projections on the sky. To figure out the complete shape, the team used data from the SAMI Galaxy Survey— conducted with the Anglo-Australian Telescope at Siding Spring Observatory in New South Wales — to look at the motions and ages of stars within 843 galaxies.

The targets span a range of ages, from sprightly blue ones, to ancient reddish ones more than 10 billion years old.{%recommended 6128%}

“We can think of almost all galaxies as being basically squashed spheres,” says van de Sande. “We found that the younger the galaxy’s stars are, the more squashed the galaxy is.”

Or, in other words: the older a galaxy, the fatter it becomes.

According to astronomer Anne-Marie Weijmans from the University of St Andrews in the UK, this finding is not unexpected.

Not only is it supported by cosmological simulations, but previous observations have also revealed that “round, red bulges of galaxies tend to be older, while the blue flatter disks tend to be younger,” she says.

“If we look at our own Milky Way galaxy we can see examples of this story,” adds van de Sande. 

“The youngest part of the Milky Way, where new stars are being formed, is the thin disk, which has a very squashed, pancake-like shape. The Milky Way also contains rounder and older components: a thick disk and a bulge.” 

But this research shows that the link between age and shape is true for galaxies of all shapes, ages and masses.

This has big implications for how they evolve.

“Did stars that were born over 10 billion years ago reside in galaxies that were already round, or did these galaxies become much rounder over time?” van de Sande muses.

“We don’t know this answer yet, but the fact that a galaxy’s shape and age go together puts strong constraints on how galaxies form and grow with time.”

Previous simulations have shown that different merger scenarios may produce different shapes. For example, the merging of two gas-rich galaxies results in rounder shapes, while more frequent mergers tend to lead to triaxial galaxies. 

The team is currently gathering further observations that — together with simulations — will build a more complete picture of the connection between age and shape.

They hope to use SAMI’s successor instrument, Hector, to study how a galaxy’s environment influences this relationship.