Galaxies grow bigger and puffier as they age: new study

An international study including the University of Sydney and The Australian National University has found galaxies grow bigger and puffier as they age.

Lead author Dr Jesse van de Sande, from the University of Sydney said it was not obvious that galaxy shape and age had to be linked, so the connection was surprising and could point to a deep underlying relationship.

“As a galaxy ages, internal changes take place and the galaxy may collide with others,” said Dr van de Sande, from the Sydney Institute for Astronomy (SIfA) in the School of Physics and the ARC Centre of Excellence in All-Sky Astrophysics in 3D (ASTRO 3D).

“These events disorder the stars’ movements.”

The findings are published today in the leading journal Nature Astronomy.

Co-researcher Professor Matthew Colless from ANU said that stars in a young galaxy moved in an orderly way around the galaxy’s disk, much like cars around a racetrack.

“All galaxies look like squashed spheres, but as they grow older they become puffier with stars going around in all directions,” said Professor Colless, who is the Director of the ANU Research School of Astronomy and Astrophysics and a Chief Investigator at ASTRO 3D.

“Our Milky Way is more than 13 billion years old, so it is not young anymore, but the galaxy still has both a central bulge of old stars and spiral arms of young stars.”

To work out a galaxy’s shape, the research team measured the movement of stars with an instrument called SAMI on the Anglo-Australian Telescope at the ANU Siding Spring Observatory.

They studied 843 galaxies of all kinds and with a hundred-fold range in mass.

The research was funded by ASTRO 3D at ANU and the ARC Centre of Excellence for All Sky Astrophysics (CAASTRO) at the University of Sydney.

Co-author Dr Nicholas Scott, from the University of Sydney and ASTRO 3D, said scientists measured a galaxy’s age through colour.

“Young, blue stars grow old and turn red,” he said.

“When we plotted how ordered the galaxies were against how squashed they were, the relationship with age leapt out. Galaxies that have the same squashed spherical shape, have stars of the same age as well.”

Dr van de Sande said scientists had known for a long time that shape and age were linked in very extreme galaxies, that is very flat ones and very round ones.

“This is the first time we’ve shown shape and age are related for all kinds of galaxies, not just the extremes – all shapes, all ages, all masses,” he said.

University of Sydney co-author Dr Julia Bryant, lead scientist for the SAMI instrument, said the team was still searching for the simple, powerful relationships like shape and age that underlie a lot of the complexity scientists see in galaxies.

“To see those relationships, you need detailed information on large numbers of galaxies,” she said. 

The Anglo-Australian Observatory (AAO) is building SAMI’s successor instrument, Hector, which is designed to observe 100 galaxies at a time.

Acknowledgements

CAASTRO is a $28 million Research Centre of Excellence funded by the Australian Research Council (ARC). CAASTRO is led by the University of Sydney, in conjunction with ANU, The University of Melbourne, The University of Western Australia, Curtin University and Swinburne University. 

ASTRO 3D is a $40 million Research Centre of Excellence funded by the ARC and six collaborating Australian universities led by ANU: the University of Sydney, The University of Melbourne, Swinburne University of Technology, The University of Western Australia and Curtin University.