Why massive galaxies don’t dance in crowds

Aug 10, 2017

Heavyweight galaxies living in a dense crowd of galaxies tend to spin more slowly than their lighter neighbours, and an Australian-led team has found out why.

“Contrary to earlier thinking, the galaxy’s mass determines its spin rate,” said team leader Associate Professor Sarah Brough (ARC Centre of Excellence for All-sky Astrophysics and University of New South Wales).

This finding, based on a detailed study of more than 300 galaxies, has been published in The Astrophysical Journal.

“We want to know which factors really drive how galaxies evolve,” said team member Dr Matt Owers (Australian Astronomical Observatory and Macquarie University).

“In this case, we’ve sorted out nature versus nurture.”

To measure how fast their galaxies rotated the researchers used an instrument called SAMI (the Sydney-AAO Multi-object Integral field spectrograph) on the 4-m Anglo-Australian Telescope in eastern Australia. SAMI ‘dissects’ galaxies, obtaining optical spectra from 61 points across the face of each galaxy, 13 galaxies at a time.

The new finding runs counter to previous studies, made with smaller samples of galaxies, which concluded that a galaxy’s spin rate is determined by how crowded its neighbourhood is

Associate Professor Brough thinks this earlier conclusion was spurious. “Once you take into account the strong association with mass, there’s no link between a galaxy’s spin rate and its environment,” she said.

The research team was drawn from the Australian Astronomical Observatory; the universities of New South Wales, Sydney, Melbourne, Queensland and Oxford; The Australian National University, Macquarie University, Swinburne University of Technology and Yonsei University; and Caltech.

The ARC Centre of Excellence for All-sky Astrophysics (CAASTRO) is a collaboration between The University of Sydney, The Australian National University, The University of Melbourne, Swinburne University of Technology, The University of Queensland, The University of Western Australia and Curtin University, the last two participating together as the International Centre for Radio Astronomy Research (ICRAR). CAASTRO is funded under the Australian Research Council (ARC) Centre of Excellence program, with additional funding from the seven participating universities and from the NSW State Government’s Science Leveraging Fund.


Brough, S. and 25 co-authors, “The SAMI Galaxy Survey: Mass as the Driver of Kinematic Morphology–Density Relation in Clusters”. The Astrophysical Journal, 844 (2017). Online: https://doi.org/10.3847/1538-4357/aa7a11.

Image: Galaxy cluster Abell 2744, imaged with the Hubble Space Telescope. The cluster lies in the constellation of Sculptor and contains several hundred galaxies. Credit:NASA, ESA, and R. Dupke (Eureka Scientific, Inc.), et al. Source:http://hubblesite.org/image/3255/news/15-galaxy-clusters

More information

Associate Professor Sarah Brough

University of New South Wales

E: Sarah.Brough@unsw.edu.au

Mob: 61 449 505 219