Why massive galaxies don’t dance in crowds

Galaxy cluster Abell 2744 HST

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Using data provided by the Sydney-AAO Multi-object Integral field spectrograph (SAMI) at the 3.9m Anglo-Australian Telescope, Australian scientists have discovered why heavyweight galaxies living in a dense crowd of galaxies tend to spin more slowly than their lighter neighbours.

How fast do galaxies spin? This is an important question for astrophysicists, as the spin rate of a galaxy provides key information about its evolution.

Now a group of Australian scientists have performed a detailed study of more than 300 galaxies to measure how fast the galaxies rotated. Their findings are published in a new research article in The Astrophysical Journal.

“We want to know which factors really drive how galaxies evolve,” says Dr Matt Owers of the Australian Astronomical Observatory and Macquarie University.

To measure how fast the galaxies rotated, the researchers used the Sydney-AAO Multi-object Integral field spectrograph (SAMI) instrument on the 3.9-metre Anglo-Australian Telescope.

SAMI ‘dissects’ galaxies, obtaining optical spectra from 61 points across the face of each galaxy, 13 galaxies at a time.

“Contrary to earlier thinking, the spin rate of the galaxy is determined by its mass, rather than how crowded its neighbourhood is,” says lead author, University of New South Wales (UNSW) Sydney scientist Associate Professor Sarah Brough, of the ARC Centre of Excellence for All-sky Astrophysics (CAASTRO).

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 the influence of other galaxies in its neighbourhood.

Associate Professor Brough says 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 says.

“In this case, we've shown that a galaxy's intrinsic nature is likely more important for its evolution than its environment,” says Dr Matt Owers.


The research team was drawn from the Australian Astronomical Observatory; UNSW Sydney; the universities of Sydney, Melbourne, Queensland and Oxford; The Australian National University, Macquarie University, Swinburne University of Technology; Yonsei University in South Korea and the California Institute of Technology.

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.

The AAO is a division of the Department of Industry, Innovation and Science.


Publication details:

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.


Further information:

Link to “SAMI Galaxy Survey” webpage: https://sami-survey.org/

Link to “Sydney-AAO Multi-object Integral-field” unit: https://www.aao.gov.au/science/instruments/current/sami

Further details about the “SAMI Galaxy Survey” can be found in our recent (26thJuly 2017) Media Release “Scientists unveil new 3D view of galaxies”: https://www.aao.gov.au/news-media/media-releases/scientists-unveil-new-3D-view-of-galaxies


Science Contacts:

- Dr Matt Owers

Future Fellow, Australian Astronomical Observatory and Macquarie University

P: +61 2 9372 4870 and +61 2 9850 8910 E: matthew.owers@aao.gov.au

- Assoc Prof Sarah Brough

Future Fellow, University of New South Wales

P: +61 449 505 219 E: s.brough@unsw.edu.au


Media contact:

- Dr Ángel López-Sánchez

Research Astronomer and Science Communication Officer, Australian Astronomical Observatory

M: +61 406 265 917 E: angel.lopez-sanchez@aao.gov.au



Galaxy cluster Abell 2744 HST 

Caption: 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.

This image is available at: https://www.aao.gov.au/files/press/galaxy-cluster-Abell-2744-HST-full-jpg.jpg

Source: http://hubblesite.org/image/3255/news/15-galaxy-clusters

Full Media Release in DOC format.