Astronomers Measure How A Galaxy’s Spin Affects Its Shape

Rotation Speed Helps Reveal Galaxy Shapes

The giant elliptical galaxy M60 and smaller spiral galaxy, NGC 4647, located in the constellation Virgo. From our vantage point both galaxies look round but they have intrinsically different shapes. Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)–ESA/Hubble Collaboration.

In a newly published study, a team of astronomers measures how a galaxy’s spin affects its shape.

It sounds simple, but measuring a galaxy’s true 3D shape is a tricky problem that astronomers first tried to solve 90 years ago. However, for the first time astronomers have measured how a galaxy’s spin affects its shape, in new research published in the journal Monthly Notices of the Royal Astronomical Society.

“This is the first time we’ve been able to reliably measure how a galaxy’s shape depends on any of its other properties – in this case, its rotation speed,” said research team leader Dr Caroline Foster of the University of Sydney.

Galaxies can be shaped like a pancake, a sea urchin or a football, or anything in between. Faster-spinning galaxies are flatter than their slower-spinning siblings, the team found.

“And among spiral galaxies, which have discs of stars, the faster-spinning ones have more circular discs,” said team member Prof Scott Croom of the University of Sydney.

The team made its findings with SAMI (the Sydney-AAO Multi-object Integral field unit), an instrument jointly developed by The University of Sydney and the Australian Astronomical Observatory with funding from CAASTRO, the ARC Centre of Excellence for All-sky Astrophysics.

SAMI gives detailed information about the movement of gas and stars inside galaxies. It can examine 13 galaxies at a time and so collect data on huge numbers of them.

Dr Foster’s team used a sample of 845 galaxies, over three times more than the biggest previous study. This large number was the key to solving the shape problem.

Because a galaxy’s shape is the result of past events such as merging with other galaxies, knowing its shape also tells us about the galaxy’s history.

Publication: “The SAMI Galaxy Survey: the intrinsic shape of kinematically selected galaxies,” C. Foster, J. Van de Sande, F. D’Eugenio, et al., Monthly Notices of the Royal Astronomical Society (2017) 472 (1) (DOI: 10.1093/mnras/stx1869).

Source: Royal Astronomical Society

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