Galaxies get more chaotic as they age

A comparison of a young (top) and old (bottom) galaxy observed as part of the SAMI Galaxy Survey. Subaru credit: Image from the Hyper Suprime-Cam Subaru Strategic Program

An international team led by Australian research centre ASTRO 3D reports that age is the driving force in changing how stars move within galaxies.

Galaxies start life with their stars rotating in an orderly pattern but in some the motion of stars is more random. Until now, scientists have been uncertain about what causes this – possibly the surrounding environment or the mass of the galaxy itself.

A new study, published in a paper today in MNRAS (Monthly Notices of the Royal Astronomical Society), has found that the most important factor is neither of these things. It shows the tendency of the stars to have random motion is driven mostly by the age of the galaxy – things just get messy over time.

“When we did the analysis, we found that age, consistently, whichever way we slice or dice it, is always the most important parameter,” says first author Prof Scott Croom, an ASTRO 3D researcher at the University of Sydney.

“Once you account for age, there is essentially no environmental trend, and it’s similar for mass.

“If you find a young galaxy it will be rotating, whatever environment it is in, and if you find an old galaxy, it will have more random orbits, whether it’s in a dense environment or a void.”

The research team also included scientists from Macquarie University, Swinburne University of Technology, the University of Western Australia, the Australian National University, the University of New South Wales, the University of Cambridge, the University of Queensland, and Yonsei University in the Republic of Korea.

The study updates our understanding from previous studies that have variously suggested environment or mass as more important factors. But the earlier work is not necessarily incorrect, says second author Dr Jesse van de Sande.

Young galaxies are star-forming super-factories, while in older ones, star formation ceases.

“We do know that age is affected by environment. If a galaxy falls into a dense environment, it will tend to shut down the star formation. So galaxies in denser environments are, on average, older,” Dr van de Sande says.

“The point of our analysis is that it’s not living in dense environments that reduces their spin, it’s the fact that they’re older.”

Our own galaxy, the Milky Way, still has a thin star forming disk, so is still considered a high spin rotational galaxy.

“But when we look at the Milky Way in detail, we do see something called the Milky Way thick disk. It’s not dominant, in terms of light, but it is there and those look to be older stars, which may well have been heated from the thin disk at earlier times, or born with more turbulent motion in the early Universe,” Prof Croom says.

The research used data from observations made under the SAMI Galaxy Survey. The SAMI instrument was built in 2012 by the University of Sydney and the Anglo-Australian Observatory (now Astralis). SAMI uses the Anglo-Australian Telescope, at Siding Spring Observatory, near Coonabarabran, New South Wales. It has surveyed 3000 galaxies across a large range of environments.

The study allows astronomers to rule out many processes when trying to understand galaxy formation and so fine-tune models of how the Universe has developed.

The next steps will be to develop simulations of galaxy evolution with more granular detail.

“One of the challenges of getting simulations right is the high resolution you need in to predict what’s going on. Typical current simulations are based on particles which have the mass of maybe 100,000 stars and you can’t resolve small-scale structures in galaxy disks,” Prof. Croom says.

The Hector Galaxy Survey will help Prof Croom and his team expand this work using a new instrument on the Anglo-Australian Telescope.

“Hector is observing 15,000 galaxies but with higher spectral resolution, allowing the age and spin of galaxies to be measured even in much lower mass galaxies and with more detailed environmental information,” says Professor Julia Bryant, lead of the Hector Galaxy Survey, University of Sydney.

Professor Emma Ryan-Weber, Director of ASTRO 3D, says, “These findings answer one of the key questions posed by ASTRO 3D: how does mass and angular momentum evolve in the Universe? This careful work by the SAMI team reveals that the age of a galaxy determines how the stars orbit. This critical piece of information contributes to a clearer big-picture view of the Universe.”


The ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) is a $40m Research Centre of Excellence funded by the Australian Research Council (ARC) and nine collaborating Australian universities – The Australian National University, The University of Sydney, The University of Melbourne, Swinburne University of Technology, The University of Western Australia, Curtin University, Macquarie University, The University of New South Wales, and Monash University.

ABOUT the SAMI Galaxy Survey

The SAMI Galaxy Survey began in March 2013, with the intention of creating a large survey of 3000 galaxies across a large range of environment. The data for the SAMI Galaxy Survey was collected using SAMI, the Sydney-Australian-Astronomical-Observatory Multi-object Integral-Field Spectrograph. SAMI is an instrument on the 4-meter Anglo-Australian Telescope at Siding Spring Observatory. Integral-field spectroscopy (IFS) allows a unique view of how stars and gas zoom around inside distant galaxies because we collect dozens of spectra across the entire face of each galaxy.


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The paper is available at:

Video and images

Video of SAMI at work at  

Image: a comparison of a young (top) and old (bottom) galaxy observed as part of the SAMI Galaxy Survey. Panels on the left are regular optical images from the Subaru Telescope. In the middle are rotational velocity maps (blue coming towards us, red going away from us) from SAMI. On the right are maps measuring random velocities (redder colours for greater random velocity). Both galaxies have the same total mass. The top galaxy has an average age of 2 billion years, high rotation and low random motion. The bottom galaxy has an average age of 12.5 billion years, slower rotation and much larger random motion.

Subaru credit: Image from the Hyper Suprime-Cam Subaru Strategic Program

An image taken by the Subaru Telescope of one of over 3000 galaxies observed by the SAMI team. This galaxy has a relatively old measured age (10 billion years). Credit: HSC-SSP/M. Koike/NAOJ.

Abstract and author list

The SAMI Galaxy Survey: galaxy spin is more strongly correlated with stellar population age than mass or environment

Scott M. Croom 1,2★, Jesse van de Sande 1,2, Sam P. Vaughan1 ,2,3,4, Tomas H. Rutherford 1,2, Claudia del P. Lagos 2,5, Stefania Barsanti 2,6, Joss Bland-Hawthorn 1,2, Sarah Brough 2,7, Julia J.Bryant 1,2,8,Matthew Colless 2,6,Luca Cortese 2,5,Francesco D’Eugenio 9,10, Amelia Fraser-McKelvie 2,5,11, Michael Goodwin 12, Nuria P. F. Lorente 12, Samuel N. Richards 1, Andrei Ristea 2,5, Sarah M. Sweet 2,13, Sukyoung K. Yi 14, Tayyaba Zafar 2,3,15


We use the SAMI Galaxy Survey to examine the drivers of galaxy spin, 𝜆Re , in a multi-dimensional parameter space including stellar mass, stellar population age (or specific star formation rate) and various environmental metrics (local density, halo mass, satellite vs. central).

Using a partial correlation analysis we consistently find that age or specific star formation rate is the primary parameter correlating with spin. Light-weighted age and specific star formation rate are more strongly correlated with spin than mass-weighted age. In fact, across our sample, once the relation between light-weighted age and spin is accounted for, there is no significant residual correlation between spin and mass, or spin and environment. This result is strongly suggestive that present-day environment only indirectly influences spin, via the removal of gas and star formation quenching. Older galaxies then have lower spin, either due to stars being born dynamically hotter at high redshift, or due to secular heating.

Our results appear to rule out environmentally dependent dynamical heating (e.g. galaxy-galaxy interactions) being important, at least within 1 𝑅e where our kinematic measurements are made. The picture is more complex when we only consider high-mass galaxies (𝑀∗ ≳ 1011 M⊙). While the age-spin relation is still strong for these high-mass galaxies, there is a residual environmental trend with central galaxies preferentially having lower spin, compared to satellites of the same age and mass. We argue that this trend is likely due to central galaxies being a preferred location for mergers.

1Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
2ASTRO3D: ARC Centre of Excellence for All-sky Astrophysics in 3D
3Astronomy, Astrophysics and Astrophotonics Research Centre, Macquarie University, Sydney, NSW 2109, Australia
4Centre for Astrophysics and Supercomputing, School of Science, Swinburne University of Technology, Hawthorn, VIC 3122, Australia 5International Centre for Radio Astronomy Research, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia 6Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
7School of Physics, University of New South Wales, NSW 2052, Australia
8Astralis-USydney, School of Physics, University of Sydney, NSW 2006, Australia
9 Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, United Kingdom
10 Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Tohmson Avenue, Cambridge, CB3 0HE, United Kingdom 11European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748, Garching, Germany
12AAO-MQ, Faculty of Science & Engineering, Macquarie University. 105 Delhi Rd, North Ryde, NSW 2113, Australia
13School of Mathematics and Physics, University of Queensland, Brisbane, QLD 4072, Australia
14Department of Astronomy and Yonsei University Observatory, Yonsei University, Seoul 03722, Republic of Korea
15School of Mathematical and Physical Sciences, Macquarie University, NSW 2109, Australia