Hungry galaxies grow fat on the flesh of their neighbours

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Modelling shows big galaxies get bigger by merging with smaller ones

Galaxies grow large by eating their smaller neighbours, new research reveals.

Exactly how massive galaxies attain their size is poorly understood, not least because they swell over billions of years. But now a combination of observation and modelling from researchers led by Dr Anshu Gupta from Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) has provided a vital clue.

In a paper published in The Astrophysical Journal, the scientists combine data from an Australian project called the Multi-Object Spectroscopic Emission Line (MOSEL) survey with a cosmological modelling program running on some of the world’s largest supercomputers in order to glimpse the forces that create these ancient galactic monsters.

By analysing how gases within galaxies move, Dr Gupta said, it is possible to discover the proportion of stars made internally – and the proportion effectively cannibalised from elsewhere.

“We found that in old massive galaxies – those around 10 billion light years away from us – things move around in lots of different directions,” she said.

“That strongly suggests that many of the stars within them have been acquired from outside. In other words, the big galaxies have been eating the smaller ones.”

Because light takes time to travel through the universe, galaxies further away from the Milky Way are seen at an earlier point in their existence. Dr Gupta’s team found that observation and modelling of these very distant galaxies revealed much less variation in their internal movements.

“We then had to work out why ‘older’, closer big galaxies were so much more disordered than the ‘younger’, more distant ones,” said second author ASTRO 3D’s Dr Kim-Vy Tran, who like Dr Gupta, is based at the UNSW Sydney.

“The most likely explanation is that in the intervening billions of years the surviving galaxies have grown fat and disorderly through incorporating smaller ones. I think of it as big galaxies having a constant case of the cosmic munchies.”

The research team – which included scientists from other Australian universities plus institutions in the US, Canada, Mexico, Belgium and the Netherlands – ran their modelling on a specially designed set of simulations known as IllustrisTNG.

This is a multi-year, international project that aims to build a series of large cosmological models of how galaxies form. The program is so big that it has to run simultaneously on several of world’s most powerful supercomputers.

“The modelling showed that younger galaxies have had less time to merge with other ones,” said Dr Gupta.

“This gives a strong clue to what happens during an important stage of their evolution.”

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More information:

Visuals:

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Paper details:

Title: MOSEL Survey: Tracking the Growth of Massive Galaxies at 2 < z < 4 Using Kinematics and the IllustrisTNG Simulation

DOI:10.3847/1538-4357/ab7b6d

Authors:

Anshu Gupta^1,2 , Kim-Vy Tran^1,2,3, Jonathan Cohn³ , Leo Y. Alcorn^3,4 , Tiantian Yuan^2,5 , Vicente Rodriguez-Gomez⁶, Anishya Harshan¹ , Ben Forrest⁷ , Lisa J. Kewley^2,8 , Karl Glazebrook⁵ , Caroline M. Straatman⁹ , Glenn G. Kacprzak^2,5, Themiya Nanayakkara¹⁰ , Ivo Labbé⁵ , Casey Papovich^3,11 , and Michael Cowley^12,13

1. School of Physics, University of New South Wales, Sydney, NSW 2052, Australia;
2. ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
3. George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
4. Department of Physics and Astronomy, York University, 4700 Keele Street, Toronto, Ontario, MJ3 1P3, Canada
5. Swinburne University of Technology, Hawthorn, VIC 3122, Australia
6. Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, A.P. 72-3, 58089 Morelia, Mexico
7. Department of Physics & Astronomy, University of California, Riverside, 900 University Avenue, Riverside, CA 92521, USA
8. Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia
9. Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium
10. Leiden Observatory, Leiden University, P.O. Box 9513, NL 2300 RA Leiden, The Netherlands
11. Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
12. Centre for Astrophysics, University of Southern Queensland, West Street, Toowoomba, QLD 4350, Australia
13. School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia

Funded by: ASTRO 3D, Australia Research Council, National Science Foundation, Nederlandse Organisatie voor Wetenschappelijk Onderzoek

More about ASTRO 3D:

ASTRO 3D is a seven-year $40 million Centre of Excellence project funded by the Australian Government through the Australian Research Council. The Centre began in June 2017 and will end in June 2024. It hosts around 200 investigators and professional staff, mostly based at six nodes: the Australian National University, Curtin University, Swinburne University of Technology, University of Melbourne, University of Sydney, and University of Western Australia. https://astro3d.org.au/

More about IllustrisTNG

The IllustrisTNG project is an ongoing series of large, cosmological magnetohydrodynamical simulations of galaxy formation. TNG aims to illuminate the physical processes that drive galaxy formation: to understand when and how galaxies evolve into the structures that are observed in the night sky, and to make predictions for current and future observational programs. The simulations use a state of the art numerical code which includes a comprehensive physical model and runs on some of the largest supercomputers in the world. TNG is a successor to the original Illustris simulation and builds on several years of effort by many people. The project description page contains an introduction to the motivations, techniques, and early science results of the TNG simulations. Presently, the project includes three primary runs spanning a range of volume and resolution; these are called TNG50, TNG100, and TNG300. https://www.tng-project.org/

More about the Multi-Object Spectroscopic Emission Line (MOSEL) survey

The MOSEL survey is an ongoing survey of star-forming galaxies around 12 billion light years away. The main objective is identify factors affecting the rise and fall of star formation activity in young galaxies.