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Dark matter on the move

January 03, 2019

Scientists have found evidence that dark matter can be heated up and moved around, as a result of star formation in galaxies. The findings provide the first observational evidence for the effect known as 'dark matter heating', and give new clues as to what makes up dark matter. The research is published today in the journal Monthly Notices of the Royal Astronomical Society.

In the new work, scientists from the University of Surrey, Carnegie Mellon University and ETH Zürich set out to hunt for evidence for dark matter at the centres of nearby dwarf galaxies. Dwarf galaxies are small, faint galaxies that are typically found orbiting larger galaxies like our own Milky Way. They may hold clues that could help us to better understand the nature of dark matter.

Dark matter is thought to make up most of the mass of the universe. However since it doesn't interact with light in the same way as normal matter, it can only be observed through its gravitational effects. The key to studying it may however lie in how stars are formed in these galaxies.

When stars form, strong winds can push gas and dust away from the heart of the galaxy. As a result, the galaxy's centre has less mass, which affects how much gravity is felt by the remaining dark matter. With less gravitational attraction, the dark matter gains energy and migrates away from the centre, an effect called 'dark matter heating'.

The team of astrophysicists measured the amount of dark matter at the centres of 16 dwarf galaxies with very different star formation histories. They found that galaxies that stopped forming stars long ago had higher dark matter densities at their centres than those that are still forming stars today. This supports the theory that the older galaxies had less dark matter heating.

Professor Justin Read, lead author of the study and Head of the Department of Physics at the University of Surrey, said: "We found a truly remarkable relationship between the amount of dark matter at the centres of these tiny dwarfs, and the amount of star formation they have experienced over their lives. The dark matter at the centres of the star-forming dwarfs appears to have been 'heated up' and pushed out."

The findings provide a new constraint on dark matter models: dark matter must be able to form dwarf galaxies that exhibit a range of central densities, and those densities must relate to the amount of star formation.

Professor Matthew Walker, a co-author from Carnegie Mellon University, added: "This study may be the "smoking gun" evidence that takes us a step closer to understanding what dark matter is. Our finding that it can be heated up and moved around helps to motivate searches for a dark matter particle."

The team hope to expand on this work by measuring the central dark matter density in a larger sample of dwarfs, pushing to even fainter galaxies, and testing a wider range of dark matter models.
-end-
Media contacts

Mr Dalitso Njolinjo
University of Surrey Press Office
Tel: +44 (0)1483 688 914
d.njolinjo@surrey.ac.uk

Dr Morgan Hollis
Royal Astronomical Society
Tel: +44 (0)20 7292 3977
Mob: +44 (0)7802 877 700
press@ras.ac.uk

Science contacts

Prof. Justin Read
University of Surrey
Tel: +44 (0)1483 689 421
j.read@surrey.ac.uk

Prof. Matthew Walker
Carnegie Mellon University
Tel: +1 412 268 2766
mgwalker@andrew.cmu.edu

Images and captions

https://ras.ac.uk/sites/default/files/dmheating_press_figure.pdf

Star formation in tiny dwarf galaxies can slowly "heat up" the dark matter, pushing it outwards. The left image shows the hydrogen gas density of a simulated dwarf galaxy, viewed from above. The right image shows the same for a real dwarf galaxy, IC 1613. In the simulation, repeated gas inflow and outflow causes the gravitational field strength at the centre of the dwarf to fluctuate. The dark matter responds to this by migrating out from the centre of the galaxy, an effect known as 'dark matter heating'. Credit: J. Read et al.

Further information

The new work appears in: "Dark matter heats up in dwarf galaxies", J.I. Read, M.G. Walker, P. Steger, Monthly Notices of the Royal Astronomical Society (2019), in press (DOI: 10.1093/mnras/sty3404).

A copy of the paper is available from: https://doi.org/10.1093/mnras/sty3404.

Notes for editors

The Royal Astronomical Society (RAS, http://www.ras.ac.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 4,000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

The RAS accepts papers for its journals based on the principle of peer review, in which fellow experts on the editorial boards accept the paper as worth considering. The Society issues press releases based on a similar principle, but the organisations and scientists concerned have overall responsibility for their content.

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