First observational evidence of ‘Dark Matter Heating’ discovered

Researchers have discovered the first observational evidence of ‘dark matter heating’ in distant galaxies, placing an important constraint on future dark matter models.

Even though astronomers and cosmologists have long been able to infer the presence of a mysterious substance that makes up the vast majority of matter in the universe- provisionally named dark matter- the characteristics of this substance have been slow to reveal themselves, mainly due to the fact that it does not strongly interact with light. The latest observed characteristic seems to confirm a long-held theory that dark matter can be heated and moved around by star-formation says research published in the journal Monthly Notices of the Royal Astronomical Society.

As dark matter doesn’t interact with light in the same way that everyday matter — or baryonic matter— does, astronomers have been forced to use its gravitational influence to perform observations of it. Using this method, researchers at 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 — small, faint galaxies that are typically found orbiting larger galaxies like our own Milky Way — may hold clues that could help us to better understand the nature of dark matter.

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’. (J. Read et al.)

When stars form, strong stellar winds push baryonic matter like gas and dust, from the centre of the galaxy in question leaving behind dark matter. This means there is less mass in the centre which means that this left-behind dark matter ‘feels’ less gravitational influence. The dark matter gathers more energy as a result of this and begins to migrate away from the centre — this effect is known as ‘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.

The Draco dwarf galaxy 270,000 light-years from Earth. Just one of the Dwarf galaxies studied by the researchers.

Professor Justin Read, lead author of the study and Head of the Department of Physics at the University of Surrey, says: “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.”

What is significant about this new finding is that it places a necessary constraint on dark matter models, dark matter must be shown to exhibit a range of densities within a dwarf galaxy that is in direct negative-correlation to the rate of star formation in that galaxy. Thus galactic discs with greater star populations should also have lower dark matter concentration.

Professor Matthew Walker, a co-author from Carnegie Mellon University, adds: “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.

Original Study: https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/sty3404/5265085