Dark matter on the move

Two blotches that look very similar
Star formation in tiny dwarf galaxies can slowly “heat up” the dark matter, pushing it outwards, a new study suggests. Left, the hydrogen gas density of a simulated dwarf galaxy, viewed from above. Right, the same for a real dwarf galaxy, IC 1613. In the simulation, repeated gas inflow and outflow causes the gravitational field strength at the center of the dwarf galaxy to fluctuate. The dark matter responds to this by migrating out from the center of the galaxy, an effect known as “dark matter heating.” Image via J. Read et al.

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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 was published January 3, 2019, in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society.

In the new work, an international team of scientists set out to hunt for evidence of dark matter at the centers of nearby dwarf galaxies. Dwarf galaxies are small, faint galaxies that are typically found orbiting larger galaxies like our own Milky Way. They are integral to discussions of dark matter because they appear to have even higher proportions of dark matter than larger galaxies, and because the primary theory of dark matter – the Cold Dark Matter Theory – suggests there should be many more dwarf galaxies orbiting our Milky Way and other galaxies than have been found.

Dark matter is thought to make up a significant percentage 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 lie in how stars are formed in galaxies.

Pie chart of universe showing 68.3% dark energy, 26.8% dark matter, and 4.9% ordinary matter.
The composition of the universe determined from analysis of the Planck mission’s cosmic microwave background data. Image via University of Oxford/© ESA.

When stars form, strong winds can push gas and dust away from the heart of the galaxy. As a result, the galaxy’s center 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 center, an effect called dark matter heating.

The team of astrophysicists measured the amount of dark matter at the centers 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 centers than those that are still forming stars today. This supports the theory that the older galaxies had less dark matter heating.

Justin Read is lead author of the study and head of the Department of Physics at the University of Surrey. He said in a statement:

We found a truly remarkable relationship between the amount of dark matter at the centers of these tiny dwarfs, and the amount of star formation they have experienced over their lives. The dark matter at the centers 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. Matthew Walker, a study 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.

Bottom line: New research suggests dark matter can be heated up and moved around as a result of star formation in galaxies.

Source: Dark matter heats up in dwarf galaxies

Via Royal Astronomical Society

January 6, 2019

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