Dark matter, that galactic drag

Invisible, nearly undetectable, it still manages to slow down the movement of metal-heavy stars

A Truly Curious correspondent

There is a bar in the middle of the Milky Way, and it provides only the most stellar stuff – literally.

Unlike the Restaurant at the End of the Universe, there is nary any alcohol to drink that we know of, but it has recently provided some food for thought. For, researchers recently discovered evidence that the stars that make up the larger Milky Way bar are slowed down by dark matter.

To those not familiar with it, dark matter is matter that cannot be directly observed, though its effects are. Consider looking through a soundproof window at a copse of waving trees. Unless you are endearingly naive, you are likely to conclude it is the wind – which you cannot see or hear – that is making them move.

Simply put, dark matter is made up of particles that do not react to electromagnetism in any form, including light. This makes it invisible to the eye and most sensors. But, like the wind, dark matter’s effects can be indirectly gauged. It does have gravity, which makes matter eddy around it. It makes up about 27% of the mass of the universe.

In this case, Rimpei Chiba and Ralph Schönrich, from Oxford University and the University of London respectively, found that the galactic bar spun slower because of dark matter.

The galactic bar is a quirk of spiral galaxies, a phenomenon that shows up as the galaxies mature – a period of about two billion years after formation – and slowly disappears over time. About half the spiral galaxies seen today have such bars. While many researchers had always believed the Milky Way was no exception, it was only in 2019 that data from the Gaia space telescope confirmed that our galaxy, too, had a signature bar.

The paper compared the stability of these stars to two clusters of small asteroids near Jupiter, poetically called the Trojans and the Greeks. These are located at stable positions, called Lagrange points, where the pull of the planet and its moon – or the sun – are equal.

Every planet has several gravitational “sweet spots” where a relatively tiny body, like an asteroid, can maintain a fairly stable position in relation to two larger bodies, such as the Sun and the planet, or the planet and its moon. The Gaia space telescope itself is located at a Lagrangian point between the Sun and the Earth, thus ensuring it a stable orbit.

Chiba and Schönrich, were studying data from Gaia on the Hercules stream, a group of stars that move together. They found that the stars were moving at the same speed as the bar. This suggested something in the bar was holding them back. That would be dark matter. They argued that the stars of the Hercules stream also circulate around such Lagrange points on the galactic bar.

The researchers also found these stars were more metal-rich. This suggests the stars came from the galactic center, usually home to more heavy metals than the outer arms. The researchers calculated that the galactic bar had slowed down 24% since it was first formed. They argued that friction – of the gravitational kind – was responsible for it.

“The counterweight slowing this spin must be dark matter,” Schönrich said in a UCL press release. “Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter. Our research provides a new type of measurement of dark matter – not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar’s spin.”

The original article can be found in the Monthly Notices of the Royal Astronomical Society