There’s Less Dark Matter at the Core of the Milky Way

Science really does keep you on your toes. First there was matter and then there were galaxies. Then those galaxies had more stuff in the middle so stars further out were expected to move slowly, then there was dark matter as they actually seemed to move faster but now they seem to be moving slower in our Galaxy so perhaps there is less dark matter than we thought after all! 

Let’s start with dark matter.  It is a strange and mysterious form of matter that doesn’t really seem to behave in any way like normal matter. It doesn’t emit light, absorb or reflect it so is to all intents invisible, hence its name. It’s thought that about 27% of the Universe is made up of dark matter but the only way we can detect it is its gravitational effect on passing light and other matter. Despite mounting evidence for its existence, we have still yet to actually detect particles that make up dark matter, whatever they are. 

Physicists at MIT (the Massachusetts Institute of Technology) have measured the speed of stars in the Milky Way galaxy and found that those further out to the edge are moving slower than expected. This suggests, rather surprisingly that the core of the milky way may be lighter in mass than first thought and thus contain less dark matter. 

The team used data from Gaia and APOGEE (Apache Point Observatory Galactic Evolution Experiment) to plot the velocity of stars against their distance. This enabled them to generate a rotation curve that shows how fast matter rotates at a given distance from the centre of a galaxy. Interpreting graphs like these allow astronomers to estimate how much dark matter there is.

Artist impression of ESA's Gaia satellite observing the Milky Way. The background image of the sky is compiled from data from more than 1.8 billion stars. It shows the total brightness and colour of stars observed by Gaia
Artist impression of ESA’s Gaia satellite observing the Milky Way (Credit : ESA/ATG medialab; Milky Way: ESA/Gaia/DPAC)

This was quite in contrast to earlier observations since the 1970’s that revealed a hint of dark matter distribution. Measurements of previous galaxies showed that stars were moving around the centre at a fairly constant velocity with distance from centre. The only way this can be explained is dark matter. This work was pioneered by Vera Rubin from the Carnegie Institution in Washington and was supported by multiple observations from other astronomers in the following years. 

The efforts to measure galactic rotation have focussed on other galaxies rather than our own. It’s actually quite difficult to achieve the same in a galaxy that you live in but undaunted; Xiaowei Ou, Anna-Christina Eilers, and Anna Frebel set about the task. Their initial observations came from Gaia data but used APOGEE data to refine their results. They were able to measure distances of more than 33,000 stars out to a distance of 30 kiloparsecs (97,846 light years). The data was then incorporated into a model of circular velocity to estimate the velocity of stars given the location of all other stars in the galaxy. This gave them an updated and refined rotation curve. 

The curve their work revealed showed a more rapid decline over distance rather than the shallow decline they expected. Stars further out are moving slower than expected and so there is less matter in the centre of our galaxy. We can observe the ‘normal’ or baryonic matter but it requires less dark matter to account for the observations. Further research is now required to explore other galaxies like the Milky Way and perhaps, change our view of the amount of dark matter in the Universe. 

Source : Study: Stars travel more slowly at Milky Way’s edge