Astronomers find new clues to the mysterious substance’s distribution across a galaxy.
Drawing on 18 years of Hubble observations, astronomers have created the most precise map ever of how stars move in a dwarf galaxy.
The new survey could be a huge boost to understanding how dark matter holds galaxies together.
Dark matter explained
Most galaxies are spinning so fast that, if they only contained the stars that we see, they should fly apart.
To explain why they don’t, astronomers have theorised that galaxies are held together by the mass of an invisible substance they call dark matter.
This phenomena was noticed by astronomer Fritz Zwicky during his observations of the Coma Cluster of galaxies.
Currently, the presence of dark matter can only be inferred from the effect it has on the motions of stars and galaxies around it, which makes accurate tracking of its distribution within a specific galaxy difficult.
While telescope observations suggest dark matter is relatively evenly distributed throughout most galaxies, simulations suggest that it should concentrate at the centre, in what is called a density cusp.
Observing the Draco Dwarf Galaxy
Uncertain if the problem lay with the simulations or the observations, a team of astronomers picked one galaxy and decided to create the most precise map ever of the motions of its stars, which could then be used to accurately trace its dark matter content.
They chose a dwarf galaxy as these show the greatest effects of dark matter, mapping out the Draco Dwarf Spheroidal Galaxy, using data taken by the Hubble Space Telescope between 2004 and 2022 for the survey.
“When measuring proper motions, you note the position of a star at one epoch and then many years later measure the position of that same star. You measure the displacement to determine how much it moved,” says Sangmo Tony Sohn from the Space Telescope Science Institute (STScI), who worked on the study.
“The longer you wait, the better you can measure the stars shifting.”
As well as the 2D motions recorded in the images, the team measured the stars’ Doppler shift to determine how quickly they were moving away from Earth, creating a full 3D model of their motions.
“Our models tend to agree more with a cusp-like structure, which aligns with cosmological models,” said Eduardo Vitral, also from STScI, who led the study.
“While we cannot definitively say all galaxies contain a cusp-like dark matter distribution, it’s exciting to have such well measured data that surpasses anything we’ve had before.”