BBC Sky at Night Magazine

Sharpest map ever made of a galaxy’s dark matter

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.

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. 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.

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.” www.hubblesite.org