Mapping the Milky Way’s Dark Matter Halo

Anytime astronomers talk of mapping the Milky Way I am always reminded how tricky the study of the Universe can be. After all, we live inside the Milky Way and working out what it looks like or mapping it from the inside is not the easiest of missions. It’s one thing to map the visible matter but mapping the dark matter is even harder. Challenges aside, a team of astronomers think they have managed to map the dark matter halo surrounding our Galaxy using Cepheid Variable stars and data from Gaia. 

If we study the nearby universe, we find that almost one-third of all disk galaxies seem to have a warp shape to their disk. Instead of resembling a perfect disk, they somewhat resemble a potato chip or crisp depending on your geographical location. Not surprisingly it is known as a disk warp and even our own Galaxy has one too. Galaxies rotate just like a spinning top and the galactic disk experiences precession or a wobble due to the torque forces from the surrounding dark matter halo. Measuring this has been a challenge for many years.

Credit and Copyright: Stefan Payne-Wardenaar; Magellanic Clouds: Robert Gendler/ESO

Dark matter is a mysterious and invisible type of matter that is thought to make up about 27% of the universe’s mass-energy content. Unlike ordinary matter, dark matter does not emit, absorb, or reflect light, making it detectable only through its gravitational effects. Its presence is inferred from the rotation speeds of galaxies, gravitational lensing, and the cosmic microwave background. Despite lots of research, the exact nature of dark matter is still unknown, and it is one of the most significant unsolved problems in physics and cosmology. It is thought that this invisible matter surrounds most galaxies. 

The international scientific journal Nature Astronomy recently published the paper “A slightly oblate dark matter halo revealed by a retrograde precessing Galactic disk warp.” This research, jointly led by the University of Chinese Academy of Sciences, Peking University, the National Astronomical Observatory of the Chinese Academy of Sciences, and Shanghai Jiao Tong University, introduced an innovative “motion picture” method to measure the precession rate of the Milky Way’s disk warp. 

On either side of the white line in the picture are two models of how dark matter is distributed in a galaxy similar to the Milky Way. At left, non-interacting cold dark matter creates satellite galaxies. At right, dark matter interacting with other particles makes the number of observed satellite galaxies smaller. Credit: Durham University

Central to the study was a type of star known as a cepheid variable. They are a type of pulsating star whose brightness varies in a regular cycle due to periodic expansions and contractions in their outer layers. They are useful because their pulsation period is directly related to their actual or intrinsic brightness so can be used for measuring distances in space. By comparing the apparent brightness of a Cepheid with its known luminosity, we can determine its distance from Earth. 

The team were able to analyse a sample of 2,600 Cepheid variable stars of different ages to observe the precession direction and rate of the Milky Way’s warp. These measurements showed that the current dark matter halo of the Milky Way is slightly oblate – like a sphere that has been squashed at the poles and is rotating backward at 0.12 degrees every million years!

Source : “Motion Picture” Method Reveals the Shape of the Milky Way’s Dark Matter Halo