Mapping the Stars in a Dwarf Galaxy to Reveal its Dark Matter

Dark matter is curious stuff! As the name suggests, it’s dark making it notoriously difficult to study. Although it’s is invisible, it influences stars in a galaxy through gravity. Now, a team of astronomers have used the Hubble Space Telescope to chart the movements of stars within the Draco dwarf galaxy to detect the subtle gravitational pull of its surrounding dark matter halo. This 3D map required studying nearly two decades of archival data from the Draco galaxy. They found that dark matter piles up more in the centre, as predicted by cosmological models.

Dark matter comprises approximately 27% of the all the mass and energy in the universe but interacts only gravitationally, emitting no light. The idea first – ahem, came to light to explain discrepancies in the rotation curves of galaxies and is detected through its gravitational effects on visible matter. Despite extensive research,  the nature of dark matter remains elusive. Understanding dark matter is crucial for comprehending the composition and evolution of the universe.

Dark matter has often been described as the invisible ‘glue’ that holds galaxies together. Although galaxies are mostly composed of dark matter, understanding its distribution within them provides an opportunity to understand its nature and relevance to the evolution of the galaxy. Computer simulations predict a dense concentration of dark matter at the core of the galaxy, forming a density cusp. However, numerous observations have shown that dark matter appears more uniformly spread throughout galaxies, contradicting these simulations. 

To study dark matter within galaxies, scientists can analyse the movements of stars, which are influenced primarily by the gravitational pull of dark matter. One common method involves using the Doppler Effect to measure the speed of objects in space—observing changes in the wavelength of light as stars move closer to or further from Earth. Along with moving toward or away from us, stars can also move across the sky. This proper motion, when combined with line of sight measurements allow for the creation of the movement of a star in 3D.

Astronomers have employed NASA’s Hubble Space Telescope to study the dynamics of stars within the Draco dwarf galaxy, located about 250,000 light-years from Earth. The Draco galaxy was used because, as a dwarf galaxy, it is relatively small and is believed to have a higher proportion of dark matter than other types of galaxy.

Over 18 years of observational data from 2004 to 2022 were examined and they painstakingly mapped the precise three-dimensional motions of these stars, drawing from extensive archival data collected by Hubble. This effort has yielded the most accurate understanding to date of how stars move within this small galaxy. Understanding precisely how stars move in galaxies allows for precise maps of dark matter to be created. 

The technique the team have developed is not only of use for the Draco dwarf galaxy but for other galaxies too. The Sculptor dwarf galaxy is already being analysed using the same technique along with the Ursa Minor dwarf galaxy.

Source : NASA’s Hubble Traces Dark Matter in Dwarf Galaxy Using Stellar Motions