An ancient galactic collision could help us find dark matter

Astrophysicists for the first time calculated the original mass and size of a dwarf galaxy that was shredded in a collision with the Milky Way galaxy billions of years ago. Rebuilding the original dwarf galaxy, whose stars now cross the Milky Way in a stellar “tidal current,” will help scientists understand how galaxies such as the Milky Way formed and could aid in the search for dark matter in our galaxy.

“We ran simulations that take this large stream of stars into account and see what it looked like before it fell into the Milky Way,” explained Heidi Newberg, professor of physics, astrophysics and astronomy at Rensselaer Polytechnic Institute. “We now have a measurement from the data, and it’s the first big step towards using the information to find dark matter in the Milky Way.”

Billions of years ago, the dwarf galaxy and others like it that were close to the Milky Way were dragged into the larger galaxy. As each dwarf galaxy merged with the Milky Way, its stars were attracted by “tidal forces,” the same kind of differential forces that make tides on Earth. The tidal forces distorted and eventually tore apart the dwarf galaxy under consideration, stretching its stars in a tidal stream hurled across the Milky Way. Such tidal mergers are fairly common, and Newberg estimates that absorbed stars in the Milky Way make up most of the stars in the galactic halo, a roughly spherical cloud of stars surrounding the spiral arms of the central disc.

The position and speed of the tidal stream stars carry information about the gravitational field of the Milky Way. Rebuilding the dwarf galaxy is a research task that combines data from stellar surveys, physics, and Newberg’s MilkyWay @ Home distributed supercomputer, which leverages 1.5 petaflops, a measure of processing speed, the power of home computers donated by volunteers. This large amount of processing power allows us to simulate the destruction of a large number of dwarf galaxies with different shapes and sizes and identify a pattern that best fits the stream of stars we see today.

Credits: NASA / ESA Hubble

“It’s a huge problem and we solve it by running tens of thousands of different simulations until we get one that actually matches what we see. And that takes a lot of power, which we get with the help of volunteers around the world who are part of the MilkyWay @ Home project, ”explained Newberg. As published February 17, 2022 in The Astrophysical Journal, Newberg’s team estimates the total mass of the original galaxy whose stars now form the Orphan-Chenab flux as 2 × 107 times the mass of our sun.

However, it is estimated that just over 1% of that mass is made up of ordinary matter such as stars. The rest is assumed to be a hypothetical substance called dark matter that exerts gravitational force, but which we cannot see because it does not absorb or emit light. The existence of dark matter would explain a discrepancy between the gravitational pull of the mass of matter that we can see and the much larger pull needed to explain the formation and movement of galaxies. It is estimated that the gravitational pull of dark matter constitutes up to 85% of the matter in the universe and that tidal flows of stars that have fallen into the Milky Way could be used to determine where dark matter is in our galaxy. “Tidal stream stars are the only stars in our galaxy for which it is possible to know their positions in the past,” said Newberg. “By looking at the current speeds of stars along a tidal current, and knowing that they were all in the same place and moving at the same speed, we can understand how much gravity changes along that stream. And that will tell us where the dark matter is in the Milky Way ”.

The research also notes that the progenitor of the Orphan-Chenab current has less mass than the galaxies measured today in the periphery of our galaxy, and if this small mass is confirmed it could change our understanding of how small star systems form and then merge together to create. larger galaxies such as our own Milky Way.

Galactic halo expert Dr. Newberg is a pioneer in identifying stellar tidal currents in the Milky Way. One day, he hopes MilkyWay @ home will help her measure more of the properties of a disintegrated dwarf galaxy. Ideally, he would like to simultaneously adapt many dwarf galaxies, their orbits and the properties of the Milky Way galaxy itself. This goal is complicated by the fact that the properties of our galaxy change over the billions of years it takes for a small galaxy to fall and be torn apart to create these tidal streams.

“By scrupulously following the path of the stars dragged into the Milky Way, Dr. Newberg and her team are constructing an image that shows us not only a long-destroyed dwarf galaxy, but also sheds light on the formation of our galaxy and the very nature of the galaxy itself. matter, ”said Curt Breneman, principal of the Rensselaer School of Science.