Astrophysicists have for the first time calculated the original mass and size of a dwarf galaxy that was ripped apart in a collision with the Milky Way billions of years ago. Reconstruction of the original dwarf galaxy, whose stars now cross the Milky Way in a stellar ‘tidal current’, will help scientists understand how galaxies like the Milky Way formed and might aid research dark matter in our galaxy.
“We ran simulations that take this big stream of stars, save it for a few billion years, and see what it looked like before it fell into the Milky Way,” said physics professor Heidi Newberg. astrophysics, and astronomy at Rensselaer Polytechnic Institute. “We now have a measurement from the data, and this is 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 near the Milky Way were swept into the larger galaxy. As each dwarf galaxy merged with the Milky Way, its stars were driven by “tidal forces,” the same type of differential forces that make tides on Earth. Tidal forces warped and eventually tore the dwarf galaxy apart, stretching its stars in a tidal current thrown through the Milky Way. Such tidal mergers are fairly common, and Newberg believes that “immigrant” stars absorbed into the Milky Way make up most of the stars in the galactic halo, a roughly spherical cloud of stars that surrounds the spiral arms of the central disk.
Critically, the position and velocities of tidal current stars contain information regarding the gravitational field of the Milky Way.
Reconstruction of the dwarf galaxy is a research task that combines data from star surveys, physics and Newberg’s MilkyWay@Home distributed supercomputer, which operates at 1.5 petaflops – a measure of processing speed computing – home computer power donated by volunteers. This great processing power makes it possible to simulate the destruction of a large number of dwarf galaxies of different shapes and sizes, and to identify a model that best matches the tidal current of stars we see today.
“It’s a huge problem, and we solve it by running tens of thousands of different simulations until we get one that actually matches. And that takes a lot of computing power, which we get with help from volunteers around the world who are part of MilkyWay@Home,” Newberg said. “We force it hard, but given the complexity of the problem, I think this method has a lot of merit. »
As published today in The Astrophysical JournalNewberg’s team estimates the total mass of the original galaxy whose stars today form the Orphan-Chenab flux at 2×107 times the mass of our sun.
However, it is estimated that just over 1% of this mass is made up of ordinary matter such as stars. The rest is assumed to be a hypothetical substance called dark matter that exerts a gravitational force, but which we cannot see because it neither absorbs nor emits 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 motion of galaxies. The gravitational pull of dark matter is estimated to account for up to 85% of the matter in the universe, and tidal currents from stars that fell with dwarf galaxies might be used to determine where the matter is. dark 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,” Dr Newberg said. “By looking at the current velocities of stars along a tidal stream, and knowing that they were all in roughly the same place and moving at the same speed, we can determine how much gravity is changing the along this current. And that will tell where the dark matter is in the Milky Way. »
The research also reveals that the ancestor of the Orphan-Chenab stream has less mass than galaxies measured on the outskirts of our galaxy today, and if this small mass is confirmed, it might change our understanding of how small star systems form and then merge. to create larger galaxies like our Milky Way.
Galactic halo expert Dr. Newberg is a pioneer in identifying stellar tidal currents in the Milky Way. One day, she hopes MilkyWay@home will help her measure more than the properties of a disintegrated dwarf galaxy. Ideally, she would like to adapt many dwarf galaxies, their orbits, and the properties of the Milky Way galaxy itself simultaneously. 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 tear apart to create these tidal currents.
“By painstakingly tracking the trajectory of stars drawn into the Milky Way, Dr. Newberg and his team are constructing an image that not only shows us a long-destroyed dwarf galaxy, but also sheds light on the formation of our galaxy and the very nature of the galaxy. matter,” said Curt Breneman, dean of the Rensselaer School of Science.
At Rensselaer, Newberg was joined in research by Eric J. Mendelsohn, Siddhartha Shelton, Jeffery M. Thompson. Carl J. Grillmair of the California Institute of Technology and Lawrence M. Widrow of Queen’s University also contributed to the discovery. ” Estimation of the mass and radial profile of the dwarf galaxy progenitor of the Orphan-Chenab flux using MilkyWay@home was published with support from the National Science Foundation and with data from the Sloan Digital Sky Survey, the Dark Energy Camera at the Inter-American Cerro Tololo Observatory, and the National Aeronautics and Space Administration/Infrared Processing & Analysis Center Infrared Science Archive.