For the first time, astronomers have determined the initial mass and size of a dwarf galaxy that was shattered billions of years ago in a collision with the Milky Way. Reconstructing the original dwarf galaxy, whose stars now flow across the Milky Way in a stellar “tidal stream,” may help scientists understand how galaxies like the Milky Way develop and may assist in the hunt for dark matter in our galaxy.
“We’ve been doing simulations that take this massive stream of stars and back it up for a few of billion years to see what it looked like before it crashed into the Milky Way,” said Heidi Newberg, a Rensselaer Polytechnic Institute professor of physics, astrophysics, and astronomy. “We now have a measurement based on data, and it’s the first significant step toward utilizing the knowledge to identify dark matter in the Milky Way.”
The dwarf galaxy and others like it around the Milky Way were pushed into the main galaxy billions of years ago. As each dwarf galaxy merged with the Milky Way, its stars were dragged by “tidal forces,” which are the same differential forces that cause tides on Earth. Tidal forces warped and finally ripped apart the dwarf galaxy, stretching its stars into a tidal torrent that blasted across the Milky Way. Such tidal mergers are rather frequent, and Newberg believes that “immigrant” stars absorbed into the Milky Way account for the majority of the stars in the galactic halo, a roughly spherical cloud of stars that surrounds the spiral arms of the central disk.
The positions and velocities of the tidal stream stars, crucially, include information on the Milky Way’s gravitational field.
Reconstructing the dwarf galaxy is a scientific undertaking that incorporates data from star surveys, physics, and Newberg’s MilkyWay@Home distributed supercomputer, which harnesses 1.5 petaflops of home computer power given by volunteers. With this much computing capacity, it is conceivable to simulate the annihilation of a huge number of dwarf galaxies of various forms and sizes and choose a model that best fits the current tidal torrent of stars.
“It’s a massive challenge that we address by performing tens of thousands of simulations until we find one that truly corresponds. And it necessitates a significant amount of computing power, which we get with the assistance of volunteers from all across the globe who participate in MilkyWay@Home “Newberg said “We’re brute-forcing it, but given the complexity of the situation, I believe our strategy has a lot of value.”
Newberg’s team calculates the total mass of the original galaxy whose stars now constitute the Orphan-Chenab Stream to be 2×107 times the mass of our sun, according to a paper published today in The Astrophysical Journal.
However, it is thought that just a little more than 1% of that mass is made up of ordinary stuff such as stars. The rest is considered to be dark matter, a hypothetical material that exerts gravitational pull but cannot be seen since it does not absorb or emit light. The presence of dark matter would explain the disparity between the gravitational pull of visible matter and the far stronger force required to account for the development and migration of galaxies. Dark matter’s gravitational pull is thought to account for up to 85 percent of the stuff in the universe, and tidal streams of stars colliding with dwarf galaxies might be used to establish where dark matter is situated in our galaxy.
“Tidal stream stars are the only stars in our galaxy for which we can know their prior locations,” Dr. Newberg said. “We can calculate how much gravity varies along a tidal stream by looking at the present speeds of stars along that stream and knowing they all used to be in about the same spot and travelling at the same pace. This will lead us to the location of dark matter in the Milky Way.”
The study also discovered that the progenitor of the Orphan-Chenab stream has less mass than the galaxies measured in our galaxy’s outskirts today, and if this small mass is confirmed, it could change our understanding of how small stellar systems form and then merge to form larger galaxies like our Milky Way.
Dr. Newberg, a galactic halo researcher, was a pioneer in finding star tidal streams in the Milky Way. She believes that one day, MilkyWay@home will assist her in measuring more than the features of a single disintegrating dwarf galaxy. In an ideal world, she would be able to accommodate countless dwarf galaxies, their orbits, and the features of the Milky Way galaxy all at once. This objective is hampered by the fact that our galaxy’s characteristics vary throughout the billions of years it takes for a tiny galaxy to fall in and be torn apart, resulting in these tidal torrents.
“By painstakingly tracking the path of stars drawn into the Milky Way, Dr. Newberg and her team are constructing an image that not only shows us a dwarf galaxy long-since destroyed, but also sheds light on the formation of our galaxy and the very nature of matter,” said Curt Breneman, dean of the Rensselaer School of Science.
Eric J. Mendelsohn, Siddhartha Shelton, and Jeffery M. Thompson assisted Newberg in his research at Rensselaer. Carl J. Grillmair of the California Institute of Technology and Lawrence M. Widrow of Queen’s University were also involved in the discovery. “Estimate of the Mass and Radial Profile of the Orphan-Chenab Stream’s Dwarf Galaxy Progenitor Using MilkyWay@home” was published with funding from the National Science Foundation and data from the Sloan Digital Sky Survey, the Cerro Tololo Inter-American Observatory’s Dark Energy Camera, and the National Aeronautics and Space Administration/Infrared Processing & Analysis Center Infrared Science Archive.