In 2018, astronomers discovered that the stars in the outer rims of galaxies would rotate once every billion years, no matter how big they are. “It’s not Swiss-watch precision,” said Dr. Gerhardt Meurer with the International Centre for Radio Astronomy Research (ICRAR), who studies the interrelationship between the interstellar medium (ISM) and the various populations of stars and dark matter; in particular, how stars form in galaxies. “But regardless of whether a galaxy is very big or very small,’ he explains, “if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round.”
For reference, our Sun is located 26 thousand light years from the center of our Milky Way, which is about 40% of the way to the outer rim, and takes roughly 240 million years to orbit the Galactic center. Stars located at the edge of the Milky Way take four times longer to complete a single revolution. The recent study discovered that stars at the edges of all galaxies, regardless of their mass or size, take a similar amount of time to orbit.
“Discovering such regularity in galaxies really helps us to better understand the mechanics that make them tick-you won’t find a dense galaxy rotating quickly, while another with the same size but lower density is rotating more slowly,” he said.
Old Stars Discovered to Extend to the Outer Edge of the Milky Way
In the solar neighborhood, young massive stars form in the spiral arms within the narrow thin disk of our Milky Way. Meanwhile, older stars that formed more than 10 billion years ago extend further out of the plane of the Galaxy, comprising the thick disk. Astronomers originally thought that the thick disk of old stars dramatically tapered with increasing distance from the center of the Galaxy. However, recent observations of nearby galaxies instead reveal populations of old stars that also extend to the outer edge.
Meurer and his team found evidence of older stars existing out to the edge of galaxies. “Based on existing models, we expected to find a thin population of young stars at the very edge of the galactic disks we studied,” he said. “But instead of finding just gas and newly formed stars at the edges of their disks, we also found a significant population of older stars along with the thin smattering of young stars and interstellar gas.”
“This is an important result because knowing where a galaxy ends means we astronomers can limit our observations and not waste time, effort and computer processing power on studying data from beyond that point,” said Meurer. “So because of this work, we now know that galaxies rotate once every billion years, with a sharp edge that’s populated with a mixture of interstellar gas, with both old and young stars.”
The ESO animation below compares rotating disc galaxies in the distant Universe and the present day. The imaginary galaxy on the left is in the nearby Universe and the stars in its outer parts are orbiting rapidly due to the presence of large amounts of dark matter around the central regions. On the other hand the galaxy at the right, which is in the distant Universe, and seen as it was about ten billion years ago, is rotating more slowly in its outer parts as dark matter is more diffuse.
Professor Meurer said that the next generation of radio telescopes, like the Square Kilometer Array (SKA), will generate enormous amounts of data, and knowing where the edge of a galaxy lies will reduce the processing power needed to search through the data.
“When the SKA comes online in the next decade, we’ll need as much help as we can get to characterize the billions of galaxies these telescopes will soon make available to us.”
The image at the top of the page from NASA’s Galaxy Evolution Explorer shows the galaxy NGC 300, located about seven million light-years away in the constellation Sculptor. It is a classic spiral galaxy with open arms and vigorous star formation throughout.
Blue represents ultraviolet light captured by the telescope’s long-wavelength detector. Green shows ultraviolet light from the short-wavelength detector, and red shows red visible light from the Las Campanas Observatory, Chile.