Rings of stars thought to surround the Milky Way are actually part of it, according to new research, meaning the galaxy is bigger than previously believed.
The findings extend the known width of the Milky Way from 100,000 light-years across to 150,000 light-years, said Yan Xu, a scientist at the National Astronomical Observatories of China and former visiting scientist at Rensselaer who was the lead author of a paper detailing the discovery in Astrophysical Journal.
"Going into the research, astronomers had observed that the number of Milky Way stars diminishes rapidly about 50,000 light-years from the center of the galaxy, and then a ring of stars appears at about 60,000 light-years from the center," Xu said. "What we see now is that this apparent ring is actually a ripple in the disk. And it may well be that there are more ripples further out which we have not yet seen."
An international team led by Rensselaer Polytechnic Institute Professor Heidi Jo Newberg, came to this conclusion after revisiting astronomical data from the Sloan Digital Sky Survey and finding that the galactic disk of the Milky Way is actually contoured into several concentric ripples.
"In essence, what we found is that the disk of the Milky Way isn't just a disk of stars in a flat plane -- it's corrugated," said Newberg, a co-author of the paper. "It's very similar to what would happen if you throw a pebble into still water -- the waves will radiate out from the point of impact."
But in this case, the pebble could be a dwarf galaxy passing through the disk. "It would gravitationally pull the disk up as it comes in, and pull the disk down as it goes through, and this will set up a wave pattern that propagates outward," she explained. "As it radiates outward from the sun, we see at least four ripples in the disk of the Milky Way."
She added that although the data only looks at part of the galaxy, it can be assumed that the pattern continues throughout.
The new research builds upon a 2002 paper in which Newberg established the existence of the "Monoceros Ring," an "over-density" of stars at the outer edges of the galaxy that bulges above the galactic plane.
At the time, Newberg noticed evidence of another over-density of stars, between the Monoceros Ring and the sun, but was unable to investigate further. With more data available from the Sloan survey, researchers recently took another crack at it.
"I wanted to figure out what that other over-density was," Newberg said. "These stars had previously been considered disk stars, but the stars don't match the density distribution you would expect for disk stars, so I thought, 'Well, maybe this could be another ring, or a highly disrupted dwarf galaxy.'"
Newberg said the findings support recent research, including a theoretical finding that a dwarf galaxy or dark matter lump passing through the Milky Way would produce a similar rippling effect. In fact, the ripples might ultimately be used to measure the lumpiness of dark matter in our galaxy.