As strange as the past year has been, it likely comes as no surprise to learn that our galaxy has a warp in it and its stars are bouncing up and down like sports fans doing the wave in a giant, rotating stadium.
The stellar motion and the galactic warp were likely caused some 3 billion years ago by a gravitational drive-by with a galaxy that orbits the Milky Way.
That’s the conclusion a trio of University of Virginia researchers came to after using a satellite-based telescope, a large-scale stellar spectroscopic survey conducted in the near-infrared portion of the electromagnetic spectrum and lots and lots of math full off letters and symbols.
“The fact that the Milky Way is warped in its rotation has been known from studies of gas and radio astronomy and it’s been known for decades. What was not so clear is that the stars are doing the same thing,” said Steven Majewski, UVa professor of astronomy and the principal investigator for the Apache Point Observatory Galactic Evolution Experiment, also known as APOGEE.
Our sun, the researchers say, is approximately 8 kiloparsecs from the middle of the Milky Way. At 3,260 light years per kiloparsec, that’s a long way to go. The galaxy, however, continues well beyond the sun and that portion has the warp.
For people of a certain age, the warp resembles an old-school vinyl record left in the sun too long.
“It’s the difference between looking at the platen and seeing that it doesn’t look quite flat and then turning it on and seeing the warp going around,” Majewski said. “We measured the speed of the warp going around and how far out from the center of the record that the warp starts. We’re in a part of the disk that is flat but just beyond where we are is where the warp is.”
If thinking about the warp and rotation is causing a bit of motion sensitivity, just wait. To get the full picture, the grooves in the record would also be in a constant up-and-down motion and spinning at a different rate than the record.
“If you are looking at the stars, they are moving up and down during the wave. At the same time, the galaxy is also turning. It’s like the spectators in a stadium doing the wave while the whole stadium is rotating,” said Xinlun Cheng, an astronomy graduate student working with Majewski and research associate Borja Anguiano.
Much of the mapping of the warp and the movements was put to rest by Cheng who performed extensive mathematical calculations using complicated formulas, Majewski said.
“The wave is a circular motion that’s made up of particles – the people – moving up and down. But we’ve got the additional movement where the stars are rotating in orbit in the galaxy and the warp is going around but it’s going around a different rate than the galaxy,” Cheng said.
The galactic bob, weave and wave was detected using the Gaia satellite space observatory of the European Space Agency, launched in 2013 and the UVa-developed APOGEE infrared spectrograph that examines chemical composition and motions of stars.
The three authored a scholarly article, “Exploring the Galactic Warp through Asymmetries in the Kinematics of the Galactic Disk,” that was published in the December issue of The Astrophysical Journal.
“We’re doing a massive survey of the stars in the Milky Way, trying to understand how they’re moving and what their chemical compositions are,” Majewski said. “If you’ve ever seen the Milky Way at night, you see it’s very splotchy looking. Those splotches are clouds of dust in the foreground that block out the light of the stars behind it. The Milky Way is a very dusty place and most of the stars are behind that dust.”
Although optical light can’t make it through such dust, the infrared light given off by stars can be seen with instruments such as APOGEE.
“Ours is the first major spectroscopic survey of its kind working in the infrared,” Majewski said. “It’s given us the view of where most of the stars inhabit and that’s in the disk of the Milky Way.”
“By combining APOGEE and information from the Gaia satellite, we’re starting to understand how the different components of the galaxy are moving,” said Anguiano. “Most of the analyses before were just counting the stars. Now we can measure the precise movement of the stars. We’ve got the galaxy warping through studying the velocity of the movements of the stars.”
The Milky Way is not a cosmic outlier, however. An estimated 50% to 70% of galaxies similar to ours are warped.
“There’s a big debate on whether warps die off, if they are a transient phenomenon or do they last a long time. If they’re excited, do they dissipate or are they always still there?” Majewski said. “The fact that 70% of galaxies have a warp tends to make you think that if you get a warp, it’s around for a long time.”
Although our galaxy’s warp has been known for years, why it’s warped has been debated. Some believe the galaxy is inherently unstable. Others believe it was created by the galaxy gobbling up intergalactic matter like a star-starved Pac Man.
“While the origin of the Galactic warp still invites controversy, the fact that the stellar warp follows the same topology as [a similar warp in Milky Way gas clouds] is evidence that the warp is gravitationally induced,” the trio’s article states.
The researchers believe the warp, which is fairly dramatic and resembles the snapped down brim of a fedora, was created by gravitational close calls and collisions with other galaxies. The most likely culprit is the Sagittarian Dwarf Spheroidal Galaxy, not to be confused with the Sagittarius Dwarf Irregular Galaxy, which is a galaxy far, far away.
Cheng’s research shows the wave likely began about 3 billion years ago. The Sagittarius galaxy has been circling the Milky Way for about the same amount of time.
“Of all of the notions laid out for potential origins of the warp, I think we side with the notion that gravitational interaction with a smaller galaxy is the most likely,” Majewski said. “For me, the reason is that we know these interactions are happening and are common around other galaxies and our Milky Way.”
The galaxies don’t need to collide. A Tom Cruise Top Gun-type fly-by is all it takes.
“Even a small galaxy, if passing close enough to the edge of the disk, can cause a tugging of the outer disk in a way to get the warp started and rippling around the disk,” Majewski said.
Although our galaxy is warped and our heavenly bodies are bouncing about, the researchers say that has no real impact on those of us living on Earth.
“We’re inside of the space where the warp begins, so it has no effect on our sun,” Majewski said. “The distances between stars in the galaxy are vast. For instance, it’s anticipated that in 5 billion years the Milky Way and Andromeda galaxies may collide and, in that collision of gigantic galaxies, not one star will hit [another] star. The stars may change their orbit around the galaxy, but that doesn’t affect how the stars evolve individually. It just changes their perspective of where they are in the galaxy.”