Read: The milky way has giant bubbles at its center
For decades, most astronomers believed that the North Polar Spur was part of our local galactic neighborhood. Some studies concluded that it connects to nearby gas clouds. Others looked at its distortion of background stars and inferred that it’s a supernova remnant—a dusty cloud marking the gravestone of a dead star.
Yet Yoshiaki Sofue, an astronomer at the University of Tokyo, has always thought the spur looked funky for a stellar debris cloud. Instead, he imagined the arc to be one stretch of a huge, mostly unseen structure—a pair of bubbles straddling the galaxy’s heart. He published simulations in 1977 that produced digital clouds lining up with the spur, and ever since then he has told anyone who would listen that the spur actually hovers tens of thousands of light-years above the disk. He has described it as an expanding shock wave from a galactic calamity dating back millions of years.
But if Sofue were right, there should also be a twin structure to the south of the galactic plane. Astronomers saw no trace of this counterpart, and most remained unconvinced.
Then in 2010, the Fermi space telescope caught the faint gamma-ray glow of two humongous lobes, each extending roughly 20,000 light-years from our galaxy’s center. They were too small to trace the North Polar Spur, but they otherwise looked just like the galactic-scale clouds of hot gas Sofue had predicted. Astronomers began to wonder: If the galaxy had at least one pair of bubbles, perhaps the spur was part of a second set?
Read: The loneliest starts in the Milky Way
“The situation dramatically changed after the discovery of the Fermi bubbles,” says Jun Kataoka, an astronomer at Waseda University in Japan who has collaborated with Sofue.
The new images have further cemented the change of opinion. They came from eROSITA, an orbiting X-ray telescope that launched in 2019 to track dark energy’s effect on galaxy clusters. In June the eROSITA team released a preliminary map: the fruit of the telescope’s first six months of observations.
The map traces X-ray bubbles that stand an estimated 45,000 light-years tall, engulfing the gamma-ray Fermi bubbles. Their X-rays shine from gas that measures 3 million to 4 million degrees Kelvin as it expands outward at 300 to 400 kilometers per second. And not only does the northern bubble align perfectly with the spur, its mirror image is obvious as well—just as Sofue predicted. “I was particularly happy to see the southern bubble clearly exhibited, so similar to my simulation,” he says.
Still, a full interpretation of all North Polar Spur observations remains complex. A nearby supernova remnant could have parked itself right in front of the X-ray bubbles by chance, for instance, giving both interpretations elements of truth. In September, Das and collaborators used state-of-the-art observations of distant stars to show that something dusty is hanging out about 450 light-years away—a stone’s throw by galactic standards.