The size of our Milky Way

 

The disk of the Milky Way Galaxy disk may actually be rippled. (Image credit: Heidi Newberg)

Two ringlike structures of stars wrapping around the Milky Way's outer disk now appear to belong to the disk itself.

The results, outlined in a new study, show that the disk is about 60 percent larger than previously thought. Not only do the results extend the size of the Milky Way, they also reveal a rippling pattern, which raises intriguing questions about what sent wavelike fluctuations rippling through the disk.

The researchers said the likely culprit was a dwarf galaxy. It might have plunged through the Milky Way's center long ago, sparking the rippling patterns astronomers have now detected for the first time.

Roughly 15 years ago, Heidi Newberg, an astronomer at the Rensselaer Polytechnic Institute in New York, and her colleagues found a group of stars beyond the disk's outermost edge. The so-called Monoceros Ring is about 60,000 light-years from the galactic center (just beyond where the disk was thought to end at 50,000 light-years). Over the years, astronomers were divided into two camps regarding the origins of the ring. Some argued that it was simply a tidal stream: The debris of a dwarf galaxy that fell into the Milky Way and was stretched in the process. Others argued that the ring is a part of the disk. The issue, however, is that the ring is slightly above the plane of the disk. So astronomers in the latter camp attributed that to the fact that the disk flares up toward the edge. Enter Yan Xu, an astronomer at the National Astronomical Observatories of China. Xu, Newberg and colleagues took a second look at the problem using data from the Sloan Digital Sky Survey. With improved data compared to previous studies, they found four total structures in and just outside what is currently considered the Milky Way's outer disk. The third structure was the highly debated Monoceros ring, and the fourth structure was the Triangulum Andromeda Stream, located 70,000 light-years from the galactic center. All four structures alternated with respect to the disk. They went from above it, to below it, to above it, to below it. Newberg, who was in the tidal stream camp, was surprised that the ring and three other structures were actually a part of an oscillating disk.

"We didn't know how a disk could go up and down," said Newberg. Luckily, computer simulations by various teams showed that a dwarf galaxy falling into the Milky Way might create a similar pattern. "When it goes through, it can disturb the disk just like a pebble disturbs water in a puddle," said Newberg. "And that wave can propagate through the disk from that event."

This new picture makes sense, said Newberg. It even matches observations of the gases in the disk, which have long been observed as rippled. But the implications extend far beyond a corrugated disk.

"If it's true that the Monoceros Ring and the Triangulum Andromeda structure are part of this oscillatory pattern, then the stellar disk goes out way further than the textbook tells us it ought to be," said Newberg. Instead of extending nearly 100,000 light-years from one side to the other, it would be more like 160,000 light-years wide.

This brings the Milky Way's size up to that of Andromeda. The Milky Way's small radius in comparison to Andromeda's larger radius has always puzzled astronomers, because the two galaxies have roughly the same mass. The team plans to further map the rippled disk of Earth's galaxy and better match their results to models. The study was detailed in the March 10 edition of the Astrophysical Journal.

Shannon Hall

https://tinyurl.com/bdhhaxs9

The Milky Way is a vast, flat, barred spiral galaxy approximately 100,000 light-years in diameter and about 1,000 light-years thick. Our Solar System is located in one of its spiral arms, about 27,000 light-years from the center, which means we are unable to take a direct picture of the entire disk. 

• If the Sun were a tiny grain of sand, the entire Milky Way's stellar disk would still be an immense 100,000 miles across.
• Compared to the nearby Andromeda Galaxy (our closest large neighbor), which is 2.5 million light-years away, the Milky Way's diameter is roughly 1/20th of that distance.
• The entire observable universe is estimated to be approximately 930,000 Milky Way diameters across, making our galaxy a tiny speck in the grand scheme of the cosmos. 

Contemplate the incredible vastness of space with this visualization of the size of our Milky Way galaxy

: NASA/JPL-Caltech