We can finally map the spiral arm on the far side of the galaxy

 

 

 

We’re putting the far side of the galaxy on the map. The most precise measurement yet of an object on the far side of the galaxy’s centre is paving the way for a definitive map of the other side of the Milky Way.

It’s difficult to observe anything on that side of the galaxy because of the dense, frenetic swarm of dust and gas at its centre. Thomas Dame at the Harvard-Smithsonian Center for Astrophysics in Massachusetts and his colleagues got around this by looking at a jet of radio waves that can outshine any emissions coming from that mess of stars.

“It’s a very bright source, indicative of a flamboyant region of star formation, and these regions are almost always located in the spiral arms of the galaxy,” says Dame. He and his team pinned down the source’s location to the Scutum-Centaurus arm of the galaxy, probably one of the Milky Way’s two major arms.

To do this, they used parallax measurements, which take into account differences in measurements from two points in space. If you hold a finger up at arm’s length and close one eye, then the other, your finger will appear to move over slightly –  that small discrepancy is called parallax. The closer the object is to the observer, and the further apart the two observing locations, the larger the parallax.

Building a better map

The object that Dame and his colleagues measured is so far away that to determine its parallax, they had to measure it over the course of a whole year, taking observations at either side of Earth’s orbit around the sun.

“These angles that are being measured are mind-boggling,” says Dame. “In this case it’s about 50 arcseconds, which is smaller than the size of Neil Armstrong’s footprint on the moon viewed from Earth.”

That tiny parallax angle corresponds to a distance of 66,500 light years. It’s the furthest distance ever measured via parallax, and the first time it’s been used for an object on the other side of the galaxy.

Since much of our understanding of structure that part of the Milky Way comes from assumptions based on knowledge of the near side, parallax measurements may allow us to build a more accurate map of our entire galaxy.

“The idea that you could be doing this for more objects on the far side of the galaxy is really exciting,” says Robert Benjamin at the University of Wisconsin, Whitewater. “How can you talk about the structure of our galaxy when you only have half of it?”

Dame says with this technique we could have an accurate and complete map of the entire Milky Way within 10 years.