The spiral arms of the Milky Way form more complex shapes than previously recognized. Not long after a whole extra arm was found within our own galaxy, astronomers have discovered a "break" in one of these enormous rivers of stars; an oddly angled 3,000 light-year-long protuberance, looking uncomfortably like bone sticking out of a badly broken limb.
Reporting in Astronomy and Astrophysics, astronomers reveal this "splinter" is the first major structure found in our galaxy at an angle so different to the arm in its in, and yet is host to many well-known nebulae, including the Eagle Nebula, home to the breathtaking Pillars of Creation.
Spiral galaxies aligned face-on to Earth reveal their structures in often exceptional detail, and for almost 60 years we've known our own galaxy has a similar form. Unfortunately, trapped inside as we are, with major features obscured by dust clouds, it's much harder to work out the exact locations of the Milky Way's spiral arms. Most objects within our galaxy don't lend themselves to precise distance measurements with ground-based instruments, leaving attempts to map the way the galaxy's arms snake around it frustratingly inexact.
Other spiral galaxies often have substructures, known as spurs, feathers, and branches, but until now no one knew if any existed in our galaxy. The newly found structure might be a spur, or something for which we have no name. It sits not far off a line between the Sun and the galactic center.
Amateur and professional astronomers have spent many hours looking at parts of the possible spur without recognizing it is there. Its famous nebulae, the aforementioned Eagle, Triffid, Lagoon, and Omega nebulas were key to its identification. Less precise distance measurements to these nebulae alerted astronomers to the Sagittarius arm's existence – and therefore the spiral structure of the Milky Way – back in the 1950s.
"A key property of spiral arms is how tightly they wind around a galaxy," lead author Dr Michael Kuhn of Caltech in a statement. Perfect circles have what is called a pitch angle of 0. The main Sagittarius arm has an estimated pitch angle of 12 degrees. “The structure we examined really stands out at an angle of nearly 60 degrees," Kuhn added.
The finding is an outcome of the Spitzer Space Telescope which has discovered more than 100,000 very young and previously hidden stars in this area. Until its retirement last year Spitzer operated in the infrared, allowing it to collect light that passes through dust clouds visible light cannot cross. These newborn stars were examined with Gaia, another space telescope that offers much more precise distances to stars than has previously been available as well as information on their movements.
"When we put the Gaia and Spitzer data together and finally see this detailed, three-dimensional map, we can see that there's quite a bit of complexity in this region that just hasn't been apparent before," Kuhn said.
Some assumptions are still required to estimate the structure's shape, notably that hot young stars and nebulae are a good indication of where the rest of an arm's material lies. Since astronomers don't know why spiral arms exist, explaining spurs and other substructures remains well beyond them – for now. It is hoped finding one so close will rectify that.