Spiral arms of Milky Way

According to a study conducted by a group of Chinese astronomers, one of the spiral arms of the Milky Way, known as Shield-Centaur arm, could extend much further than assumed, wrapping all the way around our galaxy. It would hold a rare event ever observed in other spiral galaxies nearby.
Since our solar system resides within the Milky Way, is very difficult to get a clear picture of how it globally looks our galaxy. In 1852, the astronomer Stephen Alexander was the first to hypothesize that the Milky Way had a spiral shape and since then we have followed a series of observations and discoveries that have changed the way we imagine. For several decades, astronomers have found that our galaxy was formed by four arms where there are stars and gas clouds where star formation occurs, which extend to the outer regions according to a spiral structure. In 2008, data from the Spitzer Space Telescope seemed to indicate the presence of only two arms crossed over a central structure.


Credit: NASA/JPL-Caltech/SSC

Today, however, according to a study conducted by a group of Chinese astronomers, one of the spiral arms could extend much further than assumed, wrapping the entire Milky Way. Shield-arm called Centaur, it starts from one end of the bar structure of the Milky Way crosses the orbit of the Sun and it extends to the other side of the galaxy, where it was thought ended. But in 2011, astronomers Thomas Dame and Patrick Thaddeus, the Harvard-Smithsonian Center for Astrophysics, they noticed what appeared to be an extension of this arm on the other side of the galaxy, always placed outside of our Solar System. “And what a rare occurrence. I bet that one should analyze tens of spiral galaxies to find one where you can get to see an arm that makes a 360 ° turn, “says Dame.
However, according to Yan Sun and colleagues at the Purple Mountain Observatory in Nanjing, China, the arm-shield Centaur could extend even farther. In fact, using a different approach that allows studying the clouds of gas distributed between 46,000 and 67,000 years light from the galactic center, scientists have revealed 48 new clouds of interstellar gas, compared to 24 already known previously. To deepen their research, Sun and colleagues have used some data from a radio telescope provided by researchers working on the project Milky Way Imaging Scroll Painting which aims to monitor clouds of interstellar gas and dust to reveal the radio waves emitted by carbon monoxide (CO). After hydrogen, this gas is the most abundant element that is found in the interstellar medium and it is easier to reveal it by radio telescopes.


Credit: NASA/JPL-Caltech/SSC

Therefore, combining this information with that derived from the data of the Canadian Galactic Plane Survey, which studies the time distribution of hydrogen, the researchers concluded that these 72 clouds are aligned along a segment of the arm that extends for about 30,000 light years. In addition, the scientists argue in their article that “the new arm appears to be the extension of the one found by Dame & Thaddeus, in the outer second quadrant.” This implies that the spiral arm would not only be the largest in the Milky Way, but it would be the only one to actually reach an extension that completely surrounds our galaxy. It therefore would retain an unprecedented achievement, considering the fact that nothing like that has never been observed in other spiral galaxies, at least in our part of the local Universe.
Of course, there are problems. There would be, in fact, a sort of apparent emptiness between the segment of arm found by Dame & Thaddeus and one discovered by the Chinese group, so we are talking about 40,000 light years. This could mean that the gas clouds identified by Sun and colleagues are not part of the arm Shield-Centaur but belongs to another portion, a completely new spiral arm. In short, if this proves true, perhaps the Milky Way may be characterized by different external arms, so with the upcoming observations we can, hopefully, to verify or refute this hypothesis.

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