Astronomers Just Spotted a Massive Galaxy Orbiting the Milky Way

April 18, 2016 | Joanne Kennell

Milky Way photo, silhouettes of tree
Photo credit: pixabay.com

It is just one of four newly-discovered objects that are falling into our galaxy.

A team of British astronomers has discovered an extremely luminous dwarf galaxy orbiting our Milky Way. This new member of the galaxy family has been named Crater 2 dwarf, and it lies roughly 380,000 light-years away.

Gabriel Torrealba, Sergey Koposov, Vasily Belokurov, and Mike Irwin, all from Cambridge University, UK, made the discovery while analyzing images taken by the VLT Survey Telescope in northern Chile.

About four dozen known galaxies orbit our own, and the largest of them all is Sagittarius dwarf, which was discovered in 1994. However, the reason why it is so large is because our galaxy’s gravity is ripping it apart. The next largest are the Large and Small Magellanic Clouds, followed by the newly spotted gigantic Crater 2.

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Since most galaxies do not have defined edges, astronomers often express a galaxy’s size in terms of its “half-light diameter” — a region that encloses the brightest part of the galaxy and emits half its light. The Crater 2 dwarf has a half-light diameter of 7,000 light-years, which means that if we could see it, it would look twice as big as a full moon.

Josh Simon, an astronomer at the Carnegie Observatories in Pasadena, California, told New Scientist that the galaxy is quite notable because it is brighter than almost all of the galaxies found orbiting the Milky Way during the past decade. It emits 160,000 times more light than the sun.

Although the entire Crater 2 dwarf galaxy is not visible to the human eye, individual stars within it are, and that is part of the reason why the galaxy eluded astronomers for so long. Its stars are spread so far apart from one another, it gives the galaxy a ghostly appearance which is not as eye-catching or noticeable as a more tightly packed galaxy.

According to model simulations of galaxy formation, our Milky Way grew to such an enormous size by the merging of smaller galaxies. The simulations also suggest that whole groups of galaxies can fall into a single giant at the same time — an event Crater 2 is currently experiencing.

Until now, Crater 2 has led a quiet life, never traveling near a giant galaxy. Astronomers know this because the galaxy is round. If it had encountered a giant galaxy during its lifetime, gravity would have bent the dwarf out of shape.

However, according to Torrealba, this galaxy is not alone. Crater 2 dwarf is near four other newly-found objects: the Crater globular cluster, as well as three dwarf galaxies in the constellation Leo. All of these celestial bodies may be part of a group that is just beginning to fall into the Milky Way. But don’t worry, that process takes eons.

The results of the paper were published in the Monthly Notices of the Royal Astronomical Society and online at arxiv.org.

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