Astronomers scanning the skies just got a huge surprise. They discovered a gigantic galaxy orbiting our own, where none had been seen before. It just came out of nowhere. So, just how did the recently-discovered Crater 2 succeed to pull off this feat, like a deer jumping out from the interstellar bushes to suddenly shock us? Even though the appearance may seem sudden, the Crater 2 has been there all along. We just never saw it.
Now that astronomer know it’s there, though, there are a few other crushing facts that astronomers discovered. First of all, we can’t blame the galaxy’s size for its relative insignificance. Crater 2 is so massive that researchers have already identified it as the fourth largest galaxy orbiting our own. We can’t even blame its distance, either. Crater 2’s orbit around the Milky Way puts it just precisely in our neighbourhood.
That being said, the question arises, how did we still not know it was there? A new research paper published in Monthly Notices of the Royal Astronomical Society from astronomers at the University of Cambridge has an answer for us. It turns out that, regardless of being huge and close, Crater 2 is also a pretty dark galaxy. Actually, it’s one of the faintest galaxies ever detected in the cosmos. That, along with some much perkier neighbors, let the galaxy that astronomers have nicknamed “the feeble giant” remain hidden from our eyes until now.
Now that we have observed Crater 2, nevertheless, the discovery yields some questions about what else could be out there that we are still missing. Astronomers are already talking about starting a hunt for similarly large, dark galaxies near us. It’s a good thing that there’s still so much about cosmos that we still don’t know.
About four dozen known galaxies orbit our own. The largest in terms of breadth is the Sagittarius dwarf, discovered in 1994 – but it’s big only because our galaxy’s gravity is ripping it apart. The next two largest are the Magellanic Clouds.
Now, Gabriel Torrealba at the University of Cambridge and his colleagues have found a new galaxy about 380,000 light-years away in the constellation Crater. “It’s the fourth largest satellite of the Milky Way,” Torrealba says.
Named the Crater 2 dwarf, the new galaxy is not apparent to human eyes, though individual stars within the galaxy are visible. The team was only able to find it this January by using a computer to look for over-densities of stars in data from images taken by a telescope in Chile.
How do you measure a galaxy?
Most galaxies don’t have defined edges, so astronomers sometimes express a galaxy’s size in terms of its “half-light diameter”, which encloses the brightest part of the galaxy and emits half of its light. The Crater 2 dwarf has a half-light diameter of 7000 light years – which, if we could see it, would look twice as big as the full moon.
Josh Simon, an astronomer at the Carnegie Observatories in Pasadena, California, says the galaxy is notable because it is brighter than nearly all of the many galaxies found orbiting the Milky Way during the past decade. It emits 160,000 times more light than the sun.
The Galaxy eluded detection for so long because its stars are spread out from one another, giving it a ghostly appearance.
Torrealba says it may not be alone. The Crater 2 dwarf is near four other new-found objects: the Crater globular star cluster as well as three dwarf galaxies in Leo. All may be part of a group that is just now falling into the Milky Way.
Until now, though, the new galaxy has led a quiet life, never venturing near a giant galaxy. We know this because the galaxy is round. If it had encountered a giant, gravity would have bent the dwarf out of shape.