A fairly ordinary-looking galaxy has been hiding a strange secret in plain sight. It is 10 times larger than anyone thought (a whopping 718,000 light-years in diameter), plus it's younger on the inside than it is on the out. Scientists think that it's been stitched together from the pieces of several other galaxies — hence its nickname, the "Frankenstein galaxy". Galaxy UGC 1382 in optical (pane 1) ultraviolet (pane 2) and ultraviolet and radio (pane 3) views. Image: NASA/JPL/Caltech/SDSS/NRAO/L. Hagen and M. Seibert
Under ordinary, optical light, Galaxy UGC 1382 looks smaller and fairly ordinary. In fact, the galaxy had been included in over 40 different papers before this discovery, and none of them had catalogued it as unusual. Then astronomers decided to take a closer look in the ultraviolet regime as part of a different survey, and the strange features of the galaxy began to shine through.
Image: NASA/JPL/Caltech/SDSS/NRAO/L. Hagen and M. Seibert
"We pulled up one of these galaxies and, lo and behold, there's this amazing set of spiral arms extending out," co-author Mark Seibert of the Observatories of the Carnegie Institution for Science told Gizmodo. "You simply don't expect to see spiral arms in an elliptical galaxy."
Furthermore, for almost all known galaxies, the outside edges are younger than the inside. That's because galaxies form from the centre, and expand outwards with newer, younger stars. But for UGC 1382, the galaxy's centre was younger than its outside.
"We call it the Frankenstein galaxy because it formed out of a couple different galaxies," said Lea Hagen, a graduate student at Pennsylvania State University who led the study. "The idea of galactic cannibalism, of galaxies eating their friends, is well-established. We think what happened is you had some tiny dwarf galaxies and then a few billion years later another galaxy formed nearby. At some point, those all came together."
For Galaxy UGC 1382 to form as it did — to essentially merge, leaving whole parts of the galaxies relatively intact — everything had to occur exactly as it did. Even slight changes would have been enough to dramatically change the course. "If they were galaxies of equivalent size, that would be a crash and that would have been disastrous," Seibert said. "Large galaxies don't come together like this, it's a medium sized galaxy and some little ones."
Also key was Galaxy UGC 1382's location, 250 million light years away from us. Its position in a relatively quiet patch of universe let it form undisturbed by other nearby galaxies, until it reached an incredible size. "As large as it is, it's still very thin and diffuse," Seibert said. "It requires very specific conditions to make these kinds of monsters. They have to be in very specific environments."
The unexpected size of the galaxy raises questions over just what else we're missing out there. Although the researchers believe the strange features of Galaxy UGC 1382 are very rare, there are other faint objects that could have their own secrets hidden.