Since its discovery in the 1960s, astronomers believed that in a relatively quiet suburb of our universe 250 million light-years away, there lived a boring little galaxy named UGC 1382 that was otherwise unremarkable — apart from being a system of millions of stars, gas and dust being held together by gravity. It is one of 12,921 listed in the Uppsala General Catalogue of Galaxies, or UGC.
After a quick glance, it was deemed elliptical, which is the most common type of galaxy, and lacking the spiral structure of disk galaxies like the home of our solar system, the Milky Way. But because of the technology they used, astronomers didn’t see UGC 1382’s signature huge bright blue spiral arms that proclaimed it to be something else entirely.
Using NASA’s Galaxy Evolution Explorer (GALEX), an orbiting ultraviolet space telescope that operated from 2003 to 2012, astronomers were able to use multi-wavelength surveys at a longer exposure to take a closer look at this sleepy low-light suburb of stars. A team of astronomers from the Carnegie Institution of Science, including Mark Seibert, Barry Madore, Jeff Rich and summer student Lea Hagen, were looking for the evidence of star formation still occurring in a typical “red and dead” elliptical galaxy.
After using optical data and viewing UGC 1382 in ultraviolet, they were surprised to see UGC 1382’s massive spiral arms. They also saw an enormously wide rotating hydrogen disk. It was in fact a giant low surface brightness disk galaxy, seven times as large as the Milky Way. But because it is so big and diffuse with expanding gas on the outside, its light is faint.
Further observation revealed something even more rare: The center of UGC 1382 is younger than its outer spiral. Normally, a galaxy forms stars and absorbs gas and dust, which creates more stars, and the cycle continues. That means that the oldest part and origin point is at the center, Hagen said. She is now a grad student and lead author of the study, publishing in the Astrophysical Journal.
They dubbed it a Frankenstein galaxy, where the inside and outside formed differently before merging. Rather than being a quick merge, this was a slow dance over billions of years. The old blue spiral arms are one piece of a galaxy that extend from a younger inner red disk of another galaxy. In order for the galaxy to form in the first place, its “food” usually constitutes puny, gas-rich dwarf galaxies that allow it to build outwards and stretch.
“It’s galactic cannibalism,” Hagen said. “Galaxies eat their friends. In most galaxies, where they live in groups or clusters, it’s like a city. When they’re crowded and bump into each other, they stretch and pull and disrupt each other. But in this suburb, it has lasted for several billion years without an interaction that would disrupt or destroy the disk.”
While other galaxies have formed this way, it’s Frankentstein-like nature is very rare because not many of them are so large and spread out, Hagen said.
And being a giant low surface brightness galaxy puts it in an elusive class of about a dozen that have been discovered so far. They are categorized by two components, a high surface brightness disk galaxy which is surrounded by an extended low surface brightness disk. Before the new discovery, the largest known was Malin 1, but it now has a rival in UGC 1382.
Given their faint light, isolated locations and distance from our own galaxy, these have been difficult to study before. But UGC 1382’s favorable location is closer to us, which will enable astronomers to study more about how the galaxy formed and evolved. It will also allow them to take another look at other galaxies that might also be misclassified or misunderstood.
Even though it’s much larger than the Milky Way, UGC 1382 has about the same amount of stars and gas, it’s just more spread out. And although its structure is delicate, if it has lasted this long, UGC 1382 will continue to thrive until it encounters a neighbor out in that quiet suburb of the universe.