Clash of the Galaxies

When stars collide, it’s not always in a Hollywood action film or on the red carpet. In a much more star-studded event than any movie premiere or awards could ever be, NASA has captured images of a collision of galactic proportions. Through the use of NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, researchers have been given a look at a true blockbuster of a galactic merger. The unprecedented pictures show two huge galaxies, collectively called Arp 299, coming together and expanding. Located 134 million light-years away, each of the galaxies have a supermassive black hole at its heart. NuSTAR is the first telescope that can pick up the origin of the high energy X-rays that exist within the two galaxies. Both are quite large and since they are beginning to combine, pinpointing exactly where the high-energy rays originate is difficult. Telescopes including NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton, can only detect lower-energy X-rays, but had indicated the possibility of super massive black holes actively fueling the growth of Arp 299. That data alone however could not tell scientists whether one or both of the galaxies were bulking up through a process called “accreting” where the gravity of a black hole drags gas inward, increasing its size. It is this growth that can tell researchers a great deal about the origin of galaxies and their evolution over time. Andrew Ptak of NASA's Goddard Space Flight Center in Greenbelt, Maryland and lead author of a new study accepted for publication in the Astrophysical Journal said, "When galaxies collide, gas is sloshed around and driven into their respective nuclei, fueling the growth of black holes and the formation of stars. We want to understand the mechanisms that trigger the black holes to turn on and start consuming the gas." NASA is using the data provided by NuSTAR along with available-light images of Arp 299 to visualize the two galaxies more accurately. The data was overlaid on an image captured by the Hubble Space Telescope to reveal two separate galaxies, each with a black hole. Analyzing the images and data together, NASA has determined that the black hole on the right is in the midst of a feeding frenzy. As it takes in gas, the energy created close to the black hole heats up electrons and protons to hundreds of millions of degrees, creating a superhot plasma, or corona, that converts the visible light into to high-energy X-rays. While it’s possible that the galaxy on the left is also growing, but buried too deep in gas clouds for the X rays to escape, NASA scientists believe it is more likely that it’s in a dormant state, or quiescent mode and not currently growing. "Odds are low that both black holes are on at the same time in a merging pair of galaxies," said Ann Hornschemeier, a co-author of the study. "When the cores of the galaxies get closer, however, tidal forces slosh the gas and stars around vigorously, and, at that point, both black holes may turn on." The study findings were presented at the recent American Astronomical Society meeting in Seattle.