The real monster black hole is revealed in this new image from NASA's Nuclear Spectroscopic Telescope Array of colliding galaxies Arp 299. In the center panel, the NuSTAR high-energy X-ray data appear in various colors overlaid on a visible-light image from NASA's Hubble Space Telescope. The panel on the left shows the NuSTAR data alone, while the visible-light image is on the far right.
Before NuSTAR, astronomers knew that the each of the two galaxies in Arp 299 held a supermassive black hole at its heart, but they weren't sure if one or both were actively chomping on gas in a process called accretion. The new high-energy X-ray data reveal that the supermassive black hole in the galaxy on the right is indeed the hungry one, releasing energetic X-rays as it consumes gas.
In this image, X-rays with energies of 4 to 6 kiloelectron volts are red, energies of 6 to 12 kiloelectron volts are green, and 12 to 25 kiloelectron volts are blue.