This composite image of the nearby starburst galaxy M82 shows Chandra X-ray Observatory data in blue, optical data from the Hubble Space Telescope in green and orange, and infrared data from the Spitzer Space Telescope in red. The pullout is a Chandra image that shows the central region of the galaxy and contains two bright X-ray sources -- identified in a labeled version, roll your mouse over the image to view -- of special interest.
New studies with Chandra and ESA's XMM-Newton show that these two sources may be intermediate-mass black holes, with masses in between those of the stellar-mass and supermassive variety. These "survivor" black holes avoided falling into the center of the galaxy and could be examples of the seeds required for the growth of supermassive black holes in galaxies, including the one in the Milky Way.
This is the first case where good evidence for more than one mid-sized black hole exists in a single galaxy. The evidence comes from how their X-ray emission varies over time and analysis of their X-ray brightness and spectra, i.e., the distribution of X-rays with energy.