Our galaxy, the Milky Way, is part of the Local Group of galaxies, which contains multiple galaxies with ours being the second biggest. The queen of the Local Group is the Andromeda galaxy, which shares many characteristics with us and in the future will likely collide with us, creating a massive elliptical galaxy. Using NASA's Chandra X-ray Observatory researchers have identified 26 potential black holes in Andromeda, adding to the nine previously found.
To find these black holes, Chandra had to observe the other galaxy over 150 times across 13 years. Being an X-ray telescope, Chandra is an ideal tool for finding black holes as matter falling into them will produce the high energy radiation. To help determine if the X-ray sources were possibly black holes or neutron stars, the researchers also brought in ESA's XMM-Newton X-ray observatory, which can distinguish X-rays of different energy levels. Neutron stars are other, very dense stellar objects that can emit X-rays, but only black holes could produce the signals the researchers found.
At 35 black hole candidates, the Andromeda galaxy is now second only to the Milky Way in number of potential black holes found. Many of these new candidates were found in globular clusters, remnants of ancient star formation also present in the Milky Way, but curiously no black holes have ever been identified in ours.
Source: Chandra X-Ray Observatory