Some of the most powerful objects in the entire universe are black holes. Black holes live at the center of galaxies, and as massive galaxies hurtle through space, sometimes they collide with each other and merge over hundreds of thousands of years. Astronomers have observed several pairs of galaxies in the final stages of merging after colliding. The image seen below shows these galactic mergers using techniques to peer through the dense clouds of dust and gas that surround the merging galaxies.
At the center of the merging galaxies is a pair of black holes that once lived at the center of each of the two smaller galaxies; these black holes are drawing closer and closer to each other and will eventually merge into one more massive black hole. These merging galaxies were observed using the Hubble Space Telescope (which only recently recovered from safe mode) and the W.M. Keck Observatory in Hawaii. Observations made with the Hubble telescope represent more than two decades worth of images.
Astronomers say that these high-resolution images give them a close look at a phenomenon that is believed to have been more common in the early universe when galaxies collided more often. When these two black holes do collide, scientists say that they will unleash powerful gravitational waves, which are ripples in space-time, that were detected for the first time only recently by the twin Laser Interferometer Gravitational-wave Observatory detectors.
The images are also said to give a good idea what will happen to our own Milky Way galaxy when it collides with the Andromeda galaxy in a few billion years. Our Galaxy and Andromeda both have supermassive black holes at the cores that will eventually become one. Astronomers searched ten years of data from the Burst Alert Telescope on the NASA Neil Gehrels Swift Observatory to search for visually obscured, active black holes. Hubble archive information was then used to pick merging galaxies spotted via the BAT x-ray data.
Keck was then called on to use its near-infrared vision to observe the x-ray producing black holes not found in the Hubble archive. The two merging galaxies the team is studying are an average of 330 million light-years from Earth; these galaxies are similar in size to the Milky Way and Andromeda. After analyzing 96 galaxies with the Keck telescope and 385 from the Hubble archive, the team says data suggests that 17% of these galaxies have a pair of black holes at their center that will eventually become an ultra-massive black hole when merged.