Astronomers find a puzzling pair of merging galaxies

Galaxy mergers are common throughout the universe. In fact, mergers are responsible for transforming the numerous small galaxies that existed early in cosmic history into the massive galaxies we see today. But because mergers take, on average, about 2 billion years or so to complete, astronomers must look for clues about this process in the “snapshots” we see today of galaxies in the various stages of merging. One such merger, Was 49, is challenging astronomers’ current picture of the merging process with a supermassive black hole that’s much larger than expected.

Was 49 is a merging galaxy system that can actually be broken down into Was 49a, a larger disk-shaped galaxy, and Was 49b, a small dwarf galaxy. As these galaxies merge, the dwarf Was 49b can be seen rotating within the larger disk of Was 49a, currently about 26,000 light-years from the center of the disk. Using NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR), the Sloan Digital Sky Survey (SDSS), the Chandra X-ray Observatory, and the Swift mission, a team led by postdoctoral fellow Nathan Secrest at the U.S. Naval Research Laboratory in Washington has determined that Was 49b is extremely bright in high-energy X-rays. And such high-energy X-ray luminosity is the fingerprint of emission from a supermassive black hole — one that’s larger than astronomers expect a dwarf galaxy to harbor.

“This is a completely unique system and runs contrary to what we understand of galaxy mergers,” Secrest said in a press release announcing the discovery. The results were recently published in February in the Astrophysical Journal. “We didn’t think that dwarf galaxies hosted supermassive black holes this big. This black hole could be hundreds of times more massive than what we would expect for a galaxy of this size, depending on how the galaxy evolved in relation to other galaxies.”

Current estimates place Was 49b’s supermassive black hole at more than 2 percent of the dwarf galaxy’s entire…

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