Astronomers have for the first time been able to image a pair of black holes, more massive than about a million suns, in a spiral galaxy similar to the Milky Way.
The image of the two black holes in a spiral galaxy code-named NGC 3393 was taken with NASA’s Chandra X-ray Observatory, according to the agency responsible for America's civilian space program and aeronautics and aerospace research. The pair, located near the center of the galaxy and roughly 160 million light years from Earth, is the nearest known such phenomenon.
The two black holes are separated by just 490 light years and are therefore being seen as the remnant of a merger of two galaxies of unequal mass a billion or more years ago, NASA said in a statement. Dubbed “minor mergers” by scientists, such collisions of one larger and another smaller galaxy may, in fact, be the most common way for black hole pairs to form, it added.
“If this galaxy weren’t so close, we’d have no chance of separating the two black holes the way we have,” Pepi Fabbiano of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., told United Press International. “Since this galaxy was right under our noses by cosmic standards, it makes us wonder how many of these black hole pairs we’ve been missing.”
The discovery is significant because previous observations suggested that a single supermassive black hole existed in the spiral galaxy’s center. “However, a long look by Chandra allowed the researchers to detect and separate the dual black holes. Both black holes are actively growing and emitting X-rays as gas falls towards them and becomes hotter,” Oxford-based IEWY News said.
Moreover, the pair of black holes has unusual properties. “The two galaxies have merged without a trace of the earlier collision, apart from the two black holes,” said Junfeng Wang, also from CfA. “If there were a mismatch in size between the two galaxies it wouldn’t be a surprise for the bigger one to survive unscathed.”
Astronomers believe that merging of two equal-sized spiral galaxies results in the formation of a black hole pair and a galaxy with a disrupted appearance and intense star formation. NGC 3393 could possibly be the first known occurrence of the merger of a large galaxy and a much smaller one forming a pair of supermassive black holes.
If this was a minor merger, the black hole in the smaller galaxy should have had a smaller mass than the other black hole before their host galaxies started to collide, NASA noted. “Good estimates of the masses of both black holes are not yet available to test this idea, although the observations do show that both black holes are more massive than about a million Suns.”