A team of astronomy researchers at Rochester Institute of Technology, Florida Institute of Technology and University of Sussex in the United Kingdom, find that the supermassive black hole (SMBH超重黑洞) at the center of the most massive local galaxy¹, M87, is not where it was expected. Their research, conducted using the Hubble Space Telescope, concludes that the supermassive black hole in M87 is displaced from the galaxy center. The most likely cause for this supermassive black hole to be off center is a previous merger³ between two older, less massive, black holes. "We also find, however, that the iconic M87 jet may have pushed the SMBH away from the galaxy center," says Daniel Batcheldor, Florida Tech assistant professor in the Department of Physics and Space Sciences, who led the investigation⁴.

The study of M87 is part of a wider Hubble Space Telescope project directed by Andrew Robinson, professor of physics at RIT. "What may well be the most interesting thing about this work is the possibility that what we found is a signpost of a black hole merger, which is of interest to people looking for gravitational waves（重力波） and for people modeling these systems as a demonstration⁵ that black holes really do merge²," says Robinson. "The theoretical prediction is that when two black holes merge, the newly combined black hole receives a 'kick' due to the emission⁶ of gravitational waves, which can displace it from the center of the galaxy."

David Merritt, professor of physics at RIT, adds: "Once kicked, a supermassive black hole can take millions or billions of years to return to rest, especially at the center of a large, diffuse⁷（散开的，弥漫的） galaxy like M87. So searching for displacements⁹ is an effective way to constrain¹⁰（强迫，束缚） the merger history of galaxies¹¹."

Jets, such as the one in M87, are commonly found in a class of objects called Active Galactic Nuclei¹². It is commonly believed that supermassive black holes can become active as a result of the merger between two galaxies, the in fall of material into the center of the galaxy, and the subsequent（后来的，随后的） merger between their black holes. Therefore, it is very possible that this finding could also be linked to how active galaxies—including quasars（类星体） , the most luminous¹³（发光的，明亮的） objects in the universe—are born and how their jets are formed.

This research will be presented at the American Astronomical¹⁴ Society (AAS) Conference on May 25 in Miami, Fla. It will also be published in The Astrophysical Journal Letters peer-reviewed scientific journal.

Because many galaxies have similar properties to M87, it is likely that supermassive black holes are commonly offset¹⁵（抵消，弥补） from their host galaxy centers. The potential offsets¹⁶, however, would be very subtle（微妙的，敏感的） and researchers would rely on the Hubble Space Telescope to detect them.

"Unfortunately, the highest spatial¹⁷ resolution（空间分辨率） camera onboard HST could not be revived during the recent servicing mission. This means we have to rely on the huge archive（档案） of HST data to find more of these vagrant¹⁸ supermassive black holes, as we did for M87," added Batcheldor.

Regardless of the displacement⁸ mechanism¹⁹, the implication of this result is a necessary shift in the classic supermassive black hole paradigm²⁰; no longer can it be assumed that all supermassive black holes reside at the centers of their host galaxies. This may result in some interesting impacts on a number of fundamental astronomical areas, and some interesting questions.

For example, how would an accreting²¹ (growing by the gravitational attraction of matter) or quiescent²²（静止的，沉寂的） supermassive black hole interact with the surrounding nuclear environment as it moves through the bulge²³（膨胀，凸出） ? What are the effects on the standard orientation-based unified²⁴ model of active galactic nuclei and how have dynamical models of the supermassive black hole mass been centered if the supermassive black hole is quiescent?

Especially thought-provoking, added Eric Perlman, associate professor of physics and space sciences at Florida Tech, is that our own galaxy is expected to merge with the Andromeda galaxy in about three billion years. "The result of that merger will likely be an active elliptical galaxy（椭圆星系） , similar to M87. Both our galaxy and Andromeda have supermassive black holes in their centers, so our result suggests that after the merger, the supermassive black hole may wander in the galaxy's nucleus²⁵ for billions of years."

David Axon, dean of mathematical and physical sciences at Sussex, concludes by saying, "In current galaxy formation scenarios²⁶ galaxies are thought to be assembled by a process of merging²⁷. We should therefore expect that binary²⁸（二元的） black holes and post coalescence²⁹（联合，合并） recoiling³⁰ black holes, like that in M87, are very common in the cosmos³¹."