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Bizarre behavior shown by two giant black holes

Image: Black hole in galaxy M87
Observations from the Hubble Space Telescope show that the supermassive black hole at the heart of the galaxy M87 is positioned off-center, possibly as the result of a galactic merger.
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Some strange behavior from two huge black holes at the center of two different galaxies has been noticed by astronomers.

One supermassive black hole, at the heart of a galaxy neighboring the Milky Way, has been mysteriously brightening in recent years, and scientists aren't sure why. Another of these matter-gobbling behemoths isn't where astronomers thought it was located.

Two new studies on these black holes, the results of which were presented here today at the 216th meeting of the American Astronomical Society, have turned up some surprising results and could help astronomers understand more about the evolution of galaxies and how supermassive black holes work.

Erratic behavior
In the first study, the erratic, unpredictable behavior of the supermassive black hole within the Andromeda galaxy puzzled researchers studying it. They found that the black hole became 100 times brighter following an outburst on Jan. 6, 2006.

After the outburst, however, the black hole entered another relatively dim state, but was still about ten times brighter on average than prior to 2006. The outburst suggests that a relatively high rate of matter had been falling onto the black hole, followed by a smaller, but still significant rate.

"We have some ideas about what's happening right around the black hole in Andromeda, but the truth is we still don't really know the details," said study member Christine Jones, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

The brightening since 2006 could be caused by the black hole's capturing winds from an orbiting star, or by a gas cloud that spiraled into the black hole.

But, the original cause of the outburst remains unclear. The researchers believe that it could be due to the sudden release of energy, such as magnetic fields in a disk around the black hole that suddenly connect and become more powerful.

"It's important to figure out what's going on here because the accretion of matter onto these black holes is one of the most fundamental processes governing the evolution of galaxies,' said study team member Zhiyuan Li of the Center for Astrophysics.

The results of this study imply that the feeble, but erratic behavior of the black hole in the Milky Way could potentially be typical for present-day supermassive black holes.

"The black holes in both Andromeda and the Milky Way are incredibly feeble," Zhiyuan Li said. "These two 'anti-quasars' provide special laboratories for us to study some of the dimmest type of accretion even seen onto a supermassive black hole." (Quasars are the very active centers of galaxies though to surround the central supermassive black hole.)

Mistaken location
In another unexpected finding, a separate study demonstrated that a supermassive black hole previously thought to lurk in the core of a relatively nearby giant galaxy, called M87, is not actually located at the galaxy's center.

Researchers used data from the Hubble Space Telescope to look more closely at the black hole, and found that the supermassive black hole in the M87 galaxy is, in fact, displaced from the center.

The most likely cause for the black hole's position off center is a previous merger between two older, smaller supermassive black holes (SMBH).

"The theoretical prediction is that when two black holes merge, the newly combined black hole receives a 'kick' due to the emission of gravitational waves, which can displace it from the center of the galaxy," said lead researcher Daniel Batcheldor of the Florida Institute of Technology.

"We also find, however, that the iconic M87 jet may have pushed the SMBH away from the galaxy center," Batcheldor added.

M87, which is located 50 million light-years away, has an active jet that shoots light out of the galaxy's core. The core is made up of matter that swirls closer to the black hole and approaches the speed of light, combining with tremendous magnetic fields in the process.

The material that is ejected from this core area can help astronomers understand how black holes attract and consume matter.

"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 and for people modeling these systems as a demonstration that black holes really do merge," said study member Andrew Robinson, of the Rochester Institute of Technology.

It was also discovered that this displacement often lasts for extended periods of time, which could provide a valuable tool for astronomers hoping to understand how galaxies form and grow.

"Once kicked, a supermassive black hole can take millions or billions of years to return to rest, especially at the center of a large, diffuse galaxy like M87," said David Merritt, co-author of the study and a professor of physics at the Rochester institute. "So searching for displacements is an effective way to constrain the merger history of galaxies."

Jets, such as the one in M87, are mostly found in a class of objects called Active Galactic Nuclei. Astronomers believe that supermassive black holes can become active as a result of the merger between two galaxies, the falling of material into the center of the galaxy, and the subsequent merger of their respective black holes.

So, it is possible that these findings could also shed light on how active galaxies — including faraway, developing galaxies known as quasars, the most luminous objects in the universe — are born and how their jets are formed.

About three decades ago, M87 was one of the first galaxies suggested to harbor a central black hole, but astronomers have since concluded that most large galaxies, including our own Milky Way, have supermassive black holes located at their centers.

Since many galaxies share similar properties to M87, it is likely far more common that supermassive black holes are offset from their host galaxy's centers than originally thought.

The amount that they are offset, however, would be very subtle and researchers would have to rely on observations from the Hubble Space Telescope in order to detect them.

Merger questions
These new results also raise key questions and introduce new areas of research regarding the merging of galaxies.

In particular, said Eric Perlman, one of the study's researchers and an associate professor of physics and space sciences at Florida Tech, it is believed that our own galaxy is expected to merge with the Andromeda galaxy in approximately three billion years.

"The result of that merger will likely be an active elliptical galaxy, similar to M87," Perlman said. "Both our galaxy and Andromeda have SMBHs in their centers, so our result suggests that after the merger, the SMBH may wander in the galaxy's nucleus for billions of years."

The study of M87's supermassive black hole will be detailed in an upcoming issue of The Astrophysical Journal Letters.