Could orphan black hole confirm Einstein's general theory of relativity?

Astronomers have long hunted for galaxies that might be evicting supermassive black holes at their centers. Eviction would represent an important confirmation of Einstein's theory of general relativity under extreme conditions and could help shed light on the influence such massive features have on the evolution of galaxies themselves.

Now they may have found one of those rogue black holes. A dwarf galaxy 90 million light-years from Earth hosts an unusually bright object some 2,600 light-years from its center – an object that carries many of the signatures one would expect from a supermassive black hole feasting on surrounding gas. The galaxy involved is known as Markarian 177, located within the constellation Big Dipper.

The object's position far from the center of the galaxy and the galaxy's odd shape makes it “the most promising candidate we've found” for a supermassive black hole ejected during the merger of two galaxies, says Laura Blecha, a researcher at the University of Maryland in College Park, who focuses on the interrelationship of supermassive black holes and their host galaxies as the two evolve.

She cautions that a renegade black hole is not the only explanation for the object the team has observed.

“It's either the first example we've confirmed in this very exotic class of objects, or it's the most exotic, extreme example of a very common class of objects,” exploding stars known as supernovae, says Dr. Blecha, a member of the team reporting its observations of the object in Friday's issue of Monthly Notices of the Royal Astronomical Society, published in Britain.

If it is a supermassive black hole, however, it would represent a spectacular confirmation of Einstein's general theory of relativity as it relates to the enormous gravitational fields of supermassive black holes, Blecha explains.

Black holes are objects so dense that their gravitational tug prevents even light from escaping. So-called stellar black holes form from the explosion and collapse of very massive stars. Supermassive black holes tip the cosmic scales at millions to billions of times the mass of the sun. They are thought to lurk in the centers of most, if not all, galaxies. These behemoths are thought to play a key role in galaxy evolution by regulating a galaxy's rate of star formation.

When galaxies merge, so do their central black holes. Theorists have noted that based on Einstein's theory of general relativity, such black-hole mergers should generate powerful ripples in the very fabric of space-time, ripples known as gravitational waves. One way these merger-related gravity waves would make their presence known is through a recoil effect. This effect would be powerful enough to launch the single merged central black hole out of the center of its newly enlarged galaxy into an orbit that grows ever wider. If the galaxy's gravity was weak enough, as it might be in a dwarf galaxy, the black hole could travel fast enough to leave the galaxy altogether.

Either way, it would carry with it some of the gas and stars that were in its immediate vicinity before it left the galaxy's center. This material would feed the beast. As it falls in toward the black hole, it heats up reaching temperatures that generate the radiation and revealing the black hole's presence to Earthbound astronomers.

In this case, the object, labeled SDSS1133, appears at intervals throughout the 2000s in images taken by the Sloan Digital Sky Survey. The team also looked at digitized versions of photographic images of the galaxy dating to 1950. Even then, the object appears.

In addition, the team collected observations from telescopes in Hawaii and NASA's Swift satellite, which observes the cosmos at wavelengths from gamma rays to ultraviolet light. "One exciting discovery made with NASA's Swift is that the brightness of SDSS1133 hasn't changed in ultraviolet light for a decade, which is not something typically seen in a young supernova remnant," Michael Koss, an astronomer with the Swiss Federal Institute of Technology in Zurich and the lead author of the paper describing the results in the Monthly Notices, said in a prepared statement.

Supernovae burst on the scene suddenly, then fade with time. They can appear to brighten periodically if the expanding shell of matter the explosion launched collides with dense patches of interstellar dust and gas.

SDSS1133 brightened substantially during the past six months, the researchers say. If the object remains bright for a sustained period, that would add weight to the argument that the object is a supermassive black hole.

Yet even if the object proves to be something less extraordinary than an outbound supermassive black hole, all is not lost, Blecha notes.

“Either way it's something very interesting,” she says.