A mysterious source of radiation has left astronomers contemplating exotic explanations.
Galaxy collisions produce some of the best fireworks in the universe. Interstellar gas meets head-on and explodes into star formation, and even though stars themselves rarely collide during a galactic pile-up, their orbits are forever altered as the galaxies merge.
At the center of all this activity, two supermassive black holes, one from each galaxy, coalesce into one. In fact, to make the biggest black holes in the universe, such collisions might happen over and over again. And when they do, gravitational waves released in the process often give the merged black hole a kick, like a gun’s recoil. A particularly strong recoil might even evict the black hole from its host altogether.
Yet recoiling black holes have been surprisingly hard to find — only a few known candidates exist. So when Michael Koss (ETH Zurich, Switzerland, and University of Hawaii, Honolulu) and colleagues went in search of these rare beasts, they were delighted to find an odd source of radiation near Markarian 177, a dwarf galaxy some 90 million light-years away from Earth.
The galaxy’s smooshed shape could indicate a recent galactic pile-up. And the radiation source, formally known by its telephone number, SDSS J113323.97+550415.8, lies some 2,600 light-years from the dwarf’s center, a reasonable distance for an evicted black hole.
When Koss’s team crawled through the Digital Sky Survey archives, they found this object has been going strong since 1950, and even brightened briefly by 2.5 magnitudes in 2001 before settling down again. The team obtained a bucketload of additional observations, including visible-light, near-infrared, and X-ray imaging, as well as visible-light spectra.
The following video shows some of the archival imagery of SDSS J1133 taken through a variety of filters and instruments, starting in 1950 and peaking in brightness in 2001:
Many of these observations support the ejected black hole scenario. Both the variability and the object’s broad emission lines are typical of a gas-guzzling black hole. And based on the width of the broad H-alpha emission line, which corresponds to the speed of gas whizzing around the black hole (if that’s what it is), the authors estimate a mass of 1 million Suns.
But it’s not a shoo-in. Astronomers aren’t even certain how often dwarf galaxies can grow their own supermassive black holes — such a combination might be quite rare.
“On one hand, it would be quite surprising to expect to catch the aftermath of the merger of two dwarf galaxies, both of which had supermassive black holes,” says Laura Blecha (University of Maryland), one of the paper’s coauthors. But, she adds, even modest kicks of a couple hundred kilometers per second could push the merged black hole beyond the dwarf galaxy’s weak gravitational reach.
This video shows the Sloan Digital Sky Survey data used to track some of the object's past, as well as a simulation that shows how a dwarf-dwarf galaxy merger may have evicted the black hole:
“I think it's definitely the strongest candidate [recoiling supermassive black hole] we've identified so far,” Blecha says.
Black Hole or Supernova?
But the data conflict enough that even the team members disagree on what this object is. A luminous blue variable (LBV) star, like the famous Eta Carinae, could also display broad emission lines and variability, especially if it ultimately ended its life in a supernova.
The LBV scenario provides a better explanation for some of the other observations that cannot easily be explained if the object were a black hole, such as the object’s narrow Fe II emission lines. And a supernova would also explain the object’s 2001 peak in brightness.
But an LBV-plus-supernova scenario isn’t a shoo-in either. To explain the decades of above-average brightness, the star would have had to experience the largest pre-supernova mass loss ever recorded, and even then, the broad emission lines remain difficult to explain.
Nevertheless, co-author Jon Mauerhan (University of Arizona) strongly supports this option. “This is the simplest explanation of the observational data, and one that is consistent with other objects we have recently observed.”
“The recoiling black hole is a very interesting plausibility that cannot be ruled out,” Mauerhan adds, “although this is a very exotic interpretation of the data for which there is not much precedence.”
Regardless of what it is, it’s clear that this object is rare.
“It's either a new and exotic type of object (a recoiling black hole) or the strangest, most exotic example of a very common type of object (a supernova),” Blecha says.
High-resolution radio and ultraviolet observations, or high-sensitivity X-ray observations could settle the debate once and for all — and soon. “We have good prospects for distinguishing between the two scenarios with new observations in the coming months and years,” Blecha says.
Michael Koss et al. "SDSS1133: An Unusually Persistent Transient in a Nearby Dwarf Galaxy." Monthly Notices of the Royal Astronomical Society, November 21, 2014.
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