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Exotic Object SDSS1133: Ejected Black Hole or Huge Star?

SDSS1133

Keck telescope near-infrared images of Markarian 177 and SDSS1133. Twin bright spots in the galaxy’s central region are consistent with recent star formation, a disturbance that hints this may be the merger of two galaxies.

An international team of researchers analyzing decades of observations has discovered an unusual source of light in a galaxy some 90 million light-years away. They used data taken with many facilities, including the W. M. Keck Observatory on Maunakea and the Pan-STARRS1 telescope on Haleakalā, as well as NASA’s Swift satellite.

Michael Koss, a postdoctoral fellow at the IfA during most of the time the study was ongoing, led the team. IfA astronomer David Sanders and graduate students Yanxia Li and Chao-Ling Hung also participated in the study.

The object’s curious properties make it a good match for a supermassive black hole ejected from its home galaxy after merging with another giant black hole. But astronomers can’t yet rule out an alternative possibility. The source, called SDSS1133, may be the remnant of a massive star that underwent a record period of eruptions before destroying itself in a supernova explosion.

“With the data we have in hand, we can’t yet distinguish between these two scenarios,” said Koss, now at ETH Zurich, the Swiss Federal Institute of Technology. “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.”

Koss and colleagues found that the source has brightened significantly during the past six months, a trend that, if maintained, would bolster the black hole interpretation. To analyze the object in greater detail, the team is planning ultraviolet observations with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope in October 2015.

Whatever SDSS1133 is, it’s persistent. The team was able to detect it in astronomical surveys dating back more than 60 years. The mystery object is part of the dwarf galaxy Markarian 177, located in the bowl of the Big Dipper. Although supermassive black holes usually occupy galactic centers, SDSS1133 is located at least 2,600 light-years from its host galaxy’s core.

The collision and merger of two galaxies disrupt their shapes and result in new episodes of star formation. If each galaxy possesses a central supermassive black hole, the black holes usually form a bound pair at the center of the merged galaxy before ultimately merging into one larger black hole.

Merging black holes release a large amount of energy in the form of gravitational radiation, as explained by Einstein’s theory of gravity. Waves in the fabric of space-time ripple outward in all directions from accelerating masses. If the black holes have unequal masses, the result is a lopsided gravitational wave that kicks the black hole in the opposite direction. Typically, this kick will send the ejected black hole into an elongated orbit around its galaxy. Theoretically, if the kick is strong enough, it may eject the black hole from its home galaxy, fating it to forever drift through intergalactic space. Astronomers have been searching for such a black hole, but until now have been unable to find one, so if SDSS1133 is the first found, it would be a major discovery.

If SDSS1133 isn’t a black hole, then it must have been a very unusual type of star known as a luminous blue variable (LBV). These stars undergo episodic eruptions that cast large amounts of mass into space long before they explode. Interpreted in this way, SDSS1133 would represent the longest period of LBV eruptions ever observed, followed by a terminal supernova explosion whose light reached Earth in 2001. An interaction between the ejected gas and the explosion’s blast wave could explain the object’s steady brightness in the ultraviolet.

Whether it’s a rogue supermassive black hole or the closing act of a rare star, astronomers have never before seen the likes of SDSS1133.

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