A multinational group of astronomers led by Dr Michael Koss of ETH Zurich has discovered an exotic object in a galaxy some 90 million light-years away.
The dwarf galaxy Markarian 177 (center) and SDSS1133 (blue) lie 90 million light-years away. Image credit: Sloan Digital Sky Survey.
The mystery object, named SDSS J113323.97+550415.8 (SDSS1133 for short), is part of the dwarf galaxy Markarian 177 and lies at least 2,600 light-years from the galaxy’s center.
The source may be a supermassive black hole ejected from its home galaxy after merging with another giant black hole. An alternative explanation is just as intriguing – the remnant of a giant star that erupted for a record period of time before destroying itself in a supernova explosion.
The emitting region of SDSS1133 is less than 40 light-years across.
“With the data we have in hand, we can’t yet distinguish between these two scenarios. One exciting discovery made with NASA’s Swift satellite is that the brightness of SDSS1133 has changed little in optical or UV for a decade, which is not something typically seen in a young supernova remnant,” said Dr Koss, who is the first author of the paper published in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).
He said that SDSS1133 has brightened significantly in visible light during the past six months, a trend that, if maintained, would bolster the black hole interpretation.
To analyze it in greater detail, the astronomers are planning UV observations with Hubble Space Telescope’s Cosmic Origins Spectrograph in 2015.
Whatever the object is, it’s persistent. The scientists were able to detect it in astronomical surveys dating back more than six decades.
“We suspect we’re seeing the aftermath of a merger of two small galaxies and their central black holes,” said co-author Dr Laura Blecha of the University of Maryland.
The collision and merger of two galaxies disrupts their shapes and results in new episodes of star formation. If each galaxy possesses a central supermassive black hole, they will form a bound binary pair at the center of the merged galaxy before ultimately coalescing themselves.
Merging black holes release a large amount of energy in the form of gravitational radiation, a consequence of Einstein’s theory of gravity. Waves in the fabric of space-time ripple outward in all directions from accelerating masses. If both black holes have equal masses and spins, their merger emits gravitational waves uniformly in all directions.
More likely, the black hole masses and spins will be different, leading to lopsided gravitational wave emission that launches the black hole in the opposite direction.
The kick may be strong enough to hurl the black hole entirely out of its home galaxy, fating it to forever drift through intergalactic space. More typically, a kick will send the object into an elongated orbit. Despite its relocation, the ejected black hole will retain any hot gas trapped around it and continue to shine as it moves along its new path until all of the gas is consumed.
If the object isn’t a black hole, then it might have been a very unusual type of star known as a Luminous Blue Variable (LBV).
These massive 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 our planet 13 years ago.
In this scenario, the object must have been in nearly continual eruption from at least 1950 to 2001, when it reached peak brightness and went supernova.
The spatial resolution and sensitivity of telescopes prior to 1950 were insufficient to detect it.
But if this was an LBV eruption, the current record shows it to be the longest and most persistent one ever observed.
An interaction between the ejected gas and the explosion’s blast wave could explain the object’s steady brightness in UV.
Whether it’s a rogue supermassive black hole or the closing act of a rare star, it seems astronomers have never seen the likes of SDSS1133 before.
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Koss M et al. 2014. SDSS1133: An Unusually Persistent Transient in a Nearby Dwarf Galaxy. Monthly Notices of the Royal Astronomical Society 445 (1): 515-527; doi: 10.1093/mnras/stu1673
