NASA's Swift mission probes an exotic object: 'Kicked' black hole or mega star?

Using the Keck II telescope in Hawaii, researchers obtained high-resolution images of Markarian 177 and SDSS1133 using a near-infrared filter. Twin bright spots in the galaxy's center are consistent with recent star formation, a disturbance that hints this galaxy may have merged with another. Credit: W. M. Keck Observatory/M. Koss (ETH Zurich) et al.

An international team of researchers analyzing decades of observations from many facilities, including NASA's Swift satellite, has discovered an unusual source of light in a galaxy some 90 million light-years away.

The dwarf galaxy Markarian 177 (center) and its unusual source SDSS1133 (blue) lie 90 million light-years away. The galaxies are located in the bowl of the Big Dipper, a well-known star pattern in the constellation Ursa Major.

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 erupted for a record period of time before destroying itself in a supernova explosion.

"With the data we have in hand, we can't yet distinguish between these two scenarios," said lead researcher Michael Koss, an astronomer at ETH Zurich, the Swiss Federal Institute of Technology. "One exciting discovery made with NASA's Swift is that the brightness of SDSS1133 has changed little in optical or ultraviolet light for a decade, which is not something typically seen in a young supernova remnant."

In a study published in the Nov. 21 edition of Monthly Notices of the Royal Astronomical Society, Koss and his colleagues report that the source has brightened significantly in visible light 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, a well-known star pattern within the constellation Ursa Major. Although supermassive black holes usually occupy galactic centers, SDSS1133 is located at least 2,600 light-years from its host galaxy's core.

In June 2013, the researchers obtained high-resolution near-infrared images of the object using the 10-meter Keck II telescope at the W. M. Keck Observatory in Hawaii. They reveal the emitting region of SDSS1133 is less than 40 light-years across and that the center of Markarian 177 shows evidence of intense star formation and other features indicating a recent disturbance.

"We suspect we're seeing the aftermath of a merger of two small galaxies and their central black holes," said co-author Laura Blecha, an Einstein Fellow in the University of Maryland's Department of Astronomy and a leading theorist in simulating recoils, or "kicks," in merging black holes. "Astronomers searching for recoiling black holes have been unable to confirm a detection, so finding even one of these sources would be a major discovery."

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 SDSS1133 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 Earth in 2001.

The nearest comparison in our galaxy is the massive binary system Eta Carinae, which includes an LBV containing about 90 times the sun's mass. Between 1838 and 1845, the system underwent an outburst that ejected at least 10 solar masses and made it the second-brightest star in the sky. It then followed up with a smaller eruption in the 1890s.

In this alternative scenario, SDSS1133 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 the source. 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 the ultraviolet.

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.

Source: NASA/Goddard Space Flight Center

'Eye of Sauron' provides new way of measuring distances to galaxies

This image shows the spiral galaxy NGC 4151. Credit: X-ray: NASA/CXC/CfA/J.Wang et al.; Optical: Isaac Newton Group of Telescopes, La Palma/Jacobus Kapteyn Telescope; Radio: NSF/NRAO/VLA.

A team of scientists, led by Dr Sebastian Hoenig from the University of Southampton, have developed a new way of measuring precise distances to galaxies tens of millions of light years away, using the W. M. Keck Observatory near the summit of Mauna Kea in Hawaii.

The method is similar to what land surveyors use on Earth, by measuring the physical and angular, or 'apparent', size of a standard ruler in the galaxy, to calibrate the distance from this information.

The research, which is published in the journal Nature, was used to identify the accurate distance of the nearby NGC4151 galaxy, which wasn't previously available. The galaxy NGC 4151, which is dubbed the 'Eye of Sauron' by astronomers for its similarity to the film depiction of the eye of the character in The Lord of the Rings, is important for accurately measuring black hole masses.

Recently reported distances range from 4 to 29 megaparsecs, but using this new method the researchers calculated the distance of 19 megaparsecs to the supermassive black hole.
Indeed, as in the famous saga, a ring plays a crucial role in this new measurement. All big galaxies in the universe host a supermassive black hole in their centre and in about a tenth of all galaxies, these supermassive black holes are growing by swallowing huge amounts of gas and dust from their surrounding environments. In this process, the material heats up and becomes very bright -- becoming the most energetic sources of emission in the universe known as active galactic nuclei (AGN).

The hot dust forms a ring around the supermassive black hole and emits infrared radiation, which the researchers used as the ruler. However, the apparent size of this ring is so small that the observations were carried out using infrared interferometry to combine W. M. Keck Observatory's twin 10-meter telescopes, to achieve the resolution power of an 85m telescope.

To measure the physical size of the dusty ring, the researchers measured the time delay between the emission of light from very close to the black hole and the infrared emission. This delay is the distance the light has to travel (at the speed-of-light) from close to the black hole out to the hot dust.

By combining this physical size of the dust ring with the apparent size measured with the data from the Keck interferometer, the researchers were able to determine a distance to the galaxy NGC 4151.

Dr Hoenig says: "One of the key findings is that the distance determined in this new fashion is quite precise -- with only about 10 per cent uncertainty. In fact, if the current result for NGC 4151 holds for other objects, it can potentially beat any other current methods to reach the same precision to determine distances for remote galaxies directly based on simple geometrical principles. Moreover, it can be readily used on many more sources than the current most precise method."

"Such distances are key in pinning down the cosmological parameters that characterise our universe or for accurately measuring black hole masses. Indeed, NGC 4151 is a crucial anchor to calibrate various techniques to estimate black hole masses. Our new distance implies that these masses may have been systematically underestimated by 40 per cent."

Dr Hoenig, together with colleagues in Denmark and Japan, is currently setting up a new program to extend their work to many more AGN. The goal is to establish precise distances to a dozen galaxies in this new way and use them to constrain cosmological parameters to within a few per cent. In combination with other measurements, this will provide a better understanding of the history of expansion of our universe.

Source: University of Southampton