The Incredible Wonder Of A Wandering Black Hole

In the mysterious hearts of perhaps every large galaxy in the observable Universe, supermassive black holes hide themselves in bewitching, bewildering, and sinister secret. These bizarre gravitational wonders, that weigh in at millions to billions of times more than our Sun, lie in wait for their doomed dinner–an unlucky star, perhaps, or a lost and floating cloud of very unfortunate gas–that has traveled too close to their waiting maws. But some supermassive black holes are insecure rogues themselves, that have been forced by chance to wander through Space and Time, bereft of the host galaxy that they once haunted in secretive splendor. In October 2016, a team of astronomers announced their discovery of just such a rogue–a “wandering” black hole that lurks in the outer limits of a galaxy located approximately 4.5 billion light years from Earth. Their recently released evidence suggests that this strange supermassive vagabond weighs in at approximately 100,000 times solar mass, and was originally born in the strange heart of a smaller galaxy that ultimately merged with a larger one.

The team of astronomers used NASA’s Chandra X-ray Observatory and the European Space Agency’s (ESA’s) XMM-Newton X-ray Observatory to make their discovery of an extraordinarily brilliant variable X-ray source situated far beyond the heart of its parent galaxy. Alas, this exotic celestial wanderer could be the result of the tragic event of a small galaxy falling into a larger one.

The Chandra data show that this object emitted an enormous amount of X-rays, which is why it is classified as a hyperluminous X-ray source. The brilliant burst of X-rays may have originated from a doomed star that was shredded by the powerful gravity of this wandering object.

Black holes in general are thought to come in three sizes: stellar mass, supermassive, and intermediate mass. Intermediate mass black holes are objects that sport lower masses than their supermassive cousins, but are much more massive than black holes of “merely” stellar mass. The intermediate objects weigh between 100 and 100,000 times the mass of our Sun. However, this classification system is actually somewhat more complicated. That is because black holes of various sizes are born whenever a large enough amount of mass is squeezed by its own gravity into a small enough space.

Both supermassive and intermediate mass black holes may be spotted far from the center of a host galaxy. This usually happens after a collision and merger event with another galaxy that also contains a massive black hole. As the stars, gas, and dust from the second galaxy sweep through the first one, its resident dark heart also goes along for the ride.

Dark-Hearted Vagabonds

A black hole of “only” stellar mass is born from the shattered wreckage of its progenitor star. In this model, the massive progenitor star has met its unfortunate fate in the brilliant, fiery turmoil of a supernova explosion, after having burned its entire necessary supply of nuclear-fusing-fuel–and it has collapsed as a result. Therefore, supernovae mark the grand finale of a massive star’s “life” on the hydrogen-burning main sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. After a black hole has been born from the stellar wreckage it can go on to gain more and more weight by eating its surroundings. Many cosmologists propose that by feasting on unfortunate stars, doomed clouds of floating gas, and by merging with others of its own kind, the most massive black holes of all are born. Astronomers have understood for about a decade that it is likely that every large galaxy in the observable Universe hosts a heavy, hungry heart of darkness in its core, hidden there in sinister secret.

The tattered, shredded remnants of devoured stars and blobs of gas whirl into the turbulent, swirling maelstrom surrounding supermassive black holes. This whirling banquet creates an enormous encircling disk that is termed an accretion disk. This circling, doomed material composing the accretion disk becomes increasingly hotter and hotter–and it shoots out an abundance of radiation as it sweeps ever closer to the terrible point of no return called the event horizon. The event horizon is situated at the innermost region of the accretion disk.

When observers peer deeper and deeper into Space, they are peering further and further back in Time. The more distant a shining object is in Space, the longer it has taken for its light to have made the long and difficult journey to astronomers’ waiting telescopes. No known signal can travel faster than light in a vacuum, and so the light that travels from distant objects in the Universe is unable to travel faster than this universal speed limit. In the early Universe, a large number of supermassive black holes were already in existence, haunting the hearts of the most distant and ancient galaxies. These very ancient objects reveal their presence in the form of quasars, which are really glaring accretion disks surrounding especially gluttonous and active supermassive hearts of darkness. Quasars are youthful Active Galactic Nuclei (AGN) that are powered by the tumbling matter of the accretion disk. Some astronomers hunt for celestial objects that ignited like a brilliant storm of cosmic fireflies long ago and far away. Quasars–or quasi-stellar-objects–are brilliant celestial fireflies that lit up when our Universe was young.

In astronomy, time, distance, and the wavelength of light at which observations are made, are all interconnected. Because light travels at a finite speed and, as a result, takes a finite amount of time to reach observers, extremely remote objects are seen the way that they were in the very ancient past. Astronomers use what is termed the redshift (z) to show how ancient and remote a particular luminous object is. The measurable quantity of 1 + z is the factor by which the Universe has expanded between the era when a remote, ancient source first shot its light out into the Universe and the current era when it is first being observed by astronomers. Furthermore, the redshift is also the shift of a luminous object’s spectrum towards increasingly longer and longer electromagnetic wavelengths–or towards the red end of the electromagnetic spectrum–as it travels away from us.

Supermassive dark hearts and their encircling, brilliant accretion disks can be, at the very least, as large as our entire Solar System. These gravitational beasts are described by their great hunger, messy eating habits, and heavy weight. When its outside source of energy is at last used up, a brilliant quasar switches off. It is generally thought that most galaxies went through a hyperactive quasar stage when they dazzled the ancient Universe during their flaming youth, and that these galaxies now host relic, often dormant, supermassive black holes that display only the lingering ghost of their former insatiable appetites.

This model may illustrate the way our Milky Way Galaxy’s resident supermassive beast evolved. As supermassive black holes go, our Galaxy’s dark heart, dubbed Sagittarius A* (Sagittarius-a-star), is a light-weight, with only the shadow of its former impressive appetite. Sagittarius A* weighs in at only millions, as opposed to billions, of solar masses. However, when the Universe was young, so was Sagittarius A*, and it has calmed down considerably in its old age. But, every now and then, our Galaxy’s resident supermassive black hole will wake up and go on a feeding frenzy, swallowing a large helping of mangled stars and/or badly shaken up blobs of gas that ventured to close to its gravitational grip. When this happens, Sagittarius A* again dines with the former glory of its blazing youth– before it quiets down again to nap.

The larger the galaxy, the larger its supermassive black hole. It is possible that there is a mechanism that links galactic formation to that of its resident heart of darkness. This has some very important implications for theories of galaxy birth and evolution and it is an area of ongoing research in astronomy.

It is unknown why most of the galaxies in our Milky Way’s cosmic neighborhood, including our own, possess a dormant supermassive black hole which is not particularly hungry at present.

The Incredible Wonder Of A Wandering Black Hole

The study describing the newly discovered rogue of a supermassive black hole–that is “wandering” in the direction of the outer limits of its host lenticular galaxy–has been dubbed XJ1417+52. The host lenticular, that has been named SDSSJ14711.07+522540.8 (or, GJ1417+52, for short), is located about 4.5 billion light-years from Earth. Lenticular galaxies are a type of disc galaxy intermediate between elliptical and spiral galaxies in galaxy morphological classification schemes.

The wandering wonder of a supermassive black hole was discovered during long observations of a special region, the so-called Extended Groth Strip, by a team of astronomers using the XMM-Newton and Chandra data collected between 2000 and 2002. Its extreme, dazzling brilliance suggests that it is probably a black hole with a mass of approximately 100,000 times that of our Sun–assuming that the radiation force on ambient matter is equivalent to the gravitational force.

The Chandra and XMM-Newton observations reveal that the X-ray emission of XJ1417+52 is extremely powerful–which is why astronomers have classified it as a hyperluminous-X-ray source (HLX). These sources are objects that are 10,000 to 100,000 times more luminous in X-rays than smaller black holes of “merely” stellar mass–and up to 10 to 100 times more intense than ultraluminous X-ray sources, or ULXs.

At its peak brightness, XJ1417+52 is approximately ten times more luminous than the brightest X-ray source ever observed for a rogue black hole. It is also approximately 10 times more remote than the previous record holder for a black hole vagabond.

The brilliant X-ray emission flowing from this type of black hole is derived from unlucky material that is tumbling down into its waiting maw. The X-rays from XJ1417+52 attained peak brightness between 2000 and 2002. The source itself was not seen in later Chandra and XMM-Newton observations obtained in 2005, 2014, and 2015. Overall, the X-ray brightness of the source has declined by at least a factor of 14 between 2000 and 2015.

The authors of the study propose that the X-ray outburst observed in 2000 and 2002 occurred when an unfortunate star passed too close to the waiting, wandering black hole and was shredded by its tidal forces. Some of the gaseous debris would have been heated and, as a result, brightened in X-rays, as it tumbled into the gravitational clutches of the hungry black hole–thus resulting in a spike in its emission.

The best explanation for this rogue of a wandering black hole is that it originally belonged to a tragic small galaxy that met its untimely fate when it blasted into the larger G1417+52 galaxy–stripping away most of the galaxy’s stars but leaving behind the hungry, wandering rogue of a black hole and its surrounding stars that had been situated at the center of the small galaxy.

A research paper authored by Dr. Dacheng Lin (University of New Hampshire) and his team describing this result appears in The Astrophysical Journal and is available online.

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