Hidden in the secretive, strange hearts of large galaxies, they lurk in wait for their prey–a lost and wandering star, perhaps, or a wayward cloud of floating, glowing gas. They are supermassive black holes, and they are certainly some of the most bizarre objects in our Cosmos, with almost unimaginably huge masses of millions to billions of times that of our own Sun. But some supermassive black holes are more secretive than others, and play a better game of hide-and-seek with astronomers, Black Travel veiled as they are in dense shrouds of obscuring gas and dust. In July 2015, astronomers announced their discovery of new evidence for a large population of well-hidden supermassive black holes haunting our Universe. Using NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) satellite observatory, the team of international scientists detected the high-energy x-rays emitted from a quintet of supermassive black holes previously veiled from direct observation by enormous and impenetrable clouds. The new findings were presented on July 6, 2015 at the UK’s Royal Astronomical Society’s (RAS’s) National Astronomy Meeting held at Venue Cymru, in Liandudno, Wales.
The research was led by astronomers at Durham University, UK, and it lends new support to the theory that potentially millions more supermassive black holes are lurking in sinister, fascinating secret in the dust-shrouded hearts of large galaxies dwelling in the Universe–but they are well-hidden from the prying eyes of curious observers.
The astronomers aimed NuSTAR at nine candidate hidden supermassive black holes that were believed to be extraordinarily active at the centers of their host galaxies–but where the full extent of their great activity was potentially obscured from view.
The Horror! The Horror!
In the 18th century, John Michell and Pierre-Simon Laplace predicted the existence of such dark hearted gravitational monstrosities as these. Albert Einstein’s Theory of General Relativity (1915) went on to predict the existence of bizarre objects with such deep gravitational wells that anything that was unfortunate enough to wander too close to their waiting jaws would be devoured. However, the prospect of the real existence in nature of such gravitational beasts seemed so outrageous at the time that even Einstein at first rejected the concept–even though his own calculations indicated otherwise.
In 1916, Karl Schwarzschild derived the first modern solution of the Theory of General Relativity that could describe a black hole. However, its interpretation as a region of space, from which absolutely nothing could escape to freedom, was not fully understood for another four decades. For years, black holes were considered by scientists to be merely a mathematical oddity, and it was not until the 1960s that theoretical work revealed that black holes really are a generic prediction of General Relativity.
Black holes of “only” stellar mass form when an extremely massive star collapses in the raging fury of a brilliant supernova blast. Supernovae herald the end of a star’s life on the hydrogen-burning main-sequence. After a black hole has emerged from this horrendous stellar funeral pyre, it can go on to acquire more and more mass by eating its environment. Many scientists think that by devouring unlucky stars, doomed gas clouds, and by merging with others of its own kind, the most massive black holes are born. Astronomers have realized for a decade that probably every large galaxy in the Universe hosts a central supermassive black hole, sequestered in sinister secret. These supermassive beasts are bewildering and mysterious objects, largely because they apparently were already in existence when the Universe was very young.
It is commonly thought that shredded stars and blobs of gas swirl into the merciless, violent maelstrom of supermassive black holes, and that this tumbling buffet of whirling material forms an immense disk surrounding the black hole, termed an accretion disk. This infalling material grows ever hotter and hotter and hurls out radiation, especially as it travels closer to the horrific point of no return called the event horizon. The event horizon is located at the innermost region of the disk.
When astronomers look deeper and deeper into space, they are looking farther and farther back in time. This is because the more distant a luminous object is in space, the longer it has taken its wandering light to reach our telescopes. There is no known signal in the Universe that is able to travel faster than light, and the light emanating out from remote objects in the Cosmos can travel to us no faster than this universal speed limit. In the primordial Universe, a very large number of supermassive black holes haunting the hearts of the most ancient and distant galaxies, reveal their strange presence in the form of quasars. Quasars are actually brilliantly glaring accretion disks, surrounding especially voracious and active supermassive beasts. Quasars are vigorous young Active Galactic Nuclei (AGN) that are powered by infalling material from the accretion disk. Some astronomers seek cosmic objects that caught fire like a swarm of celestial fireflies when the Cosmos was still in the earliest stages of its existence, and quasars–or quasi-stellar objects–are just such luminous celestial fireflies, that ignited very long ago.
In astronomy, time and distance, as well as the wavelength of light at which observations are made, are all inextricably tied together. Because light travels at a finite speed and, as a result, takes a finite amount of time to reach us, very distant objects are observed the way that the were in the distant past. Astronomers use what is called the redshift–or z–to show how long ago and far away a particular luminous cosmic object is. The measurable quantity of 1 + z is the factor by which the Cosmos has expanded–between the era when a distant, ancient source first hurled its light out into space, and the current time, when it is finally being observed. In addition, it is also the factor by which the wavelength of light now approaching us has been stretched as a result of the expansion of the Universe. The redshift is the shift of a luminous object’s spectrum toward longer and longer wavelengths–or towards the red end of the electromagnetic spectrum, as it travels away from us.
Supermassive black holes and their surrounding, glaring accretion disks can be–at least–as vast as our entire Solar System. These gravitational monsters are described by their hefty weight, insatiable hunger, and messy table manners. When its outside source of energy is finally depleted, the quasar shuts off. The best current estimates suggest that most galaxies experienced a quasar phase in the ancient Universe, and that they currently host remnant, frequently dormant, supermassive black holes that show only the ghost of their former appetites. This scenario possibly illustrates the way the supermassive black hole that haunts the heart of our Milky Way evolved. As supermassive black holes go, our Galaxy’s resident beast is a small one, with little appetite, and it weighs-in at “merely” millions, as opposed to billions, of solar-masses. Once, long ago, it likely dazzled the primordial darkness of the Cosmos as a brilliant quasar, but it is a quiet old monster now, except on those rare occasions when it goes on an eating binge, and devours a large helping of shredded stars and/or disrupted blobs of gas that traveled too close to its waiting jaws. The Milky Way’s resident black hole has been dubbed Sagittarius A* (Sagittarius-a-star), and it is calm and peaceful in its old age, except when it occasionally feasts on its prey with the insatiable hunger of its youth–but only for one brief shining moment.
Hidden Supermassive Black Holes Play Hide-And-Seek!
High-energy x-rays spotted from the quintet of black holes confirmed that they were veiled by shrouds of obscuring dust and gas. The five secretive, supermassive beasts were considerably brighter and more active than previously believed as they quickly and hungrily ate their surroundings and shot off immense quantities of radiation.
Such observations were not possible before NuSTAR came to the rescue of searching astronomers in this cosmic game of hide-and-seek. NuSTAR was launched in 2012 and is capable of detecting much higher energy x-rays than earlier satellite observatories.
Lead author of the research, Dr. George Lansbury, is a postgraduate student in the Centre for Extragalactic Astronomy, at Durham University. “For a long time we have known about supermassive black holes that are not obscured by dust and gas, but we suspected that many more were hidden from our view,” Dr. Lansbury noted in a July 7, 2015 RAS Press Release.
“Thanks to NuSTAR, for the first time we have been able to clearly see these hidden monsters that are predicted to be there, but have previously been elusive because of their ‘buried’ state,” he continued to explain.
“Although we have only detected five of these hidden supermassive black holes, when we extrapolate our results across the whole Universe then the predicted numbers are huge and in agreement with what we would expect to see,” Dr. Lansbury told the press.
“High-energy x-rays are more penetrating than low-energy x-rays, so we can see deeper into the gas burying the black holes. NuSTAR allows us to see how big the hidden monsters are and is helping us learn why only some black holes appear obscured,” explained Dr. Daniel Stern in the July 7, 2015 RAS Press Release. Dr. Stern is the project scientist for NuSTAR at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.