Quasars, or quasi-stellar radio sources, are the most remote and energetic members of a class of bizarre objects that are collectively termed actuve galactic nuclei. First discovered in the 1960s, quasars send dazzling storms of raging, fierce, and ferocious light out into the Universe–light that is as brilliant as a trillion stars–from a region of space that is smaller than our Solar System. Originally, quasars were thought to be mysterious, solitary star-like celestial objects, residing in the glaring hearts of ancient galaxies inhabiting the early Universe. However, today most astrophysicists believe that supermassive black holes–lurking with voracious and sinister intent in the center of their host galaxies–are the true “engines” that power quasars. Our Universe keeps its secrets well, and in June 2017, a team of astronomers announced that they have solved yet another of its many mysteries. Gas filaments, encircling stars like the strands of a “pompom” may be the answer to a 30 year-old puzzle: why do dazzling quasars twinkle?
Dr. Mark Walker of Manly Astrophysics in Australia, and his colleagues at the California Institute of Technology (Caltech) in Pasadena, California, published their new solution to this intriguing puzzle in the June 27, 2017 issue of The Astrophysical Journal.
Dr. Walker's team were carefully observing distant quasars when they noticed one dubbed PKS 1322 that had started to display a wild performance of dimming and brightening at radio wavelengths over the course of just a few hours.
“This quasar was twinkling violently,” Dr. Walker noted in a June 27, 2017 Manley Astrophysics Press Release.
Quasar radio twinkling was first noticed back in the 1980s. Most often this mysterious twinkle is gentle–involving only slow, small alterations in radio brightness. However the kind of violent twinkling that the scientists observed emanating from PKS 1322 is unpredictable–and rare.
The myriad of stars that swarm in Earth's dark sky at night twinkle when currents of air in our atmosphere focus and then defocus their traveling light. In much the same way, quasars twinkle when streams of warm gas flowing in the space between stars focus and defocus their radio signals.
However, it is a longstanding mystery where those streams of warm gas originate, as well as their true identity.
Biggest Puzzle From Afar
It is now known that quasars are powered by doomed clouds of gas and some very unfortunate shredded stars, that violently and catastrophically swirl down into the waiting, hungry, and somewhat sinister maw of a supermassive black hole impatiently waiting for its dinner. Supermassive black holes, that are millions to billions of times solar-mass, haunt the strange hearts of probably every large galaxy in the observable Universe–including our own barred-spiral Milky Way.
Despite their misleading name, black holes are not simply empty space. They are, in fact, huge quantities of matter squeezed into a very small area–and they do not come in only one size. Black holes of stellar mass are approximately the same mass as our Sun, and they are born from the wreckage of extremely massive stars that exploded as supernovae when they reached the tragic end of their nuclear-fusing “lives”, and depleted their necessary supply of fuel. The unfortunate large star's core collapsed as its outer gaseous layers were blown violently into interstellar space–leaving only a black hole behind to tell the tragic tale of a vanished star.
Supermassive black holes are a somewhat different breed of beast, mostly because of their stupendous weight–and the unanswered question of how such massive entities came to exist in the first place. There is also some evidence that intermediate mass black holes also haunt the observable Universe. Intermediate mass black holes sport masses that are far less than their supermassive kin, but much heftier than their relatively tiny cousins of “only” stellar mass.
Our Milky Way Galaxy's resident dark heart is dubbed Sagittarius A*–Sgr A*, for short (pronounced saj-a-star). Sgr A* is a relative light-weight compared to many other supermassive black holes. This is because it is mere millions–as opposed to billions–of times solar-mass.
For astronomers, long ago is the same as far away. The deeper we stare into Space, the farther we look back in Time. The more distant a luminous object is, the more ancient it is. This is because its traveling light has taken a longer time to reach Earth because of the expansion of the Universe. No known signal can travel faster than light in a vacuum, and the light that wends its weary way towards Earth, emitted by distant objects dwelling in the Cosmos, can travel no faster than this universal speed limit will allow.
Quasars are very far away, haunting the distant hearts of young, and very active, galaxies in the primeval Universe–and they hurl into Space as much as a thousand times the energy output of our entire Galaxy. This radiation is emitted across the entire electromagnetic spectrum almost uniformly, ranging from the far-infrared to X-rays, showing a peak in the ultraviolet-optical bands. Some quasars are powerful radio sources, as well as sources of gamma-rays.
The first observations of quasars, derived from early optical images, showed them to be point sources indistinguishable from stars–except for their strange spectra. In addition, the luminosity of some quasars changes rapidly in the optical range of the electromagnetic spectrum–and even more rapidly in the X-ray range. Because these changes occur so rapidly, they reveal an upper limit on the volume of a quasar, indicating that quasars are not much larger than our Solar System. This suggests an immense energy density. The mechanism that causes the alterations in brightness likely involves the relativistic beaming of jets that are pointed almost directly at our planet.
It appears that the larger the galaxy, the larger its resident supermassive beast. For this reason, it is generally thought that there must be some mechanism that links the formation of a host galaxy to that of its central black hole and vice versa. This has important implications for theories of galactic formation and evolution, and it is an ongoing area of research in astronomy.
However, one very big question remains unanswered–why is it that galaxies in our Milky Way's cosmic neighborhood appear to host dormant supermassive black holes no longer funneling in large quantities of matter today? Indeed our own Sgr A* is just such an inactive, dormant black hole. The flaming youth of its ancient glory days have long since passed, and Sgr A* only occasionally is shaken out of its deep, dark sleep when an appetizing cloud of gas, or a delectable shredded star, passes too close to where the hidden beast slumbers. When this occurs, Sgr A* gobbles up the infalling buffet, just like it did, long ago, when it was still a furiously flaming young black hole in the ancient Universe. For a short time, the old black hole rages with some of the relic fires of its glory days–before it doses off again.
Many astronomers think that Sgr A* was once a quasar when both it, and the Universe, were young.
Twinkle, Twinkle Quasi-Star
Back in the 1960s advances in x-ray and radio astronomy revealed, to the prying eyes of curious astronomers, a fascinating new class of objects that had never been seen before. These mysterious objects turned out to be quasars–especially bright Active Galactic Nuclei (AGN)–even though originally they were thought to be associated with stellar objects–hence their name: quasi-stars.
Active galaxies have a small core of dazzling emission that is embedded at the very heart of an otherwise typical galaxy. This core is usually highly variable and extremely bright when compared to the rest of its host galaxy.
For “normal” galaxies, astronomers think of the total energy they hurl out into space as the sum of the emission from each of the stars dancing around within their galactic host. However, in the case of active galaxies, this is not true. Instead, there is a great deal more energy emitted in active galaxies than there should be and this overabundance of energy can be observed in the infrared, radio, ultraviolet, and X-ray regions of the electromagnetic spectrum. The energy that is thrown out into intergalactic space by an active galaxy is not “normal.” The question is this: What is happening in these galaxies to churn out such an enormous energetic output?
Since most, if not all, normal galaxies hold a supermassive black hole in their secretive, hot hearts, it seems that in the case of an active galaxy its resident supermassive beast is accreting material from its host galaxy's central region. As the material crashes down, down, down into the waiting gravitational jaws of the central black hole, angular momentum causes it to sprial in and create a disk. This disk is termed an accretion disk, and it becomes hotter, and hotter, and hotter as the result of the gravitational and frictional forces at work.
Most models of AGN also show a region of cold gas and dust. This region is generally thought to be in the shape of a gigantic donut (torus) with the hungry black hole lurking within the donut's hole. In approximately one out of ten AGN, the supermassive beast and accretion disk create narrow beams of energetic particles that are tossed outward in opposite directions away from the accretion disk. These jets emerge at nearly the speed of light, and become a strong source of radio wave emission.
The properties of an active galaxy result from the resident supermassive black hole's mass, the rate of accretion onto this voracious gravitational beast, whether or not it has a strong jet, and the angle at which astronomers observe the galaxy. Radio galaxies, or quasars, are AGN with powerful jets that rush outward into large portions of the space between galaxies.
Matter that is being pulled inward by the voracious supermassive black hole can also be observed hurling out brilliant light and if the speed of this crashing matter can be measured, it is possible for astronomers to measure the black hole's mass. This is not an easy task. This is because it requires the capabilities of sophisticated technology, such as that of the Hubble Space Telescope (HST) to carry out these difficult measurements.
Indeed, HST measurements have proven to be fundamental in astronomers' observations of the jets and accretion disks of matter surrounding a number of supermassive black holes. HST has revealed black holes 3 billion times as massive as our Sun at the secretive hearts of some host galaxies. While this particular observation was expected, the unexpected observation provided by HST was the surprising revelation that supermassive gravitational beasts hungrily haunt the hearts of all large galaxies–as well as small galaxies.
Dr. Duccio Macchetto, a European Space Agency astronomer, and Head of the Science Policies Division of the Space Telescope Science Institute (STScl) in Maryland, explained:
“Hubble provided strong evidence that all galaxies contain black holes millions or billions of times heavier than our Sun. This has quite dramatically changed our view of galaxies. I am convinced that Hubble… will find that black holes play a much more important role in the formation and evolution of galaxies than we believe today. Who knows, it may even influence our picture of the whole structure of the Universe… ?”
How I Wonder What You Are
The first hint that stars could be playing a role in the mysterious quasar twinkling came when Dr. Walker and his colleagues began to study their quasar, PKS 1322-110, using one of the 10-m Keck optical telescopes in Hawaii.
“At that point we realized this quasar is very close on the sky to the hot star Spica,” study co-author, Dr. Vikram Ravi of Caltech, commented in the June 27, 2017 Manly Astrophysics Press Release.
Dr. Walker remembered that a different violently twinkling quasar, dubbed J1819+3845, is situated very close on the sky to the searing-hot star Vega–something previously observed by other scientists. A duo of hot stars, a duo of twinkling quasars—could that be a mere a coincidence?
Additional studies indicated that it was not.
Dr. Walker and his colleagues re-examined earlier data collected on J1819+3845 and yet another violently twinkling quasar dubbed PKS 1257-326. The astronomers found that this second quasar also resides close on the sky to a searing-hot star named Alhakim.
At this point it looked like much more than mere coincidence was involved. The chance of having both twinkling quasars close to searing-hot stars is one in ten million, the scientists calculated.
“We have very detailed observations of these two sources. They show that the twinkling is caused by long, thin structures,” explained study co-author Dr. Hayley Bignall in the June 27, 2017 Manly Astrophysics Press Release. Dr. Bignall is of the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia.
The astronomers suggest that every hot star is encircled by a throng of warm gas filaments–all pointing towards the star.
“We think these stars look like the Helix Nebula,” Dr. Walker added in the Manly Astrophysics Press Release.
In the Helix a star is situated in a swarm of cool globules of molecular hydrogen gas, and each globule is about as big as our Solar System. Ultraviolet radiation emanating from the star crashes into the globules, giving each one a thin coating of warm gas and a long gas tail flowing outwards.
The star that resides in the Helix has come to the end of that long stellar road, and is in the process of dying. Astronomers usually assume that the globules formed late in the doomed star's “life”. However, Dr. Walker thinks such globules might also exist around younger stars that are not as close to their tragic end as the Helix star. “They might date from when the stars formed, or even earlier,” he added in the Manly Astrophysics Press Release.
“Globules don't emit much light, so they could be common yet have escaped notice so far. Now, we'll turn over every rock to find more signs of them,” he added.