The vast expanse of the universe resembles an empty ocean, dotted with rare islands known as galaxies. However, this perception is deceiving. Only a small fraction of all atoms reside within galaxies, while the majority drift in the intergalactic medium, the space between them. Like the roots of a massive tree, gas extends from each galaxy, pulled by gravity to replenish the dense cosmic forest. Within this intergalactic medium lie the building blocks of creation: hydrogen and helium, intricately woven into sheets and filaments that flow into galaxies, eventually giving birth to stars. Yet, upon closer examination, we discover the true architects of the cosmos: quasars, the most powerful entities in existence. Despite being as minuscule as grains of sand in comparison to the mighty Amazon River, quasars reside at the centres of some galaxies, emanating a radiance equivalent to a trillion stars. They emit tremendous jets of matter, reshaping the universe itself and possessing the ability to extinguish entire galaxies.
So, what exactly are quasars, and how do they shape the structure of the universe according to their whims? Back in the 1950s, astronomers noticed perplexing phenomena in the sky: the enigmatic emission of loud radio waves from various locations. These peculiar sources, observed in radio waves instead of visible light, were dubbed "quasi-stellar radio sources" or "quasars" due to their star-like appearance. Their behaviour was truly extraordinary. Some quasars exhibited fluctuations, while others emitted high-energy X-rays alongside radio waves. Moreover, they displayed remarkable velocities, moving at speeds exceeding 30% of the speed of light. The only plausible explanation for their rapid motion was that they resided at immense distances, with their apparent speed actually resulting from the expansion of the universe propelling them away from us.
These colossal distances implied that quasars could not merely be stars; they were the active cores of galaxies located billions of light-years away. However, the astonishing part is yet to come. In order to exhibit such extraordinary brightness and intensity, considering their vast distances, quasars had to be thousands of times brighter than the entire Milky Way galaxy. These were cosmic monsters, erupting and bellowing into the void with an unprecedented violence.
As our sky mapping efforts continued, we uncovered the existence of over a million quasars, all seemingly situated in far-flung reaches of the cosmos. In the realm of astronomy, "far away" equates to "long ago," as the light from these distant objects requires an immense span of time to reach us. Quasars were abundant in the early universe, reaching their peak around 10 billion years ago, when galaxies and the universe itself were in their infancy.
Let us venture back in time, approximately 3 billion years after the Big Bang, and explore the events that transpired during this epoch.
The power of quasars during their early stages is truly awe-inspiring. How could a young, fledgling galaxy emit such tremendous brilliance and exhibit such violence? The sheer amount of light and radiation generated by quasars surpassed what could be attributed to stars alone, as there were nowhere near enough stars within these galaxies to account for the immense luminosity. Given that galaxies tend to grow over time through mergers, the starlight emitted by smaller galaxies should not surpass that of present-day galaxies. Consequently, the only plausible explanation for the prodigious amounts of energy emanating from quasars is their voracious appetite for supermassive black holes.
Although the exact mechanisms of their formation remain elusive, it appears that every galaxy harbors a supermassive black hole at its core. Surprisingly, the brilliance of a quasar does not originate from within these black holes. Instead, it emanates from the surrounding space, specifically from a colossal disk of gas known as an "accretion disk." Similar to stars, quasars utilize matter as their fuel source for illumination. However, black holes are the most efficient engines for converting matter into energy in the entire universe. The energy released by matter falling into a black hole can exceed the energy released by nuclear fusion in a star's core by a factor of 60. This extraordinary release of energy stems from gravity, rather than nuclear reactions.
As matter hurtles toward a black hole, approaching speeds close to that of light, it carries an immense amount of kinetic energy. Regrettably, once it crosses the event horizon, that energy is lost within the black hole's depths. One can only witness this energy spectacle if matter is introduced into the black hole in a specific manner. Falling directly downward denies the external universe the opportunity to partake in the energy release. However, when a substantial amount of matter spirals rapidly toward the event horizon, it forms a disk, occupying a space not much larger than our solar system. Particle collisions and friction within this disk heat the matter to hundreds of thousands of degrees. In this compact region, the galactic core can generate energy multiple times greater than that produced by all its stars combined. This is the essence of a quasar—an extravagant feast for a supermassive black hole. These cosmic behemoths consume an astonishing amount of matter, ranging from one to a hundred times the mass of Earth per minute. Given that the universe, ten billion years ago, was only a third of its current size, the intergalactic medium was more concentrated. Consequently, the gas filaments surrounding quasars could provide an abundant banquet, resulting in the emission of unfathomable quantities of light and radiation.
The brightest quasars manifest as powerful jets that twist and weave through the magnetic field of the surrounding matter, confined to narrow cones. Like colossal particle accelerators, they project immense beams of matter that traverse the circumgalactic medium, giving rise to vast plumes spanning hundreds of thousands of light-years. The scale of this phenomenon is nearly beyond comprehension—a tiny spot within a galaxy sculpting patches of the universe that stretch over hundreds of thousands of light-years. However, the reign of quasars is relatively short-lived, lasting only a few million years, as their extravagant feast ultimately proves fatal for their host galaxy.
But how does a quasar bring about the demise of a galaxy? Granted, "killing" might be an exaggerated term, as the galaxy persists even after the quasar's light extinguishes. However, the galaxy is forever transformed. As one of the hottest and brightest entities in the universe, a quasar exerts such intense heat that it halts the process of star formation within its host galaxy. Hot gas is incapable of giving birth to stars because its atoms move rapidly and collide with significant force, resisting the gravitational forces necessary for collapse. Contrarily, the most conducive environment for star formation resides in cold gas, unencumbered by the vigorous motions that impede the gravitational collapse required to initiate the birth of new stars.
Additionally, quasars expel gas from their galaxies, starving themselves while simultaneously depriving their host galaxy of the raw materials essential for star formation. Though this may seem disheartening, it is, in fact, beneficial for life. The alternative scenario, an abundance of stars, can be far more perilous, as the birth of new stars is often accompanied by the explosive demise of massive stars in supernovae, potentially sterilizing any nearby planets.
However, this intricate cosmic dance is far from simple. Similar to the complexities of our own planet's biosphere, every component of a galaxy interconnects and influences every other facet of its environment. While hotentities like quasars and supernovae tend to expel gas from galaxies, shockwaves and quasar jets can also compress gas, briefly providing conditions suitable for the birth of new stars. Gas that exits the galaxy eventually mixes with incoming gas, undergoing recycling processes that reintegrate it back into the galactic system.