Black holes have been a hot topic among the scientific community for decades. If the scientific community had its own twitter, #blackholes4eva would be a trending hashtag for years to come. This is justified, of course, because in the past twenty years, we’ve made leaps and bounds in terms of our understanding of black holes. From being an obscure scientific theory left to the musings of sci-fi writers, we now have pictographic evidence of the existence of black holes. What’s more, we’ve discovered what are known as ‘cloaked’ black holes. By definition, they weren’t supposed to be found, but we found them all the same!
Cloaked black holes are basically black holes that are obscured from view. They’re in an early stage of life and obscured by a dense cloud of gas. Eventually, as they ‘grow up’, they’ll consume the thick gas curtain and come into full view as a mature supermassive black hole.
That is the simple, succinct, dinner-conversation version of the answer. However, the life of a black hole from cloaked to supermassive is a bit more complicated than that.
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Formation of a black hole
A black hole is most commonly the result of the death of a star. A star loses the fuel that’s keeping it ablaze and the gravitational pull at its core becomes too much for it to handle. Eventually, the star collapses into itself and forms a region of near infinite density known as a black hole.
However, that’s just one of the scenarios. There has been evidence to suggest that black holes and the very first stars co-existed at the beginning of the universe. Certain black holes are so massive that they would have had to be present close to the beginning to have enough time to grow to their current size. These black holes have masses of over a billion times that of our sun and are popularly known as supermassive black holes. No one knows with certainty how the first black holes came into existence, but a popular theory suggests that right after the Big Bang, there were pockets of extremely high density that collapsed gravitationally, leading to the creation of the universe’s very first black holes.
Yet another theory of supermassive black hole formation suggests that when the universe was in its infancy, smaller black holes coalesced at the center of galaxies to form their supermassive siblings. That’s perhaps the only realistic means by which these celestial behemoths grew to the size of “supermassive”.
These gigantic black holes pull all forms of debris and gas from surrounding areas towards their gravitational center. Just like water going down your bathroom drain, the debris swirls down towards the center of this galactic drain. The swirl creates a sort of disk of gas and debris around the black hole called the accretion disk. These accretion disks are responsible for one of the brightest phenomenon in the universe—quasars.
What are Quasars?
Quasars are relatively compact objects with large luminosities. In fact, they’re the brightest objects in the universe, apart from a few of the most powerful supernovae. The word quasar comes from the acronym QSO, which is short for “Quasi-Stellar Object” since their bright shine makes them resemble stars. The powerful luminosity comes from the intense light and energy generated by friction between debris of the accretion disk of supermassive black holes as it swirls around, eventually falling into the black hole. Typically, these strongholds of luminescence are at least a hundred times brighter than their host galaxies. Which begs the question—why can’t we see quasars like we see stars in the night sky? If these objects are the brightest objects in the universe, what’s stopping them from, as Rihanna would put it, “shining bright like a diamond”?
This apparent invisibility is because quasars are not only the brightest objects in the universe, but also some of the most distant. The closest are at least a few billion light-years from earth, so no matter how brightly they shine, the can only be observed through telescopes as a faint glow in the sky. Furthermore, the debris and gas creating the quasar also create a sort of ‘cloak’ in front of it that blocks a significant portion of light from passing through it. That brings us to our primary topic at hand, ‘cloaked’ black holes.
What are ‘cloaked’ black holes?
Supermassive black holes in their infancy, when they are still devouring the dense curtain of gas in their vicinity, swirling debris, and creating huge accretion disks leading to the formation of quasars, are called ‘cloaked’ black holes. Eventually, these black holes will have consumed the entirety of the gaseous veil cloaking them, and quasars will burn through all of the fuel that’s powering them; the black holes will then turn into the stereotypical image that we have in our mind. Cloaked black holes have been trending in the news recently, after NASA’s Chandra X-Ray telescope discovered what is perhaps the oldest black hole ever observed. Billions of light-years away, hidden behind an impenetrable cloak of gas, Chandra discovered a black hole that is just 6% younger than the age of the universe. That’s about 13 billion years old!
It’s extraordinarily rare to observe a quasar in the cloaked phase, as current scientific instruments find it close to impossible to detect them. Chandra found this quasar, named PSO167 – 13, almost by accident. They weren’t even looking for cloaked black holes billions of years old. The discovery of PSO167 was pure serendipity.
The discovery was led by Fabio Vito, a Fellow at the Pontificia Universidad Católica de Chile, in Santiago, Chile. PS0167-13 was one of nine quasars the team was observing, assuming that they were all uncloaked. They observed PS0167-13 for about 16 hours. At the end of which, they found that just three photons of high-energy X-rays had returned from this black hole. Through this observation, the team of scientists inferred that this was a cloaked black hole, as the gaseous cloak absorbed all of the low-energy X-rays and only let the high-energy rays pass.
A black hole in such an early stage of its development is an incredible find. Observing it even more intently over the years will give scientists significant insight into the early days of the universe, and it will certainly be exciting to see what they find out!