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Supermassive black holes: monsters in the early Universe

When matter is squashed into a tiny volume the gravitational attraction can become so huge that not even light can escape, and thus a black hole is born. A star such as the Sun will never leave a black hole because the quantum forces between matter stop this from squeezing into a sufficiently small volume. Once the Sun dies it will merely leave a white dwarf star, which slowly cools and dims over billions of years. But when the nuclear furnace of a star weighing more than approximately 20 times the Sun exhausts itself, the quantum forces cannot halt its immense gravitational collapse, sparking its explosion as a supernova and often leaving a stellar mass black hole in memory of its previous glory. Such a black hole weighs from a few Suns to perhaps a few dozen. But black holes weighing millions of Suns have been found. And these monstrous things are aptly called super-massive black holes.

As a result of very arduous observations, astronomers have discovered that more-massive elliptical galaxies have bigger super-massive black holes and that these super-massive black holes appear extremely rapidly after the birth of the Universe. This raises three hitherto baffling questions:

  1. How can super-massive holes even form?
  2. Why do heavier elliptical galaxies contain heavier super-massive black holes? The most-massive galaxies, weighing more than a hundred times the Milky Way in stars, have super-massive black holes that nearly reach the mass of the Milky Way.
  3. How can such incredibly heavy super-massive black holes form within a few hundred million years after the Big Bang when this is basically in the first blink of the cosmic baby’s eye?

The biggest problem is that there is no known physics that can be used squash the fantastic amount of normal matter into a sufficiently small volume to make a super-massive black hole within a hundred million years. The matter resists: if squashed too much it becomes relativistic and radiates much of its mass in the form of photons. So, to try and tackle this issue, some wild theories have been developed in the hope of finding an explanation.

Some of these theories have tried to utilise exotic primordial black holes or with dark matter, while others have attempted to construct hyper-massive stars that collapse to black holes weighing more than a hundred solar masses. Some have even tried to use combinations that also invoke special ways of accreting normal matter onto such a seed-massive-black hole. All of these theories are problematical as they are based on hypotheses that have not been confirmed by observation. For example, the evolution of the first putative hyper-massive stars is extremely uncertain because such objects should blow off most of their mass within the first million years.

In spite of the seemingly unsolvable nature of this issue, a team of astrophysicists from Charles University in Prague, Bonn University, the European Space Agency, and the University of Tokyo may have solved all three problems simultaneously.

The team noted that when two black holes merge then approximately 95% of the mass combines to the new more massive black hole with only a tiny amount of the rest being radiated as gravitational waves. This is very nicely shown by the ongoing detection of merging black holes using gravitational wave detectors. They also noted that stellar populations that form in the very early Universe, when there is only hydrogen and helium, are dominated by massive stars weighing more than 20 and up to 150 or so Suns. Additionally, they also indicated that more-massive elliptical galaxies formed more quickly thereby transforming their gas more vigorously into stars. The final ingredient of this theory is that the more vigorously the galaxy forms its stars, the heavier the star clusters are that form in the galaxy at its centre.

Figure 1: The stages of formation of supermassive black holes: the first extremely massive cluster forms at the centre of the later galaxy and looks like a quasar for about 50Myr. Gas inflow then collapses the remnants of the stars (the black hole cluster) into a super-massive black hole seed which continues to grow as gas falls onto it as long as the galaxy keeps forming (the true quasar phase).

The astrophysicists computer-coded the above and calculated what happens when an elliptical galaxy begins to form when, roughly two hundred million years after the Big Bang, the gas in the cosmos had sufficiently cooled through its expansion to gravitationally collapse. The first-formed extremely massive star clusters were full of heavy stars, leaving many millions of normal black holes in regions spanning not much more than 10 light years across. This was over after about 10 to 20 million years when the last massive star died. But around these first clusters of black holes, the elliptical galaxies were just starting to form. The formation of a whole galaxy continued for about a billion years and, during this extremely violent time, very large amounts of gas fell onto the cluster of black holes near its centre. The gas squeezed the cluster together until the black hole velocities within the cluster became relativistic. At this point, the cluster of black holes started radiating a critical amount of its energy as gravitational waves. It consequently collapsed within a dozen million years to a combined black hole mass of a hundred thousand to millions of Suns. As the elliptical galaxy continued to form around this massive black hole, gas kept falling onto it so that it grew more in mass as a quasar, with additional massive elliptical galaxies hosting more massive central black holes that grew faster.

Using conventional physics and the most updated observational data, this theory thus answers all three questions noted above and makes two predictions:

  1. The very first quasars may not be actually accreting super-massive black holes, but the extremely early hyper-massive star clusters containing many millions of brilliant massive stars could be the precursors of real accreting super-massive black holes.
  2. When the cluster of black holes reaches its relativistic state, its hundreds of thousands and millions of black holes will be radiating gravitational waves in bursts as the black holes pass each other ever more frequently. This type of signal still needs to be calculated in detail and should allow this theory to be tested the future gravitational wave observatories.

 

Featured image: Using the Event Horizon Telescope, scientists obtained an image of the black hole at the center of galaxy M87, outlined by emission from hot gas swirling around it under the influence of strong gravity near its event horizon. (By Event Horizon Telescope et al. via Event Horizon Telescope.)

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