So far, we have looked at the extremes of astrophysical black hole scale, stellar-mass black holes, and supermassive black holes. There is a good reason for this. These extremes are the most well-known cases. If we consider all the measured masses of black holes to date, we see a cluster of objects in the range of about 5-20 solar masses with some reaching as high as possibly 70-80 solar masses. These are the stellar-mass black holes. There are also a large number of objects towards the right side of this plot at the highest mass end. These are the supermassive black holes that are thought to reside in the centers of most galaxies. Masses have also been obtained for many other low-mass compact objects. These low-mass stellar remnants populate the far left of this plot and are classified as either neutron stars or white dwarfs. In the middle of this plot, there is a great deal of empty space. Should there not be something lying in the middle? This is one of the many questions perplexing astronomers today. If we were to find black holes lying in the center of this plot, they would be known as intermediate-mass black holes. Although the search for these sources is ongoing, intermediate-mass black holes have proven themselves to be very elusive. Intermediate-mass black holes are just that. They have masses that lie between the heaviest stellar-mass black holes and the lightest supermassive black holes, making them intermediate on the mass scale of the astrophysical black holes, hence the name. But what are these objects, how are they made and why should we care about them? Intermediate-mass black holes weigh in at more than 100 times the mass of our sun reaching up to 100,000 times the mass of our sun. They are thought to be too big to form from the death of stars that exist in our universe today. If this is the case, how are they made? The first intermediate-mass black holes were discovered recently in 2019 by two gravitational wave observatories; LIGO located in the United States and VIRGO located in Italy. Stellar-mass black holes were first observed in the 1970s, and the first confirmation of supermassive black holes took place in the 1990s. We had to wait almost 30 years to see an intermediate-mass black hole. This long wait was for many reasons. For instance, they're probably much more rare than stellar-mass and supermassive black holes. But the most important reason is that we needed a new type of telescope. The confirmation of stellar-mass black holes only required the use of ground-based telescopes to observe and measure the orbits of the companion star. Supermassive black holes are much farther away. The launch of the Hubble Space Telescope allowed the 1994 observation of gas orbiting the center of the galaxy M87 to confirm the existence of a supermassive black hole in that galaxy. Gravitational wave observatories can detect the motion of the black holes even when there is no light emitted. The first gravitational wave observatory named LIGO first detected stellar-mass black holes in 2015. In 2019, a collaboration between LIGO and VIRGO detected two black holes with masses of 66 and 85 solar masses merging into a larger black hole with a mass of 142 solar masses large enough to be classified as an intermediate-mass black hole. This observation shows us how intermediate-mass black holes can be built by merging together smaller mass black holes. In fact, since 66 and 85 solar masses are already fairly high, it is hypothesized that these two black holes are the result of mergers of a previous generation of lower mass black holes. As newer gravitational wave observatories in Japan, India, and Australia start operating, it is likely that more intermediate-mass black holes will be detected, which will give us a better understanding of the population of black holes that grow through the merger of smaller black holes. Now that we found evidence for intermediate-mass black holes with a mass close to the lower boundary of 100 solar masses, what about higher masses, such as 1,000 or 10,000 solar masses? When we look at far-away objects like quasars, we're seeing the objects as they looked at the time that the light was emitted, which was at a time when the universe was young. For instance, with some of the farthest away quasars, we're seeing the quasars as they appeared when the universe was only about 800 million years old. These quasars are galaxies with supermassive black holes that are a few million times more massive than the intermediate-mass black hole that LIGO and VIRGO discovered. Current calculations suggest that there isn't enough time to construct these supermassive black holes through the mergers of millions of a 100 solar mass black holes. Instead, theorists predict that an earlier population of intermediate mass black holes with masses around 1,000- 10,000 solar masses were formed in the early years of the universe. These were the seeds of the supermassive black holes. If this idea is correct, there should be some leftover intermediate mass black holes. There are two known methods for forming larger intermediate mass black holes; direct collapse and runaway formation. One theory suggests that these behemoths were formed early in the universe when it was a much simpler place, chemically speaking, that is. The first stars formed when the universe was only about 100 million years old. At this time, the universe contained only the simplest elements, so there was just hydrogen and helium. This early in the universe, stars could become much larger than they are today, sometimes containing upwards of hundreds of solar masses. We have already learned that massive stars burn hotter, brighter, and quicker than their low mass counterparts. This was true for those first stars too. Such huge stars would have very short lives indeed. We have also seen that massive stars can lose their mass through winds. What we have not yet mentioned though, is that the power of this wind is a function of the star's chemistry. Astronomers have found that the metallicity of a star or the amount of metals it contains affect the strength of the stars' wind. Here is where I should point out a quirk of astronomy. Forget the high school chemistry class for the moment. According to astronomers, the universe is made up of hydrogen, helium, and metals. Anything that contains more than two protons is a metal. Strange but true in the astronomical circles. Anyway, back to stellar winds. Therefore, the metallicity of a star or a region give you an indication of how much of these metals are present. When the metallicity is essentially zero, we find that a star loses little to no mass via its wind, irrespective of its size. This means that those first stars would have lost very little mass by the ends of their lives and at the end of the short life. Well, here's another place where it changes from the life of stars today. Given the huge mass contained in these first stars, astronomers think that they may not have ended in a huge explosion as massive stars do today. Instead, it is thought that once the star ran out of fuel, the force of gravity would be so strong that all of the star would collapse directly down to a black hole. The outer envelope of the star would not be blown away as it is today, it will be dragged down into the black hole to join the core of the star. This stellar death is called direct collapse. This means that the first stars in the universe may have collapsed to form black holes weighing hundreds of solar masses. They would have created intermediate mass black holes. Some theoretical astronomers have suggested that intermediate mass black holes could also form by a process known as runaway formation. Runaway formation can only occur in dense regions. Dense regions are areas in space where many stars are clumped closely together as they are in some stellar clusters. Within the central region of the cluster, you can think of the stars as dancers in a club. They are moving around each other as they travel under the influence of gravity. If two of these stars get too close together, they can start orbiting as binaries do, or they can spiral in towards each other and merge. This new star will have more gravity and attract other nearby stars. As they spiral in and merge, the object at the center will have even more gravity and the cycle will continue, allowing this object to grow and grow until the gravity of this object is so strong that the supermassive star is forced to collapse to make an intermediate mass black hole.
