Article by Professor Jorick Vink
Before addressing Black Hole sizes, we should first go back to see what astronomers mean when they use the term “Black Hole”. This may be easiest to understand when considering the end products of stars. When a star runs out of fuel its core contracts to become a compact object.
For stars like the Sun, the final product is that of a white dwarf, a compact object contracted to the size of the Earth. For stars that are about 8 times the mass of the Sun, cores contract to a size no larger than an average city. These objects are called neutron stars. When neutron stars emit radio emission through two opposite beams – sweeping around like a regular clock – they are referred to as pulsars, discovered by Jocelyn-Bell Burnell in the late 1960s.
For the most massive stars in the Universe – up to at least 300 times the mass of the Sun – it is thought that their cores collapse to just a point in space, called a ‘singularity’. Such singularity is usually referred to as a Black Hole, where the force of gravity is so humongous that even light particles cannot escape. This is why the object is Black, you simply cannot see it!
One may then ask the question how we know black holes exist if we cannot see them even when looking with the world’s largest telescopes? The first evidence for the existence of black holes came from an indirect method using double star motions. Some stars were known to be wobbling around their axes. From the orbit of the primary star it was possible to infer the mass of the companion. However, as this companion was not seen, it became clear that the only explanation for this puzzle was the existence of a Black Hole. Typical masses of Black Holes in these X-ray systems, such as Cygnus X-1 in our Milky Way, were anywhere between 5 and 10 times the mass of the Sun.
When in early 2016 LIGO announced the discovery of gravitational waves (see Fig 1), or ripples in space-time, from two merging Black Holes, one of the largest surprises was that these black holes were heavy, of order 30 – 40 times the mass of the Sun, much larger than the most massive black holes in our Milky Way.
Theoretical studies by the current author and colleagues showed that such heavy Black Holes may be produced in environments where the iron contents is notably lower than within our Solar environment. These pristine conditions are rather more similar to those in the early Universe.
So are these heavy LIGO Black Holes the most massive Black Holes that we know of? The answer is a resolute “no”. As there exists an entirely different category of black holes that are found at the centres of galaxies. One such example is Sagittarius A at the centre of our own Milky Way. This is thought to host a Black Hole as massive as a million times the mass of our Sun. In fact, it has been hypothesised that all galaxies contain a massive black hole at their centre, with a relationship between the size of the Black with the size of the entire galaxy. These supermassive black holes have been inferred to be anywhere between about a hundred thousand to several billions of solar masses. Supermassive black holes are also referred to as Active Galactic Nuclei (AGN) or Quasars.
The current record holder is TON 618 with an estimated mass of 66 billion times the mass of the Sun (see Fig 2).
Arguably the most interesting question is how Supermassive black holes grow to such humungous masses? One possibility is to start with just a stellar mass seed and slowly grow the Black Hole to larger & larger masses by eating up more & more objects. The big puzzle remains how this process could get out of hand so quickly, with the Black Hole growing to such enormous proportions?
Hints to this puzzle may be revealed from quasar observations at high redshift in Early Universe. As large quasar masses are already noticed at a relatively young age of the Universe, the existence of very massive First Stars of 100 solar masses or more has been suggested. Moreover, the existence of massive quasars in the early Universe seems to suggest the presence of supermassive stars with over one hundred thousand solar masses. Yet, the formation and evolution of such supermassive stars remains an open question in modern day Cosmology.
Alternative hints to the formation of supermassive black holes may be gained from the local Universe by identifying a intermediate mass population of Black holes on the order of 100 to ten thousand solar masses. One possible group of objects that may fall in this mass range are the ultra-luminous X-ray sources, or ULXs, but it is as yet an open question as to whether any observed ULX truly hosts one of these illusive intermediate mass black holes, which are so urgently required to be able to solve the puzzle of the formation of supermassive black holes.
It might be clear that there is still a lot more work ahead of us both from the observational and theoretical side. Until that point, I am afraid we shall all remain in the dark!