Researchers solve the mystery of small galaxies with supermassive black holes

black hole eating star
This illustration shows a glowing stream of material from a star as it is being devoured by a supermassive black hole in a tidal disruption flare. (Credits - NASA/JPL-Caltech)

Nearly every galaxy in the universe appears to have a black hole in the centre including our Milky Way galaxy. There is a strange relationship between the mass of black hole at the galactic centre and properties of the galactic host itself. So a bigger galaxy refers to a bigger black hole. However, some exceptions have been found to this trend and researchers have detected an important link between the evolution of galaxies and black holes.
We do not know which came first, the black hole or the galaxy. About thirteen and a half billion years ago galaxies and black holes formed nearly at the same time. According to astronomers, regular communication takes place between galaxies and their black holes. Bigger galaxies have more matter to feed their giant black holes. On feeding, the black holes become active. Gases swirl around it with high energies before falling onto the event horizon. Some part of the gas heats and radiation is emitted which escapes the vicinity. Gases can also swirl around the outer edges without falling in which form of long jets extending thousands of light-years in the nearby medium.
So, this energy which is dumped in the surrounding heats up the nearby gases and the heated gas has low inclination to sink into the centre where it might encounter the black hole. Thus the gas stays away and the black hole stabilizes in size as it cannot feed on anything. If a galaxy grows larger, then it can press more gas to the centre feeding the black hole followed by a feedback episode which holds the continued growth of black hole from getting out of control.

Thus smaller galaxies end up with smaller black holes and the larger ones with big black holes. However, there are exceptions to it, as some galaxies host the black holes way out of their ability to feed them. The reason could be due to a peculiar type of galaxy known as a blue nugget. These galaxies are found very far away meaning they were part of a younger universe. They are blue and small as their name suggests. They have an enhanced rate of star formation. Due to this, they form bigger and brighter stars than average. They are able to form stars at an exceptional rate since they feed on the nearby gas streams pumping material to the undersized galaxy.
However, this influx of raw material has its impacts and due to these activities, a massive black hole formation takes place at the centre. It feeds all over the galaxy shutting the further formation of stars. Due to their size, this event affects the entire galaxy terminating the growth at an early stage. So the stars which were formed eventually die as old, dim, red stars turning the galaxy into a red nugget. Thus it has an exceptionally large black hole at the centre. There are only handful of examples of such cases which can give insights into the relationship between galaxies and their black holes.
Journal Reference: arxiv


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