Lecture 27 - Supermassive Black Holes

"Black Holes are macroscopic objects with masses varying from a few solar masses to millions of solar masses. Macroscopic objects are governed by a variety of forces derived from a variety of approximations to a variety of physical theories. In contrast, the only elements in the construction of Black Holes are our basic concepts of space and time. They are, thus, almost by definition, the most perfect macroscopic objects there are in the universe. - S. Chandrasekhar"

The compact sources at the centre of radio galaxies have incredible luminosities. Invoking the "Eddington Luminosity Limit", astronomers concluded that the central engine must have a mass of the order of a billion solar masses. The time variability of the compact central source led one to conclude that it must be a very compact object. Thus emerged the paradigm that the central engines of Quasars and Radio Galaxies must be supermassive black holes. In April 2019, a stunning image was made of the shadow of the giant black hole at the centre of the giant elliptical galaxy M 87. This was done using the "Event Horizon Telescope" - an intercontinental baseline radio interferometer. This Lecture describes the technical advances that were needed to make such an image and explains how one might understand the morphology of the image.