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(ii) Two of the most striking pieces of evidence we have for black holes in the universe are (a) supermassive black holes in galactic centres, and (b) the phenomenon of quasars. Explain in each case what is seen, what is the physical explanation of what is seen, and how this provides evidence for black holes.
BLACK HOLE EVIDENCEàSUPERMASSIVE BLACK HOLES IN GALATIC CENTERS
To prove that a black hole exists is tricky: they do not emit any light. The only way we can know they exist is by observing how they alter their surroundings in the nuclei of galaxies. Only their presence is felt. In their vicinity, orbiting stars and gas clouds behave differently; they move at high speeds around some invisible massive object. For example, when astronomers peer into the heart of the Milky Way with radio, infrared and X-ray telescopes, they see swirling clouds of gas and the motions of millions of stars near the Milky Way’s center (in the direction of the constellation Sagittarius). In the exact center, there is a radio emission named Sagittarius A, which differs from any other radio emission in our galaxy. Curiously, hundreds of stars crowd this area, which is within 1 light year or so of Sagittarius A. These motions of stars and gas near Sgr A show that there are a few million solar masses within a tiny space, suggesting that a black hole exists, possibly 4 million times more massive than the sun, all packed into a region a little larger than our solar system. A massive object like that, and in such a small space, is more than likely a black hole: the relentless pull of gravity, whose energy is also said to be the source for quasars.30
The best evidence for black holes arises from observing the centers of many galaxies (the galactic nuclei) using high-resolution radio-to-ultra-violet wavelengths. These observations show that:
• There are super massive black holes at the center of many active and non- active bright galaxies (i.e. the spiral galaxy NGC 4258 and some dozen bright galaxies near the Milky Way).
• The mass of a black hole has millions to billions solar masses.
• There is a size correlation between black holes and galaxies: the most massive black holes belong to the most massive galaxies suggesting that the growth of
these black holes is closely related to the process of galaxy evolution.
• The emission of extreme amounts of radiation and powerful jets of matter in
these galactic centers suggest the presence of massive, compact objects.
• They affect a companion star’s orbit.
• They tear mass from its surface.
• They form an accretion disk around themselves that emits intense X rays.
• Once matter falls into a black hole, it cannot be seen from the outside.
There is also a process of elimination whose argument posits:
30 This information has been compiled and studied from Bennet, Donahue, Schneider, Voit. The Cosmic Perspective. Volume II. Stars and Galaxies. 6th Edition. 599, and from Chaisson, McMillan. Astronomy Today. Volume II. Stars and Galaxies. 646- 650.
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