Lesson 56: The Role of Supermassive Black Holes
In this lesson,
we will look at the idea that
some or perhaps even all galaxies contain a supermassive black hole at their
center. We will also discuss the role these black holes may play in regulating
star formation in galaxies.
A small percentage of galaxies, known as active galaxies, emit extreme amounts
of radiation from their central regions. The central region of such a galaxy is
known as an active galactic nucleus. The most luminous examples of such
galactic nuclei are known as quasars, and can be more than a trillion times
more luminous than our Sun.
Scientists believe that
these active galaxies contain supermassive black holes, usually no bigger than
our solar system. This black hole is probably surrounded by a swirling
accretion disk of very hot gas, similar to what we see in X-ray binary star
systems. Matter falling into these supermassive black holes is converted into
the radiation we see from billions of light years away.
Evidence for black holes is based on looking at the orbits of objects around
the centers of such galaxies. Based on the characteristics of these orbits,
scientists have calculated that the mass of the relatively small objects in the
centers of these galaxies must be billions of times greater than our Sun. The
only object known to pack so much mass into so small a space is a black hole.
Evidence collected during the last two decades indicates that it is possible
that ALL galaxies have a supermassive black hole at their centers.
Studies have revealed that
the size of a galaxy's bulge is related to
the mass of its central black hole. In other words, galaxies with large bulges
have large black holes, while those with smaller bulges have smaller black holes.
This may indicate that black holes play a part in
a galaxy's evolution. The study of radio galaxies, which are
galaxies with unusually strong radio-wave emissions, reveals that active
galactic nuclei can release powerful jets of matter that heat the surrounding
gas. By heating the gas, the central black holes can delay or prevent the
formation of new stars.