4. Galaxies and Supermassive Black Holes

Summary Billions of stars group together to form gravitationally bound structures which are called galaxies. Galaxies often have a spiral or an elliptical shape, can orbit each other, collide and merge with each other. In the center of each galaxy a supermassive black hole is located, with some of them being responsible for the brightest objects on an absolute scale in the early stages of the Universe.

Keywords Black Holes; Dark Matter; Galaxies; Stars; Theory of Relativity

UGC 1810 and UGC 1813 in Arp 273 (captured by the Hubble Space Telescope)

The pair of colliding spiral galaxies Arp 273 at a distance of 300 million light-years from Earth. Galaxies can orbit each other, collide and merge with each other. While spiral galaxies are common in the Universe, the merger of two such galaxies can lead to the formation of a larger elliptical galaxy. (© NASA, ESA, and the Hubble Heritage Team (STScI/AURA) / Wikimedia Commons / Public Domain)

Stars are not evenly distributed in space but are rather grouped together in gravitationally bound structures which consist of ten millions (107) to hundred trillions (1014) stars. These structures are called galaxies, while smaller galaxies with up to a few billions (a few 109) of stars are also named dwarf galaxies. Most galaxies in the Universe are dwarf galaxies, and larger galaxies are often orbited by a few of them. Galaxies can collide and merge with each other, and galaxy mergers are the reason why galaxies tend to become bigger over cosmological time scales. The first galaxies formed about the same time when the first stars were created, a few hundred million years after the Big Bang. They formed in denser areas whose signature is already visible in the Cosmic Microwave Background. Most of matter in galaxies is dark matter which does not seem to interact with light and whose nature is unclear. There is about five times more dark matter in the Universe than ordinary matter based on protons and neutrons.

Most of the large galaxies in the Universe are spiral galaxies. They have a center with an ellipsoid shape, also called the galactic core or bulge, and a thin but large disk component around it. Inside the disk there are spiral arms in which relatively many new stars are born. The stars and the interstellar medium orbit the galactic center, similar to the planets orbiting the Sun in our Solar System. The fact that also the Sun orbits the center of our galaxy and not the other way around is known since 1918 through measurements of American astronomer Harlow Shapley. The disk typically rotates faster than the galactic center. Several globular clusters, which are spherical collections of relatively old stars, additionally orbit the galactic center out of the disk plane. In some spiral galaxies the galactic center has a barred instead of an ellipsoid shape, in which case it is called a barred spiral galaxy. It is believed that these bars are normal and temporary phenomena in the life of an unbarred spiral galaxy.

The second major type of galaxies are elliptical galaxies. Inside these relatively spherical galaxies typically no structures can be identified, and the stars orbit the galactic center in various non-closed orbits. This type of galaxies only possesses a dilute interstellar medium and has a low star formation rate if compared to spiral galaxies. Elliptical galaxies typically evolve out of spiral galaxies which merged with each other, which results in this galaxy type being found more commonly in dense environments with many galaxies where many galaxy interactions took place. In the center of clusters of galaxies there are typically supergiant elliptical galaxies, often with several cores, with one example being Messier 87 in the Virgo Cluster.

Artist impression of a supermassive black hole at the centre of a galaxy

An artist's impression of a supermassive black hole in a galactic center. While no light can escape the black hole itself, its gravitational attraction heats the surrounding gas in its accretion disk, with a part of the gas ultimately being sucked in and another part being blown away in extremely bright jets along its poles. The darkest objects in the Universe are thus responsible for the brightest objects in the Universe on an absolute scale. (© ESO/L. Calçada / Wikimedia Commons / CC-BY-4.0)

In the core of each galaxy a giant black hole is located, called a supermassive black hole. Black holes are dense and massive regions in spacetime which, due to the locally overwhelming attractive gravitational interaction, do not allow any light, matter or information to escape from it. As described by the General Theory of Relativity which was formulated by German-born physicist Albert Einstein in 1915, they bend spacetime in extreme ways by slowing down time and stretching length scales in their vicinities. Black holes are formed by dying massive stars when their nuclear fuel is used up and nothing is left to withstand their gravitational self-attraction. The masses of their largest individuals, the supermassive black holes, range from hundreds of thousands to billions of solar masses. These objects probably acquired their huge masses by sucking in gas clouds or nearby stars, or by colliding with other black holes.

Black holes can float through space like stars without disturbing their environments significantly. However, especially in the early Universe around 10 billion years ago when supermassive black holes consumed particularly much matter, their enormous gravitational influences on their environments led to the formation of extremely hot accretion disks in their horizontal planes and to relativistic jets along their rotation axes, which outshined their host galaxies by several orders of magnitude. These phenomena are called quasars and are the brightest objects in the Universe on an absolute scale, being an example of active galactic nuclei. The closest known active quasar is 3C 273 at a distance of 2.4 billion light-years. Such, while supermassive black holes themselves are not visible by nature, their massive impacts on their environments are responsible for the brightest known objects in the Universe.

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