What are some amazing facts about quasar

Active galaxy nuclei

A supermassive black hole is believed to be located in the middle of large galaxies. Although these compact objects themselves are not visible, the matter flowing towards them can shine astonishingly brightly. In this case, astronomers speak of an active galactic nucleus, or AGN for short. Leonard Burtscher from the Max Planck Institute for Extraterrestrial Physics in Garching explained in our podcast which physics takes place in such systems.

The so-called active galactic nuclei shine brighter than any star, supernova or galaxy. Because of their English name - Active Galactic Nuclei - these are also called AGN for short.

Leonard Burtscher: “An AGN is a galaxy that has a black hole in its center - like probably most galaxies. The special thing about an AGN is that this black hole accretes a relatively large amount of matter. Relative means that the radiation emanating from this matter obscures all other emissions from this galaxy. That means, the AGN is brighter than the entire rest of the galaxy. "

Leonard Burtscher from the Max Planck Institute for Extraterrestrial Physics

Depending on the angle of observation and distance from Earth, researchers also refer to the active galactic nuclei as quasars, blazars, Seyfert or radio galaxies. The areas from which the radiation escapes are in all cases tiny compared to the entire galaxy. The so-called Schwarzschild radius, which is directly proportional to the mass of the black hole, is used as a measure of the extent of a black hole. If our sun were to be squeezed further and further, it would also become a black hole as soon as the spherical radius falls below the Schwarzschild radius.

“The Schwarzschild radius of the sun is three kilometers and the Schwarzschild radius of a black hole is three billion Solar masses amounts to for example three billion kilometers. This size is not uncommon in the center of the largest elliptical galaxies. "

A typical AGN is roughly the size of our solar system, while the home galaxy is several million times as large. The black hole in the center is surrounded by a so-called accretion disk: This consists of the matter - dust and gas - that the black hole attracts due to its enormous gravity.

“When it hits the black hole, matter has to reduce its angular momentum. It's like the whirlpool in the bathtub. The water does not just flow out of the bathtub, but first of all swirls around the drain and only then does it flow away. Now, the Schwarzschild radius of a black hole is so tiny compared to the galaxy that matter cannot be aimed directly at the black hole even if it flows into the center. Therefore it flows by first of all. So an orbit is formed and then the gas has to laboriously reduce this twist, the torque, until it can really flow into it. This is a very complicated process that is not really fully understood yet. "

The central machine

Before the matter finally disappears in the black hole, it heats up strongly and emits high-energy radiation, which also stimulates the gas in the vicinity to glow. Astronomers also refer to the black hole and its accretion disk as the “central engine”, the central machine that drives an active galactic core and lets it shine. In addition, some AGNs have another distinctive feature.

“About ten percent of all AGNs are, as they say, radio loud. This means that the ratio of luminosity in the radio range to the luminosity in the optical range is greater than a certain factor. And when AGNs are radio loud, they have a jet. When AGNs have a jet and why they have a jet is not clear. "

Region around the quasar ULAS J1120 + 0641

What is clear, however, is that the particles in these bundled matter beams are accelerated almost to the speed of light and can extremely heat up the gas there on their way through the galaxy. Researchers found the first active galactic nucleus named 3C273 in the constellation Virgo in 1959. Hundreds of thousands have been added since then. Due to their tremendous luminosity, the light from the galactic nuclei can travel billions of light years through space before it hits earthly telescopes. It is therefore not surprising that one of the oldest celestial objects discovered to date is an AGN: It emitted its radiation only 770 million years after the Big Bang, around 13 billion years ago. However, AGNs are by no means only part of the young universe - they can also be found in our cosmic neighborhood, only a few megaparsecs away, whereby one megaparsec corresponds to approximately 3.2 million light years.

“One of the next AGNs is Centaurus A. The distance is known very precisely: 3.83 megaparsecs. But there are other AGNs a few megaparsecs away, for example the Circinus Galaxy. It lies exactly behind the Milky Way and was therefore only discovered in the 1960s or 1970s. "

Scientists therefore appreciate the active galactic nuclei not only because they allow insights into a distant and long-forgotten universe. She is also interested in the interaction between an AGN and the surrounding galaxy.

The black hole in the Milky Way

“How does the AGN affect the rest of the galaxy? There is no answer to that yet. There are two camps. Some claim that this so-called AGN feedback is very important for the rest of the galaxy. These Feedback could ultimately even lead to the entire molecular material for example in the form of a wind or a jet is thrown out of the galaxy and then star formation comes to a standstill. And then the galaxy, as they say so romantically, will be red and dead - red and dead. "

However, there are also doubters who do not ascribe such a destructive role to the active galactic nuclei in the evolution of the galaxy.

“The other variant is that AGNs are ultimately irrelevant for this are. That is what the other camp means. Their opinion: Although this effect does exist, it does not play a role in the global budget of the galaxy. It plays a much bigger role Merging of Galaxies, which was very common in the early universe. Through this merging process, the matter is strongly compressed and many stars are born. And then the material is used up. So you don't need an AGN to remove the material, but the material is gone because it is bound in the stars. "

Radio galaxy Centaurus A

The Milky Way also has a supermassive black hole in its center. Sagittarius A - as the black hole is called - is currently emitting mainly in the radio range of the electromagnetic spectrum. Otherwise, however, it is relatively quiet. It could have looked very different in the past.

“The Milky Way is believed to have been an AGN once. There are a few pointers to this. A clue comes from the Fermi gamma telescope, which has discovered a so-called Fermi bubble that drives a shock front above the Milky Way. This is in the X-ray range and all possible wavelengths been observed. So you see a great energy explosion that once took place there. Interestingly, the age calculated for the Fermi Bubble fits relatively well with the age of the stars in the center of the Milky Way. These are really very close to the black hole. So maybe something happened at that time. It could be that there was a major eruption about ten million years ago, which on the one hand led to the shock wave and on the other hand to the formation of stars. "

If the black hole in the Milky Way was actually active in the past, at least the effects on the earth on the edge of the galaxy seem to have been quite minor: at this point in time it was already inhabited - especially after the dinosaurs died out of mammals. Researchers do not know whether the Milky Way core will be active again in the future. In general, AGNs seem to be more of a feature of young galaxies. So it is quite possible that the Milky Way has already passed its brightest times.