Quasars and Active Galactic Nuclei
Francesca Philips
29th September 2016
Quasars and Active Galactic Nuclei
A Galaxy’s Centre
From any photograph, it is easy to recognise that stars typically appear to be most concentrated in the centre, or nucleus, of a galaxy. In our own galaxy, there is a higher density of stars closer to the Galactic Centre. Our Sun is approximately 25,000 light yearsfrom the nucleus, and in the vicinity of it, there are 0.006 stars per cubic light year. The concentration of stars in the Milky Way’s nucleus is estimated to be 300 times this(Begelman & Rees, 2010).
The nucleus of the Milky Way is a mere 25,000 light years away, but most of the optical and ultraviolet light from it is completely hidden by gas and dust. Radio, infrared and gamma-ray wavelengths, however, allow us to map out the region and see the dense star cluster. In 1974, an almost stationary compact source named “Sagittarius A*” was discovered in the centre of our galaxy(Begelman & Rees, 2010). After having observed it for decades, astronomers began to theorise that Sagittarius A* was, in fact, a black hole. Astronomers used infrared cameras to track the motion of stars in the nucleus of our galaxy, and discovered that the stars in the centre of the Milky Way orbit the same compact space; almost as if there was a small object there with an enormous mass. When scientists calculated the object’s mass, they discovered that it was approximately 400 million times the mass of our own Sun (Institute of Astronomy X-Ray Group, 2005). This, coupled with an unusual radiation spectrum that is similar to what one would expect from gas swirling into a black hole at a slow rate, provides significant evidence to support the black hole theory. It is now estimated that there is a supermassive black hole at the heart of almost all galaxies. These black holes can cause phenomena such as quasars and other active galactic nuclei, which can be some of the most luminous objects in the known universe(NASA, 2011).
The Discovery of Quasars
In the 20th century, advances in radio astronomy led to a wealth of new findings about activity which could previously not be observed, particularly regarding the nuclei of galaxies that were often obscured from optical and ultraviolet light by dust and gas. Astronomers began to realise that unusual phenomena were occurring in the centre of many of these galaxies. Observations of odd, compact objects with particularly strong luminosity, high concentrations of blue light, and high energy emissionsbaffled astronomers(Robson, 1996).
Originally discovered as a result of the first radio surveys of the sky in the late 1950s, ‘quasars’ were to remain a mystery for many decades. These objects were initially mistaken for stars, primarily because they were bright. However, other features of these objects were not consistent with features of stars that had been previously observed. The objects had unusual colouring; they had a high redshift and were brighter in ultraviolet light than normal stars(Begelman & Rees, 2010). A high redshift is an effect of the expansion of space between the Earth and the object, and so it was curious that such distant objects could appear so bright.
After decades of scientific controversy and mystery, astronomers deduced that quasars are very distant objects that often lie in young, active galaxies billions of light years away. The quasars can emit almost a thousand times the energy output of our universe across the electromagnetic spectrum. However, the question of why these quasars are capable of producing such energy and luminosity, and what they really are, remains.
Active Galactic Nuclei
Quasars are the most energetic and extreme forms of “active galactic nuclei”, or AGN.An active galactic nucleus occupies the centre region of a galaxy, and has a luminosity that is far higher than usual over a section of the electromagnetic spectrum. This luminosity is often non-thermal, and varies between AGN(Krolik, 1999). The radiation of these AGN is the result of accumulation of matter by a supermassive black hole in the centre of a host galaxy.
The host galaxies of these spectacular phenomena are called “active galaxies”. While the centre of our galaxy does contain a supermassive black hole, the Milky Way is inactive. In order for a galaxy to be considered active, the nucleus must produce a substantial luminosity that is dominated at some wavelength other than starlight. While there are many different types of nuclei within these active galaxies, they are typically defined as a compact region in the centre of a galaxy which has a higher luminosity over the electromagnetic spectrum than can be accounted for only by light from the stars that it contains.
Radio-Loud, Radio-Quiet, and Jets
Though each AGN varies considerably, astronomers normally class them under two broad terms: “radio-quiet” and “radio-loud”(Institute of Astronomy X-Ray Group, 2005). The main difference between these two classes of AGNs is determined by the characteristics of theastrophysical jets.
Jets are narrow streams of radiation and particles emitted at high speeds along the axis of rotation of a compact object(National Radio Astronomy Observatory, 2010). In the case of black holes, matter is attracted by a strong gravitational force and falls towards the black hole; however, instead of falling in, some of the particles can be accelerated and emitted in a beam along the axis of rotation (Krolik, 1999).Massive galactic central black holes will typically have the most powerful jets, and these jets will often ‘inflate’ lobes which can extend outside the galaxy itself, producing phenomena similar to that observed in Figure 3.
Approximately 10% of all known active galactic nuclei are radio-loud, meaning that they produce large-scale radio jets and lobes. These jets and lobes make up a significant amount of the luminosity of the nuclei. In contrast, radio-quiet objects only eject weak radio jets that are insignificant in regards to luminosity. This differencein the objects highlights the variety of AGN that exist.For example, types of AGN under these classification labels are: blazars, Seyfert galaxies, quasars etc. (National Radio Astronomy Observatory, 2010).
Unified Models
Scientists have been seeking a unified model to describe AGN for the last few decades, and are only now coming close to understanding the relationship between the different types of nuclei. Some unified models propose that the different observational classes of AGN are, in fact, a single type of object that we are merely seeing under different conditions. These models are ‘orientation based unified models’, where it is simply the different orientation of the object to the observer that causes the object to seem different(Peterson, 1997). This can be seen in Figure 4 and 5, where it is the angle from which one observes the AGN that determines what type of AGN is seen. There is controversy, however, about such models. A select group of AGN do not fit this model and others that appear to be extreme versions of AGN raise the question of why there is so much variety between the nuclei.
Conclusion
The study of the evolution of active galactic nuclei allows us to make assumptions about the early universe and the formation of galaxies. For example, scientists predicted that in the early universe, the most luminous forms of AGN, such as quasars, were more common. This means that the conditions of the formation of these AGN were also widespread. If this were the case, then it implies that there were massive black holes formed in the early Universe, and that there was an abundance of cold gas near the centre of galaxies, allowing the supermassive black holes to feed on matter(Krolik, 1999). It also suggests that there is something about younger galaxies which promotes the creation of active galactic nuclei.
Quasars and other AGN are also used as ‘lighthouses’ in astronomy. Hubble investigated the presence of helium in the early Universe by looking at the ways that light from quasars is altered as it travels through the Universe. As the light goes through matter, the change in the light can indicate the composition of the gas and reveal the composition of the Universe. Instruments, such as the Cosmic Origins Spectrograph, can break up the ultraviolet light from distant AGN into the component wavelengths so that the way that the intervening matter absorbs each wavelength can be studied(NASA, 2011). AGN are fascinating, not only because they are some of the most luminous objects in the Universe, but because they help us to piece together a picture of the early Universe and better understand it.
Bibliography
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Institute of Astronomy X-Ray Group. (2005, April 28). Active Galactic Nuclei (AGN). Retrieved from X-Ray Astronomy:
Keel, B. (2000, October). Introducing Active Galactic Nuclei . Retrieved from UA Astronomy:
Krolik, J. (1999). Active Galactic Nuclei. Princeton: Princeton University Press.
NASA. (2011, January). Active Galaxies and Quasars. Retrieved from Imagine the Universe:
National Radio Astronomy Observatory. (2010). Active Galactic Nuclei. Retrieved from National Radio Astronomy Observatory:
Peterson, B. (1997). An Introduction to Active Galactic Nuclei. Retrieved from Extragalactic Astronomy:
Robson, I. (1996). Active Galactic Nuclei. Chichester: Praxis Publishing.