Chapter 25. The Unified Model of Active Galaxies

25.1 Introduction

Credit: Holland Ford, Space Telescope Science Institute/John Hopkins University

The goals of this module: After completing this exercise, you should be able to:

1. Describe the observational characteristics of blazars, quasars & radio galaxies.
2. Explain the unified model and how it explains each type of active galaxy.

In this module you will explore:

1. The spectra of various types of active galaxies.
2. How changing our view of an active galaxy causes it to appear as a blazar, a quasar, or a radio galaxy.

Why you are doing it: Active galaxies, such as the one pictured above, at first appeared strange and different from most of the galaxies we knew. They could be shaped like spirals or elliptical galaxies, with some similarities but a few important differences. It is important to understand how one model can explain most of the features of active galaxies.

25.2 Background

An artist's rendition of the center of an active galaxy, with jets being emitted perpendicular to the accretion disk surrounding a supermassive black hole.

The invention of radio astronomy by Grote Reber in 1936 soon led to the discovery of a number of new and strange objects in the sky. These objects were very strong radio emitters, yet appeared to be very far away from us; some of the most distant objects in the universe. Their spectra were different than normal galaxies, which emit thermal spectra due to the presence of their stars. (To learn more about thermal spectra, take a look at the activity "Spectra and Kirchhoff's Laws.") The spectra from the radio sources appeared to originate from nonthermal sources; objects that emit light for a reason other than they are hotter than their surroundings.

The light emitted from these odd objects varied very quickly, implying that they were compact. How could we possibly figure out the nature of these very distant radio sources?

25.3 Blazars

BL Lac, a blazar located 900 million light-years from Earth, has unusual properties

The first blazar to be identified, BL Lac, is located in the constellation Lacerta. This is the designation for a star, since when BL Lac was named in 1929 it was mistaken for a variable star due to periodic change in brightness over a few-month period. It took another forty years for careful observations to reveal faint emission surrounding it, and Doppler measurements of its spectrum to tell us that this object is located on the order of a billion light-years away. For this and other blazars that far away to be as bright as they are in the radio spectrum, they have to be some of the most energetic sources in the entire universe, emitting as much energy as a thousand Milky Way galaxies. Further observations revealed that most blazars have faint emission surrounding them: these bright radio sources are located in the centers of much fainter galaxies.

The spectra of blazars are intriguing. Blazar spectra are nearly featureless (while absorption and emission lines are present, the main spectra of blazars are dominated by nonthermal radiation), unlike galaxies which have absorption spectra due to the combined light from all of the stars contained within them. Blazar spectra are the result of synchrotron radiation. Synchrotron radiation is produced by fast-moving electrons as they spiral around strong magnetic field lines. The spectra of blazars are very different from stellar spectra, and do not contain prominent emission or absorption lines.

Synchrotron radiation has a spectrum quite unlike stellar spectra

25.4 Radio Galaxies

In 1936, Grote Reber detected a number of strong radio emissions from a variety of sources. While some of them originated from within our Galaxy, one source proved to be a mystery. In the constellation of Cygnus, this object became known as Cygnus A (shown above). Located 740 million light-years away, Cygnus A gives off as much energy as 10 million Milky Way galaxies.

Cygnus A, an interacting galaxy located between a pair of enormous radio lobes, which are generated by jets emitted from the center of the galaxy, is one of the most energetic radio sources in the entire universe!

25.5 Quasars

3C 273, a bright quasar 2 billion light-years away.

As astronomers examined more and more bright radio sources at great distances, they discovered another class of radio objects. Observations of these sources (such as 3C 273) showed that, like blazars and radio galaxies, these objects are extremely far away. In fact, they are among the most distant objects observed in the universe, on the order of billions of light-years away. Astronomers called these objects quasars (or quasi-stellar radio sources) due to their large radio output and starlike appearance. Similar objects, found at large distances and starlike in appearance yet having little or no radio emission, have also been detected. Astronomers now distinguish between the two by the terms radio-loud quasars and radio-quiet quasars.

Quasar spectra suggest that they also aren't galaxies. Their spectra are characterized by broad emission lines, indicative of rapid, turbulent motion within the energy-emitting region. Also, quasars emit more ultraviolet light than normal galaxies. In the 1980's, astronomer determined that similar to blazars, quasars are the bright centers of much fainter galaxies. Radio-loud quasars are typically found in elliptical galaxies, while radio-quiet quasars are also in ellipticals, but are in some nearer spiral galaxies.

25.6 A Unified Model

With the discovery of different types of active galaxies, yet all with similar characteristics, astronomers began looking for a connection between the various active galaxies. The images below demonstrate the model that astronomers believe unifies the various active galaxies; suggesting that they are all active galactic nuclei, driven by a central supermassive black hole seen from different angles.

The images above show a supermassive black hole with an accretion disk surrounding it, giving off jets perpendicular to the disk. Depending on how the accretion disk and jets are oriented relative to our line of sight, we either see the jets coming straight towards us or we see them at some angle.The series of images show the possible orientation angles and you can see how a change in view of the galaxy can alter how we perceive it.

When the jets of energy are pointed directly towards us, we detect a nonthermal spectrum void of emission or absorption lines, indicative of blazars. Fast moving electrons are spiraling along the magnetic field lines, which spiral outward perpendicular to the accretion disk. The magnetic fields focus the charged particles into the jets of energy that we observe. When they are pointed directly towards us, we detect mostly the synchrotron energy given off by the electrons.

As our angle of observation increases we now observe the turbulent accretion disk, whose rapid rotation produces broad spectral lines in addition to thermal radiation from the disk and synchrotron radiation from the jets. This is consistent with observations of radio-loud quasars (if no jet is present the observer would see a radio-quiet quasar). When we look at the accretion disk close to edge-on, the torus or doughnut shaped region surrounding the black hole obscures our view of the accretion disk. We can see emission from hot gas rising out of the accretion disk, but its spectrum is neither blue- nor red-shifted, and so we detect narrow emission lines. In addition, we still detect the synchrotron emission from the jets. Combined, this leads us to detect the galaxy as a radio galaxy.

25.7 Quick Check Quiz

Indepth Activity: The Unified Model of Active Galaxies