Unit 16: Quasars
 
 

OVERVIEW

The similarity between Active Galactic Nuclei (AGN) and quasars is reviewed. The events leading to the discovery of quasars are discussed, along with the initial controversy about quasar redshifts. The evidence for generation of tremendous amounts of energy in a small volume are discussed. Some of the physical processes in quasars are reviewed. The problem of understanding quasar geometries is discussed. The phenomenon of gravitationally-lensed quasars and their uses is discussed.

LEARNING OBJECTIVES

At the end of this unit you should be able to:

1. Discuss the similarities and differences between quasars and AGN.

2. Review how quasars were discovered.

3. Summarize the argument that concludes that large energies are produced in small volumes in quasars.

4. Discuss the sources of light in a quasar

5. Explain the reason for the difference in a radio loud and radio quiet quasar.

6. Explain the nature and consequences of the problem of understanding quasar geometries (viewing angles)

7. Describe how a gravitationally-lensed quasar arises.
 

 KEY WORDS Active Galactic Nuclei

AGN

Quasars

QSOs

broad emission lines

broad line clouds

radio jets

supermassive black hole

thermal radiation

luminous

large redshift

accretion disks

molecular torus

broad emission lines

synchrotron emission

radio-loud quasar

radio-quiet quasar

quasar/AGN geometry

gravitational lens
 

 WRITTEN NOTES Although the Earth's atmosphere limits the resolution of optical telescopes, it is usually possible to just barely see the size of a very large, distant, high redshift galaxy.

The various types of active galaxies (which we will not discuss in detail) often contain very bright nuclei (the "nucleus" just means the center of hte galaxy). These are called Active Galactic Nuclei or AGN. AGN are too small and distant to be resolved. A galaxy surrounding an AGN is often so faint that it is difficult to see in the glare of the point-like, bright AGN.

Quasars (sometimes referred to as QSOs or quasi-stellar objects) are high redshift objects that appear only as unresolved points of light through most earth-based telescopes. Quasars are the most luminous type of AGN.

When quasars were first discovered in the late 1950s and early 1960s, it was not known what they were, since the galaxies they "live" in could not be seen for the bright glare of the quasar itself.  Back then, it was not known if quasars even had a galaxy surrounding them or not.  As telescope technology improved, there have been many examples where a faint galaxy was discovered surrounding a quasar. Quasars were finally known to be a kind of AGN.

 Because quasars are so luminous, they can be seen even at very large redshifts (which also means very far back in time). With telescopes equipped with modern cameras, quasars can be used to probe the Universe back to a time when the Universe was about 10% of its current age (i.e., back to a time when the Universe was only about 1.5 billion years old). The Discovery of Quasars Quasars were discovered in 1963. Cyril Hazard (now a Mellon Professor here at the University of Pittsburgh) was trying to identify the exact location in the sky of a bright radio source. Since single-dish radio telescopes have poor resolution, he recorded the exact time when the radio source was covered by the nearby Moon. By doing this he was able to determine the position of a point-like (star-like) bright object which emitted the strong radio waves. The object is called 3C273. Martin Schmidt (at Caltech) took a spectrum of the quasar 3C273 and discovered its high redshift. Quasar Redshifts In the early days of quasar research, many astronomers were reluctant to believe that quasars were at the distances which their redshifts implied.

The redshift, usually designated by the letter z, is calculated by dividing the shift in wavelength of some spectral emission or absorption line by its true (rest) wavelength. By using the Special Theory of Relativity, the redshift can be used to calculate the speed at which a quasar appears to be moving away from an observer.

The redshifts of some quasars were so high, corresponding to velocities of nearly 90% the speed of light, that they implied that the most distant quasars were as far away as 10 billion light years (the exact number depends on the adopted value of the Hubble constant and the cosmological model of the Universe  the astronomers used).

When these large distances were used with observations which showed that some quasars could double their luminosity within a day (which implies that quasars are relatively small--the bigger an object is, the longer it takes it to change its brightness in this way), it meant that if quasars were indeed at very large distances, they were capable of generating as much energy as could 100 Milky Way Galaxies in a volume not much bigger than the size of the Solar System!

 Initially it was not clear how such tremendous amounts of energy could be generated in such a small volume. Now the types of processes required to generate such energy are better understood, and most astronomers are confident that quasars are at the distances indicated by their redshifts. Processes in Quasars Extremely energetic processes may be seen in quasars at all wavelengths: · strong X-ray, ultraviolet, optical, infrared and radio continuum light may be generated by accretion disks and synchrotron emission (rapidly moving electrons in a magnetic field)

· broad emission lines arise due to gas clouds moving nearly or exceeding one-tenth the speed of light.

· jets show matter being ejected at nearly the speed of light.
 

 The basic model for quasars/AGN is a super massive black hole (one million to one billion solar masses) surrounded by an accretion disk of hot gas. The energy which is generated ultimately comes from the incredibly strong gravitational field of the black hole.  As the gas in the disk whirlpools inward, towards the black hole, it heats up from friction and glows:  thermal radiation--remember the thermal or "black body" spectrum from earlier in the course.  The outer edges of the disk are the coolest, emitting most of their light in the infrared.  As you look closer to the center of the disk, you see the gas get hotter, glowing red, then yellow, the in the ultraviolet, and finally in X-rays (near the center of the disk).  Circling outside the accretion disk is a ring of gas & dust called the molecular torus.  "Molecular" because the gas here is cool enough to form molecules, and "torus" means donut-shaped.  It has such thick dust that we cannot see through it in visible light (although infrared & radio waves go through it).

The gas in the center of the disk is so hot that it becomes ionized (electrically charged) and blows off a hot wind from the disk's surface.  This wind gets funneled, possibly by magnetic fields or by the disk's shape, into two jets moving in opposite directions away from the disk.  One blows off the top of the disk and the other blows off the bottom of the disk.  These two jets carry a magnetic field along with them, making the charged particles spiral around slightly as they shoot outward.  Whenever charged particles do this, they glow.  This is called synchotron emission, and it is mostly radio waves (also a little infrared and visible light, but mostly radio).  So these jets broadcast a radio signal, and we call them radio jets.  The light from synchotron emission mostly shines in the direction the jet is moving, so you will only see the jet if it is moving somewhat towards you.  We do not always see jets in quasars.

There are clouds of gas orbiting the black hole in several different directions.  They get blasted by the high energy X-ray and ultraviolet light from the accretion disk, making them emit light in chemical spectra--specific spectral lines.  Because some clouds will be moving towards us and some will be moving away from us at any given time, these spectral lines are both redshifted and blueshifted, making them blur a bit and broaden.  So we refer to these as the Broad-Line Clouds.  They are not a significant source of light from the quasar.
 

There are therefore two major sources of light from the quasar: (1) thermal radiation from the accretion disk and (2) synchotron radiation from the radio jets.  The broad-line clouds don't add much light.  Since thermal radiation is mostly X-ray, UV, and some visible light, and the synchotron radiation is mostly radio waves, with a little infrared & visible light, then adding these together gives a quasar a fairly flat, even spectrum.  It emits light all the way from the longest wavelengths of the electromagnetic spectrum (radio) to the shortest wavelengths (gamma rays & X-rays--although I haven't mentioned it, quasars also emit gamma rays).

Quasars are divided into two major categories: radio-loud and radio-quiet.  Radio-loud quasars emit strong radio signals, and radio-quiet quasars emit little or no radio waves.  Why is this?  Since the radio waves are emitted mostly by the radio jets, then if the jets are weak or non-existent, there won't be a radio signal.  Also, if the jets do not aim towards us at all, we might not see the light from them.  So it could be a simple matter of our viewing angle, or it could be that some quasars do not have jets.  This problem has not been solved yet.

Viewing angle might also play a role in how we see the light from the accretion disk.  If we look at the quasar so that the disk is edge-on, then the dusty molecular torus may completely block our view of it.  If we instead look at the quasar so the disk is face-on, then we are looking through the hole of the dusty torus, and we see the quasar just fine.  So there may be "hidden quasars" in some galxies where we have not yet seen them.

Galaxy-galaxy collisions is also probably an important process in some part of the quasar/AGN phenomenon.  The quasar's engine needs fuel (gas to fall into the accretion disk), and eventually it could run out.  Gas normally would orbit the quasar's galaxy far away from the nucleus where the quasar is, so something needs to make it fall in to the center.  Collisions with other galaxies might be needed to push more gas down into the quasar and get it running again.
 

 Quasars and Gravitational Lenses Some quasars have been found to be gravitationally lensed, in agreement with predictions of Einstein's General Theory of Relativity.

Gravitational lensing occurs when light from a distant, high redshift quasar is bent around a lower redshift, foreground object like a galaxy. This gives rise to multiple quasar images.

 The gravity of the matter in, for example, the foreground lensing galaxy can be studied by observing such configurations. This includes the non-luminous dark matter. READING ASSIGNMENT

For this lesson, you're responsible only for the material I covered, but use these parts of the book to help explain it:

Ch. 20:  section 20.5.  The paintings on pages 637 & 638 may be useful in showing you the accretion disk, jets, and dusty molecular torus.

HOMEWORK

No homework, just an in-class extra-credit assignment.