Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).

Galaxies With Active Nuclei Chapter 17 Galaxies With Active Nuclei

Guidepost This chapter is important for two reasons. First, it draws together ideas from many previous chapters to show how nature uses the same basic rules on widely different scales. Matter flowing into a protostar, into a white dwarf, into a neutron star, or into the heart of a galaxy must obey the same laws of physics, so we see the same geometry and the same phenomena. The only difference is the level of violence. Second, this chapter is important because the most distant objects we can see in the universe are the most luminous galaxies, and many of those are erupting in outbursts and are thus peculiar. By studying these galaxies, our attention is drawn out in space to the edge of the visible universe and back in time to the earliest stages of galaxy formation. In other words, we are led to think of the origin and evolution of the universe, the subject of the next chapter.

Outline I. Active Galaxies A. Seyfert Galaxies B. Double-Lobed Radio Sources C. Testing The Black Hole Hypothesis D. The Search for a Unified Model E. Black Holes and Galaxy Formation II. Quasars A. The Discovery of Quasars B. Quasar Distances C. Evidence of Quasars in Distant Galaxies D. Superluminal Expansion E. A Model Quasar F. Quasars Through Time

“Active Galactic Nuclei” (= AGN) Active Galaxies Galaxies with extremely violent energy release in their nuclei (pl. of nucleus). “Active Galactic Nuclei” (= AGN) Up to many thousand times more luminous than the entire Milky Way; energy released within a region approx. the size of our solar system!

The Spectra of Galaxies Taking a spectrum of the light from a normal galaxy: The light from the galaxy should be mostly star light, and should thus contain many absorption lines from the individual stellar spectra.

Seyfert Galaxies Unusual spiral galaxies: Very bright cores Emission line spectra. Variability: ~ 50 % in a few months NGC 1566 Most likely power source: Accretion onto a supermassive black hole (~107 – 108 Msun) Circinus Galaxy

Interacting Galaxies Seyfert galaxy NGC 4151 Seyfert galaxy NGC 7674 Active galaxies are often associated with interacting galaxies, possibly result of recent galaxy mergers. Often: gas outflowing at high velocities, in opposite directions

Cosmic Jets and Radio Lobes Many active galaxies show powerful radio jets Radio image of Cygnus A Hot spots: Energy in the jets is released in interaction with surrounding material Material in the jets moves with almost the speed of light (“Relativistic jets”).

Radio image superposed on optical image Radio Galaxies Centaurus A (“Cen A” = NGC 5128): the closest AGN to us. Jet visible in radio and X-rays; show bright spots in similar locations. Infrared image reveals warm gas near the nucleus. Radio image superposed on optical image

Radio Galaxies (2) Radio image 3C129: Evidence for the galaxy moving through intergalactic material Radio image of 3C 75 3C 75: Evidence for two nuclei  recent galaxy merger

Jet Deflection (SLIDESHOW MODE ONLY)

Formation of Radio Jets Jets are powered by accretion of matter onto a supermassive black hole Black Hole Accretion Disk Twisted magnetic fields help to confine the material in the jet and to produce synchrotron radiation.

The Jets of M 87 M 87 = Central, giant elliptical galaxy in the Virgo cluster of galaxies Jet: ~ 2.5 kpc long Optical and radio observations detect a jet with velocities up to ~ 1/2 c.

M31 at Many Wavelengths (SLIDESHOW MODE ONLY)

Evidence for Black Holes in AGNs Elliptical galaxy M 84: Spectral line shift indicates high-velocity rotation of gas near the center. Visual image NGC 7052: Stellar velocities indicate the presence of a central black hole.

Model for Seyfert Galaxies Seyfert I: Strong, broad emission lines from rapidly moving gas clouds near the BH Gas clouds Emission lines UV, X-rays Seyfert II: Weaker, narrow emission lines from more slowly moving gas clouds far from the BH Supermassive black hole Accretion disk Dense dust torus

Other Types of AGN and AGN Unification Observing direction Cyg A (radio emission) Radio Galaxy: Powerful “radio lobes” at the end points of the jets, where power in the jets is dissipated.

Other Types of AGN and AGN Unification (2) Quasar or BL Lac object (properties very similar to quasars, but no emission lines) Emission from the jet pointing towards us is enhanced (“Doppler boosting”) compared to the jet moving in the other direction (“counter jet”). Observing direction

The Dust Torus in NGC 4261 Dust Torus is directly visible with Hubble Space Telescope

Black Holes in Normal Galaxies X-ray sources are mostly accreting stellar-mass black holes. The Andromeda galaxy M 31: No efficient accretion onto the central black hole

Black Holes and Galaxy Formation Interactions of galaxies not only produce tidal tails etc., but also drive matter towards the center  triggering AGN activity. Such interactions may also play a role in the formation of spiral structures.

Quasars Active nuclei in elliptical galaxies with even more powerful central sources than Seyfert galaxies Also show strong variability over time scales of a few months. Also show very strong, broad emission lines in their spectra.

Spectral lines show a large red shift of The Spectra of Quasars Spectral lines show a large red shift of z = Dl / l0 = 0.158 The Quasar 3C 273

Quasar Red Shifts z = Dl/l0 Our old formula Dl/l0 = vr/c Quasars have been detected at the highest red shifts, up to z ~ 6 z = 0 z = 0.178 z = 0.240 z = Dl/l0 z = 0.302 Our old formula Dl/l0 = vr/c is only valid in the limit of low speed, vr << c z = 0.389

Quasar Red Shifts (2) Several Gpc. The full, relativistic expression always gives speeds less than c, but extremely large distance: Several Gpc.

Studying Quasars The study of high-redshift quasars allows astronomers to investigate questions of: 1) Large scale structure of the universe 2) Early history of the universe 3) Galaxy evolution 4) Dark matter Observing quasars at high redshifts: distances of several Gpc Look-back times of many billions of years The universe was only a few billion years old!

Probing Dark Matter with High-z Quasars: Gravitational Lensing Light from a distant quasar is bent around a foreground galaxy → two images of the same quasar! Light from a quasar behind a galaxy cluster is bent by the mass in the cluster. Use to probe the distribution of matter in the cluster.

Evidence for Quasars in Distant Galaxies Quasar 0351+026 at the same red shift as a galaxy  evidence for quasar activity due to galaxy interaction

Galaxies Associated with Quasars Two images of the same quasar, 1059+730 New source probably a supernova in the host galaxy of the quasar

Host Galaxies of Quasars Host galaxies of most quasars can not be seen directly because they are outshined by the bright emission from the AGN. Blocking out the light from the center of the quasar 3C 273, HST can detect the star light from its host galaxy.

Gallery of Quasar Host Galaxies Elliptical galaxies; often merging / interacting galaxies

Superluminal Motion Individual radio knots in quasar jets: Sometimes apparently moving faster than speed of light! Light-travel time effect: Material in the jet is almost catching up with the light it emits

New Terms radio galaxy active galaxy active galactic nucleus (AGN) Seyfert galaxy double-lobed radio source double-exhaust model hot spot unified model BL Lac object blazar quasar relativistic Doppler formula gravitational lens superluminal expansion relativistic jet model

Discussion Questions 1. Do you think that our galaxy has ever been an active galaxy? Could it have hosted a quasar when it was young? 2. If a quasar is triggered in a galaxy’s core, what would it look like to people living in the outer disk of the galaxy? Could life continue in that galaxy? (Begin by deciding how bright a quasar would look seen from the outer disk, considering both distance and dust.)

Quiz Questions 1. Which characterizes the visible part of the spectrum for most galaxies? a. They have absorption lines of singly ionized calcium (Ca II). b. They have absorption lines of neutral atomic hydrogen (H I). c. They have emission lines of carbon monoxide (CO) molecules. d. Both a and b above. e. All of the above.

Quiz Questions 2. How are the spectra of Seyfert galaxies different from most galaxies? a. They have broad absorption lines of highly ionized elements. b. They have broad emission lines of highly ionized elements. c. They have narrow absorption lines of highly ionized elements. d. They have narrow emission lines of highly ionized elements. e. They have a continuous spectrum.

Quiz Questions 3. What conditions can create broad emission lines of highly ionized elements? a. High-temperature gas must be present. b. Low-density gas must be present. c. The gas must be rotating at high speeds. d. Both a and b above. e. All of the above

Quiz Questions 4. Seyfert galaxies have a spectrum with broad emission lines of ionized elements. What other unusual features do Seyfert galaxies have? a. They have small, highly luminous nuclei that fluctuate rapidly. b. They have small, dark nuclei that fluctuate rapidly. c. They have large, highly luminous nuclei that fluctuate rapidly. d. They have large, dark nuclei that fluctuate rapidly. e. They all have highly red shifted spectral lines.

Quiz Questions 5. What is the difference between type 1 and type 2 Seyfert galaxies? a. Type 1 Seyferts are very luminous at ultraviolet wavelengths. b. Type 1 Seyferts are very luminous at X-ray wavelengths. c. Type 2 Seyferts have broader emission lines. d. Both a and b above. e. All of the above.

Quiz Questions 6. Galaxies in close pairs are three times more likely to be Seyfert galaxies than are isolated galaxies. What general conclusion can be drawn from this statistical fact? a. Most Seyfert galaxies found in galaxy pairs are type 2. b. Seyfert galaxies in pairs are smaller than isolated Seyfert galaxies. c. The majority of Seyfert galaxies in pairs are spiral galaxies. d. Isolated Seyfert galaxies are most likely to be type 1. e. Seyfert galaxies are very likely the result of galaxy interactions.

Quiz Questions 7. What is at the center of Seyfert galaxies? a. Globular star clusters. b. Clusters of about one million neutron stars. c. Supermassive black holes. d. Dwarf elliptical galaxies. e. None of the above.

Quiz Questions 8. In the double-exhaust model, how does a double-lobed radio source form? a. The high-energy source at the center of the central galaxy is transforming energy into matter and antimatter that flow out in opposite directions to form the lobes. b. The magnetic field of the central galaxy pulls the hot ionized intragalactic matter toward the two galactic poles and forms two feeding lobes. c. The central galaxy experiences gravitational harassment due to a near collision with another galaxy. d. Tidal interaction with a nearby galaxy drags matter out into the two opposing radio lobes. e. The lobes are inflated by bipolar jets of excited gas emerging from the central galaxy.

Quiz Questions 9. What observational evidence leads us to believe that AGNs contain supermassive black holes? a. Broad emission lines of ionized gases indicate that gas near the center of an AGN is orbiting at high speeds. b. Short-duration fluctuations in brightness limit the size of the object at the center of an AGN. c. High-resolution imaging reveals dark regions the size of an event horizon at the center of some AGNs. d. Both a and b above. e. All of the above.

Quiz Questions 10. How do blazars (BL Lac objects) differ from type 1 Seyfert galaxies? a. Blazars are much more luminous than type 1 Seyfert galaxies. b. The luminosity of blazars fluctuates more rapidly than type 1 Seyfert galaxies. c. The luminosity of blazars fluctuates more slowly than type 1 Seyfert galaxies. d. Both a and b above. e. Both a and c above.

Quiz Questions 11. Blazars are more luminous and fluctuate much more rapidly than both type 1 and type 2 Seyfert galaxies. How does the unified model of an AGN’s supermassive black holes and accretion disk explain these differences? a. Blazars and Seyfert galaxies are different views of the accretion disk of an AGN. b. Blazars are the face-on view of the accretion disks of AGNs. c. Seyfert type 1 galaxies are AGNs with the accretion disk tipped slightly from face-on. d. Seyfert type 2 galaxies are AGN accretion disks with an edge-on view. e. All of the above.

Quiz Questions 12. Why are galaxies with active nuclei more often found in close galaxy pairs and in rich clusters of galaxies? a. Galactic harassment is more likely under these circumstances. b. Galactic merger is more likely under these circumstances. c. Galactic cannibalism is more likely under these circumstances. d. Galactic interactions transfer material onto the central supermassive black holes. e. All of the above.

Quiz Questions 13. Which of the following are characteristics of quasars? a. Quasars have bright emission line spectra. b. The spectra of quasars have large blue shifts. c. Quasars are larger than the largest spiral galaxies. d. Quasars are more abundant now than at anytime in the history of the universe. e. All of the above.

Quiz Questions 14. What evidence do we have that quasars are small? a. Some quasars emit strongly at radio wavelengths. b. They have rapid fluctuations in brightness. c. They have large red shifts in their spectra. d. They are very distant. e. All of the above.

Quiz Questions 15. What evidence do we have that quasars are very far away? a. Their spectral lines have large red shifts. b. Some are gravitationally lensed by distant galaxies. c. Some light from quasars contains less red shifted absorption lines of distant galaxies. d. Some quasars have nearby galaxies with similar red shifts in their spectra. e. All of the above.

Quiz Questions 16. How do quasars resemble the AGN in Seyfert galaxies? a. They have jets and pairs of opposing radio lobes. b. They are small and very luminous. c. They have new chemical elements never found on Earth. d. Both a and b above. e. All of the above.

Quiz Questions 17. How can a quasar jet eject material at apparent superluminal speed? a. The accretion disk surrounding the supermassive black hole at its center can add more than one solar mass per year. b. The intense magnetic field of a quasar can accelerate ions to a speed greater than the speed of light. c. The jet ejects material at nearly the speed of light almost directly toward Earth. d. Both a and b above. e. All of the above.

Quiz Questions 18. What does it mean that quasars are most common at a red shift of about 2? a. The average quasar has a recessional speed that is twice the speed of light. b. Most quasars cannot be imaged at visible wavelengths. c. Quasars were more plentiful in the past. d. Both a and b above. e. All of the above.

Quiz Questions 19. Where are all the dead quasars? a. They have dissipated into the ether. b. They lurk quietly at the hearts of galaxies. c. They have decayed to become gas and dust. d. Fortunately, they are many 1000s of Mpc from Earth. e. The residents of cosmic wormholes have consumed them.

Quiz Questions 20. Why are most quasars so far away? a. There is a greater volume of the universe that is far away than is nearby. b. Quasars were more abundant in the past, when galaxies were close together. c. The jets have moved most of them to great distances from us. d. Both a and b above. e. All of the above

Answers 1. d 2. b 3. e 4. a 5. d 6. e 7. c 8. e 9. d 10. d 11. e 12. e 13. a 14. b 15. e 16. d 17. c 18. c 19. b 20. d