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Galaxies With Active NucleiChapter 17

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.

Guidepost

I. Active GalaxiesA. Seyfert GalaxiesB. Double-Lobed Radio SourcesC. Testing The Black Hole HypothesisD. The Search for a Unified ModelE. Black Holes and Galaxy Formation

II. QuasarsA. The Discovery of QuasarsB. Quasar DistancesC. Evidence of Quasars in Distant GalaxiesD. Superluminal ExpansionE. A Model QuasarF. Quasars Through Time

Outline

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 GalaxiesTaking 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

NGC 1566

Circinus Galaxy

Unusual spiral galaxies:

• Very bright cores

• Emission line spectra.

• Variability: ~ 50 % in a few months

Most likely power source:

Accretion onto a supermassive black

hole (~107 – 108 Msun)

Interacting GalaxiesSeyfert 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

Seyfert galaxy NGC 4151

Cosmic Jets and Radio LobesMany active galaxies show powerful radio jets

Radio image of Cygnus A

Material in the jets moves with almost the speed of light (“Relativistic jets”).

Hot spots: Energy in the jets is released in

interaction with

surrounding material

Radio Galaxies

Radio image superposed on optical image

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 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

Twisted magnetic fields help to confine the material in the jet and to produce synchrotron radiation.

Accretion Disk

The Jets of M 87M 87 = Central, giant elliptical galaxy in the Virgo cluster of galaxies

Optical and radio observations detect a jet with velocities up to ~ 1/2 c.

Jet:

~ 2.5

kpc l

ong

M31 at Many Wavelengths

(SLIDESHOW MODE ONLY)

Evidence for Black Holes in AGNsElliptical 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

Accretion disk

Dense dust torus

Gas clouds

UV, X-rays

Emission lines

Supermassive black hole

Seyfert I:Seyfert I:

Strong, broad Strong, broad emission lines from emission lines from rapidly moving gas rapidly moving gas clouds near the BHclouds near the BH

Seyfert II:Seyfert II:

Weaker, narrow Weaker, narrow emission lines from emission lines from more slowly moving more slowly moving gas clouds far from gas clouds far from

the BHthe BH

Other Types of AGN and AGN Unification

Radio Galaxy:

Powerful “radio lobes” at the end points of the jets, where power in the jets is dissipated.

Cyg A (radio emission)

Observing direction

Other Types of AGN and AGN Unification (2)

Emission from the jet pointing towards us is enhanced (“Doppler boosting”) compared to the jet moving in the other direction (“counter jet”).

Quasar or BL Lac object (properties very similar to quasars, but no emission lines)

Observing direction

The Dust Torus in NGC 4261

Dust Torus is directly visible with Hubble Space Telescope

Black Holes in Normal Galaxies

The Andromeda galaxy M 31:

No efficient accretion onto the central black hole

X-ray sources are mostly accreting stellar-mass black holes.

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 very strong, broad emission lines in their spectra.

Also show strong variability over time scales of a few months.

The Spectra of Quasars

The Quasar 3C 273

Spectral lines show a large red shift of

z = = 0.158

Quasar Red Shifts

z = 0

z = 0.178

z = 0.240

z = 0.302

z = 0.389

Quasars have been detected at the highest

red shifts, up to

z ~ 6

z = /

Our old formula

/= vr/c

is only valid in the limit of low speed,

vr << c

Quasar Red Shifts (2)

The full, relativistic expression always gives speeds less than c, but extremely large distance:

Several Gpc.

Studying QuasarsThe 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 quasar behind a galaxy cluster is bent by the mass in the cluster.

Use to probe the distribution of matter in the cluster.

Light from a distant quasar is bent around a foreground galaxy

→ two images of the same quasar!

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 QuasarsHost 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 MotionIndividual 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

radio galaxyactive galaxyactive galactic nucleus (AGN)

Seyfert galaxydouble-lobed radio source

double-exhaust modelhot spotunified modelBL Lac objectblazarquasarrelativistic Doppler formula

gravitational lenssuperluminal expansionrelativistic jet model

New Terms

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.)

Discussion Questions

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. d2. b3. e4. a5. d6. e7. c8. e9. d10. d

11. e12. e13. a14. b15. e16. d17. c18. c19. b20. d