Galaxies with Active Nuclei

Chapter 17

You can imagine galaxies rotating slowly and quietly making new stars as the eons pass, but the nuclei of some galaxies are sites of powerful eruptions that eject high-speed jets in opposite directions. As you study these active galaxies, you will be combining many of the ideas you have discovered so far to answer five essential questions:

• What evidence shows that some galactic nuclei are active?

• What is the energy source of this activity?

• What triggers the nucleus of a galaxy into activity?

• What are the most distant active galaxies?

• What can active galaxies reveal about the history of the universe?

Guidepost

There are billions of the galaxies in the sky, and astronomers can’t study every one. Rather they must use statistical evidence, and that raises a common question about the scientific method:

• If statistics isn’t certainty, how can scientists use it to understand nature?

The active galaxies are the last pieces of evidence you need before you try to understand the birth and evolution of the entire universe and the galaxies that fill it. You will start that journey in the next chapter.

Guidepost (continued)

I. Active Galaxic NucleiA. Seyfert GalaxiesB. Double-Lobed Radio SourcesC. Exploring Supermassive Black HolesD. The Search for a Unified ModelE. The Origin of Supermassive Black Holes

II. QuasarsA. The Discovery of QuasarsB. The Distance to QuasarsC. 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 GalaxiesUnusual spiral galaxies:

• Very bright cores

• Emission line spectra from core region.

• Variability: ~ 50 % in a few months

Most likely power source:

Accretion onto a supermassive black

hole (~107 – 108 Msun)

Interacting Galaxies

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 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 GalaxiesCygnus A: A giant pair of radio jets.

Jet visible in radio and X-rays; show bright spots in similar locations.

Centaurus A

(= “Cen A” = NGC 5128):

Infrared image reveals warm gas near the

nucleus.

Radio Image

Radio Galaxies (2)

NGC 1265: Evidence for the galaxy moving through

intergalactic material

Radio image of 3C 75

3C 75: Evidence for two nuclei recent galaxy merger

Radio Galaxies (3)

3C31: Member of a chain of galaxies.

Twisted jets, probably because two galactic nuclei are orbiting each other.

Formation of Radio JetsJets 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

Evidence for Black Holes in AGNsNGC 4261: Radio image reveals double-lobed jet structure; close-up view by Hubble Space Telescope reveals a bright

central source embedded in a dust torus.

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

Seyfert II:Seyfert II:

Weaker, Weaker, narrow narrow

emission emission lines from lines from

more slowly more slowly moving gas moving gas

clouds far clouds far from the BHfrom the BH

The Dust Torus in NGC 4261

Dust Torus is directly visible with Hubble Space Telescope

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

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.

Bursts of Activity of Supermassive Black Holes

A star wandering too close to a supermassive black

hole can be disrupted and trigger an X-ray outburst.

Active Galaxies in Galaxy Clusters

The powerful radio lobes of radio galaxies can push away intergalactic gas in galaxy clusters.

Even hundreds of millions of years after the Galaxy’s activity has calmed down, there are still “ghost

cavities” in the X-ray emission from intergalactic gas.

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, beyond

z ~ 6

z = /

This indicates distances of several Gigaparsec

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

Host Galaxies of Quasars

The radio image of the quasar 3C175 shows a double-lobe jet structure, indicating its association

with an active galactic nucleus.

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

(velocity is close to c)

Relativity should be consider in the calculation