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Galaxies with Active Nuclei

Active Galaxies

Galaxies with extremely violent energy released 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!

Line 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

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)

NGC 7742

Interacting Galaxies

Seyfert galaxy NGC 7674

Active galaxies are often associated with interacting

galaxies, possibly result of recent galaxy mergers.

Seyfert galaxy 3C219

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

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 (II)

Evidence for the galaxy moving

through intergalactic

material

Radio image of 3C75

3C75: Evidence for two nuclei recent galaxy

merger

Visual + radio image of 3C31

Radio image of NGC 1265

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 M87

M87 = 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

The Dust Torus in NGC4261

Dust torus is directly visible with Hubble Space Telescope

Model for Seyfert Galaxies

Supermassive black holeAccretion disk

dense dust torus

Gas clouds

Seyfert I:Seyfert I:

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

holehole

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 black holethe black hole

UV, X-rays

Emission lines

Radio Galaxy:

Powerful “radio lobes” at the end points of the jets, where

power in the jets is dissipated.

Cyg A (radio emission)

Other Types of AGN and AGN Unification

Observing direction

Emission from the jet pointing towards us is enhanced

(“Doppler boosting”) compared to the jet moving in the other

direction (“counter jet”).

Other Types of AGN and AGN Unification

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

emission lines)

Observing direction

The Origin of Supermassive Black Holes

Most galaxies seem to harbor supermassive black holes in their

centers.

Fed and fueled by stars and gas from the near-central

environment

Galaxy interactions may enhance the flow of matter onto

central black holes

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 3C273

Spectral lines show a large redshift 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

redshifts, up to

z ~ 6

z = /

Our old formula

/= vr/c

is only valid in the limit of low speed,

vr << c

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

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!

Gravitational Lensing of Quasars

Gallery of Quasar Host Galaxies

Elliptical galaxies; often merging / interacting galaxies

1. What evidence suggests that the energy source in a double-lobed radio galaxy lies at the center of the galaxy?

Firstly, the geometry suggests that the lobes are inflated by gas jets emerging from the central galaxy.

This is supported by the presence of synchrotron radiation which suggests magnetic fields that confine the emitted jets to narrow beams, and hot spots which suggest gas is being pushed into the surrounding gas causing the heating. Also, we know that matter falling onto a massive compact object (e.g., a black hole) can cause these jets.

2. How does the peculiar rotation of NGC5128 help explain the origin of this active galaxy?

There is a spherical cloud of stars orbiting about an axis which is perpendicular to the axis of rotation of the disk. This strongly hints that this is the result of a merger.

3. What statistical evidence suggests that Seyfert galaxies have suffered recent interactions with other galaxies?

They are three times more common in interacting pairs of galaxies than in isolated galaxies.

25% have shapes that suggest tidal interactions with other galaxies.

4. How does the unified model explain the two kinds of Seyfert galaxies?

It all depends on how the accretion disk is tipped WRT your line of sight. Tipped slightly you are able to observe the hot, fast moving gas in the central galaxy, thus the x-rays and higher Doppler shifts produce the smeared, broad spectral lines. Not tipped at all the disk blocks the radiation from the central galaxy. Plus, this gas is moving slower and explains the narrow spectral lines.

5. What observations are necessary to identify the presence of a supermassive black hole at the center of a galaxy?

Observations of size and motion… the short time period it takes to fluctuate in brightness small

Motion of stars near the center allow for use of Kepler #3 and hence the mass. Thus Doppler shifts combined with other observations that allow for the distance to be calculated (e.g., Cepheids). Basically everything to allow for us to use Kepler #3.

6. How does the unified model implicate collisions and mergers in triggering active galaxies?

Tidal interactions with other galaxies not only can rip matter from a galaxy, but also can throw matter inward, toward the center of the galaxy. In this case, you would have a flood of matter falling into the black hole increasing the intensity of the bipolar flow.

7. Why were quasars first noticed as being peculiar?

How could quasars be so luminous that they emit 10 to 1000 times the energy of a galaxy, yet reside in a region only the size of our solar system?

8. How do the large redshifts of quasars lead astronomers to conclude they must be very distant?

, if z is large, then d must also be large.0

czz d

c H

9. What evidence suggests that quasars are ultraluminous but must be very small?

They are very distant, yet easily photographed very luminous. The small time to fluctuate in brightness (a few days) they must be smaller than a few light days in diameter.

10. How do gravitational lenses provide evidence that quasars are distant?

The spectra of quasars are similar, yet each are as unique as fingerprints. In 1979, the object 0957+561 was observed. It consists of two quasars separated by 6” of arc. These two objects share the exact same spectra, which implies that they are the same object! Yet the closer lensing galaxy is so far away that it is difficult to detect.

11. What evidence is there that quasars occur in distant galaxies?

Astronomers recorded the spectra of objects near quasars. Those objects share the same spectra of normal galaxies, and they have the same redshift as the quasar.

12. How can our model quasar explain the different radiation received from quasars?

The two kinds of radiation are a continuous spectrum and some emission lines. The continuous spectra fluctuates rapidly, which suggests that the object emitting it is small – probably a central black hole with an accretion disk. The emission lines don’t fluctuate rapidly, suggesting that they emanate form a larger region many light-years in diameter – probably clouds of gas excited from the synchrotron radiation from the central black hole. Also, as with AGNs, how the disk is tipped will affect what kind of quasar you’ll see.

13. What evidence is there that quasars must be triggered by collisions and mergers?

Galaxies were closer when the universe was young. They would have collided more often, and we know that interactions can cause matter to flow inward. Also, quasars are often located in distorted galaxies which suggests they interacted with other galaxies.

14. Why are there few quasars at low redshifts and at high redshifts but many at redshifts of about 2?

A redshift of 2 corresponds to a time in the universe when galaxies were most actively forming, colliding, and merging.