1. Galaxies With Active Nuclei

2. “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!

3. 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.

4. 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

5. 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

6. 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”).

7. 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

8. 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

9. 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.

10. 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.

11. 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.

12. 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

13. 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.

14. Other Types of AGN and AGN Unification (2)
Quasar
Emission from the jet pointing towards us is enhanced (“Doppler boosting”) compared to the jet moving in the other direction (“counter jet”).
Observing direction

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

16. 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

17. 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.

18. 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.

19. 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

20. 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

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

22. 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!

23. 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.

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

25. Galaxies Associated with Quasars
Two images of the same quasar,
New source probably a supernova in the host galaxy of the quasar

26. 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.

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

28. 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