1. Galaxies with Active Nuclei Chapter 17

2. 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

3. 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)

4. I. Active Galaxic Nuclei A. Seyfert Galaxies B. Double-Lobed Radio Sources C. Exploring Supermassive Black Holes D. The Search for a Unified Model E. The Origin of Supermassive Black Holes II. Quasars A. The Discovery of Quasars B. The Distance to Quasars C. Evidence of Quasars in Distant Galaxies D. Superluminal Expansion E. A Model Quasar F. Quasars Through Time Outline

5. 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!

6. 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.

7. Seyfert Galaxies Unusual 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 (~10 7 – 10 8 M sun )

8. 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

9. 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

10. Radio Galaxies Cygnus 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

11. 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

12. Radio Galaxies (3) 3C31: Member of a chain of galaxies. Twisted jets, probably because two galactic nuclei are orbiting each other.

13. 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

14. The Jets of M 87 M 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 long

15. Evidence for Black Holes in AGNs NGC 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.

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

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

18. 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

19. 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

20. 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.

21. 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.

22. 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.

23. 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.

24. The Spectra of Quasars The Quasar 3C 273 Spectral lines show a large red shift of z =   = 0.158

25. 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

26. 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!

27. 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!

28. 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

29. 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.

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

31. 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 (velocity is close to c) Relativity should be consider in the calculation