Active Galactic Nuclei

1. Active Galactic Nuclei Astronomy 315 Professor Lee Carkner Lecture 21

2. Strange Galaxies • Some galaxies have a compact, powerful source of energy at their core • Among the most energetic objects in the universe • Need multiwavelength observations to understand them

3. M87

4. Jet From M87

5. Seyfert Galaxies • Seyfert galaxies are spirals with optically bright, concentrated nuclei • Can change in brightness very quickly • Small, but very bright

6. Other Seyfert Properties • Some Seyferts have broad emission lines • These same Seyferts also are bright in high energy X-ray and UV radiation • Called Type 1 Seyferts

7. Seyfert Clues • Are there any common properties shared by Seyfert galaxies? • Are Seyferts caused by gravitational interactions?

8. Radio Galaxies • Some galaxies are flanked on either side by a pair of radio lobes • Some central galaxies are also bright radio sources and some are not

9. Cygnus A Radio Galaxy

10. Mapping the Lobes • Radio lobes often show hot spots of enhanced emission • Lobes are material ejected from the star and impacting the intergalactic medium • Radio galaxies produce bipolar jets (like young stars)

14. Source of Radio Waves • Radio emission is due to synchrotron radiation • Lobes must have magnetic field and galaxy must be ejecting electrons • Total energy stored in lobes is huge

15. Radio Galaxy Properties • Central galaxy is often giant elliptical and in a crowded cluster • Often deformed • Jets sometimes are twisted • Some radio galaxies have broad and narrow lines, some just narrow lines

16. BL Lac Objects • BL Lac objects look like stars but show rapid variations • They don’t show the broad or narrow lines we see in Seyfert galaxies • Don’t have radio lobes

17. Quasars • Some sources of radio galaxies look like stars • Quasars have very large red shifts and very large distances • Since quasars are billions of light years away, we are seeing what they looked like billions of years ago

20. Quasar Properties • In most cases you can’t see the host galaxy • Core must be brighter than regular AGN • Quasars are younger than “normal” AGNs • Type 1 quasars have broad emission lines, Type 2 quasars do not

21. AGN Power Source • We have two questions about AGNs • Different types of AGNs are due to viewing matter falling into a black hole from different angles

22. Massive Black Holes • AGN black holes are a million to a billion times the mass of the Sun • Why do we think they have black holes? • Computed densities indicate black hole • AGNs vary so rapidly that the emitting region must be very small (small+massive=BH)

23. Structure of the Core • Black hole pulls matter into an accretion disk • Outer disk is thick can block view of center • The moving material twists up the magnetic field creating a magnetic flux tube that the jets follow out the poles

24. Unified Model • How does this model account for the basic properties of AGNs? • We thus see the jet and disk regions in different ways, producing the observed type of AGN

25. Case 1 -- Face on • Can see the radio jets, but no lobes • Can’t see broad or narrow lines • Type of AGN

26. Case 2 -- Inclined • Can see radio emission and lobes • Can also see broad and narrow lines • Types of AGN:

27. Case 3 -- Edge on • Can see radio emission and lobes • Only see narrow lines • Types of AGN

28. Unified Model for AGNs

29. Quasars and AGNs • Quasars act very much like extra powerful versions of radio galaxies or BL Lac objects • Quasars may be young active AGN, low red shift active galaxies may be AGN that have been refueled

30. AGN and Non-AGN • Why are some galaxies active and others not? • We think all galaxies have black holes • What makes a galaxy active is matter falling into it • Collisions and tidal forces may disrupt the center of galaxies and move material into the black hole

31. Next Time • Read 24 .4-24.5