1. Active Galactic Nuclei Chapter 26 Revised 2007

2. Active Galactic Nuclei Come in several varieties; Starburst Nuclei – Nearby normal galaxies with unusually high star formation rates in the nucleus. Seyfert Galaxies – identified by Carl Seyfert in 1943. Essentially normal, nearby galaxies, with unusually bright nuclei Quasars – quasi-stellar objects, distant, high luminosity first recognized as galaxies by Martin Scmidt in 1963.

3. NGC 7742 – A Seyfert Galaxy

4. 3C 48 – A Quasar

5. To learn more we have to understand the emission spectrum

6. A typical quasar spectrum

7. But, of course, it’s redshifted…

8. So redshifted that astronomers must use the relativistic form of the Doppler Equation  = √ [(1 + v/c)/ (1- v/c)] - 1 For the quasar 3C 273  = 2 so, v/c = 0.16 or 16% of the speed of light Thus, the quasar must be very far away, using v = H o d d = 900 Mpc

9. Quasar Luminosities are very high. Quasars are observed with redshifts as high as v = 0.8c The fact that they are so far away and so bright must mean that they have very high luminosities ~ 10 11 L o which is 1000 times more luminous than a normal spiral galaxy.

10. The most distant quasars have the largest redshifts and very broad emission lines.

11. We also see evidence for Jets

12. Jets power lobes of radio emission

13. A closer look at the nucleus reveals disks

14. All of which leads to a standard model for AGN’s

15. and the broad emission lines are radiated from the disk which allows an estimate of the black hole mass v M BH r m Balancing forces; mv 2 /r = G M BH m /r 2 which re-arranges to M BH = 2.32 x 10 5 v 2 (km/s) r(kpc) M o

16. Time Variability Some quasars are observed to change in brightness on timescales of days, which can be used to set a limit on the size of the emitting region.

17. Correlating the continuum variations with the emission line variations provides the light travel time  t which yields the size of the emitting region r ~ c t r Variability observed on a timescale of a day leads to the following size; r = 3 x 10 8 m/s 24 hrs/day 60min/hr 60s/min r ~ 10 13 – 10 14 m or 100 – 1000 AU !

18. If the width of the broad lines, typically 10,000 km/s wide, reflects the rotational velocity of an accretion disk around a super massive black hole, then the line width together with a size for the emitting region leads to an estimate for the mass of the black hole using, M BH = 2.32 x 10 5 v 2 (km/s) r(kpc) M o M BH = 2.32 x 10 5 (10,000) 2 (1 x 10 14 m / 3 x 10 19 m/kpc) M BH ~ 10 8 M o The large mass inferred from the broad lines combined with the small size inferred from the rapid time variability combined with the high luminosities inferred from the large redshifts all point to an exotic object at the heart of a quasar - a Massive Black Hole

19. Basic Black Hole Physics Escape Velocity The escape velocity from a Black Hole is calculated the same way as the escape velocity from any massive object m v 2 esc = GM BH (just K.E. = P.E.) 2 r In the case of a Black Hole, the maximum escape speed is c, the speed of light, so c 2 = 2 GM BH /R which can be re-arranged to make R the subject of the formula R = 2GM BH / c 2 also known as the Schwarzschild radius or “the event horizon”

20. The Schwarzschild radius for a typical quasar is R = 2GM BH / c 2 R = 2. (6.67 x 10 -11) 10 8 (1.99 x 10 30 )/ (3 x 10 8 ) 2 R = 2.94 x 10 11 m or ~ 2 AU for a 10 8 M o Black Hole

21. Energy Source It is the release of gravitational potential energy as matter falls into a black hole that drives the high luminosities observed from quasars  P.E. = - G M BH R s L = G M BH dm/dt R s where dm/dt is the mass inflow rate

22. For a Typical Quasar L   L , M BH ~ 10 8 M , R s ~ 2 AU then dm/dt = L R s = 10 12 L  2.94 x 10 11 G M BH 6.67 x10 -11 10 8 M  and L  /M  = 1.99 x 10 30 kg/3.9 x 10 26 W Then dm/dt ~ 2.72 x 10 29 kg/yr Or ~ 0.1 M  /yr which is a trivial amount !

23. Conversion Efficiency The conversion of gravitational PE into light is not expected to be 100% efficient. But, even if the efficiency is ~ 10%, then dm/dt increases to only 1 M  /yr which is still a trivial amount. So, the high luminosity is not due to a high infall rate but rather is due to the enormous depth of the gravitational potential well that the material is falling into. One final word on singularities. Black holes have finite mass but zero volume, so the density is infinite, and the laws of physics as we know them break down. A common misconception among the general public is that black holes have infinite mass – they don’t. The mass is finite.

24. Quasar Evolution Quasars are observed only at high redshifts – at large lookback times, so they existed in the distant past. There are no nearby quasars. The lower luminosity AGN’s, the Seyfert and starburst galaxies, bridge the gap between us and the more distant quasars

25. There is evidence for a super-massive Black Hole at the center of our Galaxy Ghez, A, 1998, ApJ...509..678 M BH = (2.6 +/- 0.2) x 10 6 M 

26. Tidal disruption Stars passing closer than a distance d = r (3M BH /M * ) 1/3 are torn apart by the enormous gradient in the gravitational field strength between the front-side of the star facing the BH and the back-side of the star furthest from the BH.

27. The next nearest supermassive black hole is in M81

28. Which has a curious nuclear spiral…

29. The gas provides a means to determine a mass for the nucleus by measuring the rotational velocity of the gas using the Doppler effect. modelobserved Devereux, N., 2003, AJ 125, 1226 M BH = (7.0 +2 -1 ) x 10 7 M  The principle evidence that the mass concentration is a black hole is that if the mass were stars, the nucleus of the galaxy would be 5.3 magnitudes, or about a factor of 130 times, brighter than is actually observed!

30. Finally, Black Hole the movie ……….