1. AY202a Galaxies & Dynamics Lecture 14: AGN: The Unified Model

3. Response of astrophysicists to a new idea (McCray 1979)

5. The Unified Model
(Due primarily to Elvis & Lawrence 1982 with help from Blandford 1980)
All AGN are manifestations of the same basic model:
Black Hole + Accretion Desk (Disk) + Surrounding Stuff
(obscuring torus, broad line region, narrow line region)
Thermal Emission from the Disk, Synchrotron & Line Emission from Ionized Gas, Pair Production from the Black Hole
Luminosity depends on accretion rate M
Classification depends on Aspect
Radio emission depends on presence of jets

8. Edge On = Sy 2
Face On = Sy 1
Down the Pipe = Blazar
(jet, relativistic beaming)

12. Accretion Disk To first order, assume it radiates as a black body
where T(r) is the disk temperature at radius r
2h
c eh/kT(r) -1

13. 2r2 Consider a mass, m, falling from r +∆r to r: ∆E = - ≈
and we assume BH mass dominates. If half the energy is converted into heat and we assume the energy is emitted locally
ΔL = GM• m ∆r , m = accretion rate
GM• m GM• m GM• m ∆r
r r+∆r r r • •
2r2

14. T(r ) = (3GM• m / 8 rS3)1/4 (r/rS)-3/4
In steady state, the accretion rate is independent of radius so the same amount of matter flows through any cylindrical radius.
If the disk is optically thick, it emits as a BB
∆L = 2 x 2r r  T(r )4
the disk has two sides
Which gives
T(r ) = (GM• m / 8 r3)1/4
A more accurate derivation, and scaling by rS
T(r ) = (3GM• m / 8 rS3)1/4 (r/rS)-3/4 • •

15. Scales

16. Multi-component Models
Malkan 1983

17. Blandford’s Characterization
AGN described by
BH Mass M•
Accretion rate dM/dt = M
Viewing angle 
H = 2 M• H dimensionless
angular momentum
of the spinning BH .

18. Eddington ratio = 1
Begelman 1985

19. Blandford 1985

20. Blandford’s classification scheme

21. Radiation vs Gravity
Energy produced by accreting BH produces radiation pressure on the surrounding medium. If we assume that the medium is ionized, then the opacity is characterized by Thomson Scattering. The force on an e- is
Frad = T
where T = (8/3) ( e2/mec2 )2 = 6.653x10-25 cm2
is the Thomson cross section (low energy interactions of photons and electrons) e- dominate L
4 r2 c

22. The gravitational force is primarily on the protons
Fgrav = GM• (mp +me)/r2
(electron mass is irrelevant, e and p are coupled in the plasma)
To remain gravitationally bound
Frad < Fgrav, so
T L GM• mp
4r2c r2
<

23. So we can rewrite this as L < Ledd = M•
≈ x10 38 (M•/M) ergs/s
≈ 30,000 (M•/M) L
which is the definition of the Eddington Luminosity for a Black Hole of Mass M•
4 G c mp T

24. For accretion to occur M• must be larger than Medd M• > Medd = L
≈ 8 x 107 ( ) M
which gives a lower limit on the mass of the Black Hole. T
4  G mp L
10 46 erg/s

25. QSO Evolution Density vs Luminosity Data show increased density with z
(2dF)

26. QSO
Density
Evolution
QSO Epoch
at z~2.5

27. Ly α Forest
Clouds along the l.o.s.

28. Damped Lyα Systems
If N(HI) > 2x1020 cm-2, the Lyα is optically thick even in the wings. This is the typical density in the outer disks of spiral galaxies, ~ 1.5 M pc-2
The # densities of
these clouds at high
z tell us about the
#’s and sizes of
protogalactic disks.

29. Gunn-Peterson Trough HI absorption in the IGM seems to appear
around z~6
(SDSS)

30. References
B. Peterson, An Introduction to Active Galactic Nuclei (Cambridge 1997)
J. Krolik, Active Galactic Nuclei (Princeton 1999)

31. Next Week's Paper
Abundances, Masses, and Weak-lensing Mass Profiles of Galaxy Clusters as a Function of Richness and Luminosity in CDM Cosmologies
Stefan Hilbert & Simon White
ArXiv:0907:4371 July 24, 2009