Lecture 15: Active Galactic Nuclei
•  Discovery
–  Seyferts and Quasars
•  Properties
•  Unification
•  SMBHs
–  MW SMBH
–  The Eddington Limit
–  SMBH correlations
•  AGN activity through the ages
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Discovery: Seyferts
• 
1943 Carl Seyfert publishes list of odd galaxies:
–  Mostly spirals with point-like nuclei
–  Broad emission lines
–  Also high ionisation states (O[VI])
–  Doppler interpretation implied >1000 km/s
• 
Later two classes of Seyferts proposed:
–  Seyfert Is: Broad hydrogen lines & narrow forbidden
lines (e.g., O[III])
–  Seyfert IIs: Only narrow lines
• 
Assumed lines originate from distinct regions:
–  Broad lines from Broad Line Region
–  Narrow lines from Narrow Line Region
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1
NGC5548 (Seyfert I)
NGC3277 (Normal Spiral)
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2
Discovery: Quasars
-  Strong radio sources known to correlate with point-like objects
-  Maarten Schmidt collected the first spectrum for radius source 3C273
- Contained unexplained broad lines, identified as redshifted hydrogen
- Eventually deduced a redshift of 0.16 (Schmidt, Nature, 1963)
- Soon other Quasars were discovered with redshifts up to 2
- Current record holder around z=6.0
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3C273
Optical jet
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3
•  z = 6.4 object found by Fan et al., using Sloan Digital
Sky Survey
–  seen as it was 0.8 Gyr after the Big Bang
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Properties
Point-like
= compact
Distant
= luminous
Broad lines
= high velocities
High excitation lines
= energetic
Variable
= small (<1 lyr)
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images from
www.cv.nrao.edu/~abridle/
at 100 Mpc
at 1 Gpc
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Emit over all wavelengths:, e.g.,Mk421
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Mean Quasar Spectrum (Optical)
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Radio jets: e.g., Cygnus A
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Other AGN types
• Quasars, radio-loud
• QSO (Quasi-stellar objects), radio quiet
• Seyfert I (Broad & narrow lines)
• Seyfert II (Narrow lines)
• Blazers (Highly variable systems), super-luminous
– BL Lacs (BL Lacertae)=no features
– OVV (Optically violently variable)
• LINERS
– Weak Seyferts (no broad lines)
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Theories
•  Two competing theories:
–  Nuclear starbursts
–  Super-massive black holes
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Evidence for black holes
•  gas moving at ~10,000 km/s, inconsistent with stellar orbits
•  emission lines change in brightness over days to weeks
–  light travel time implies emission from region only ~
light-weeks across (~0.01 pc)
•  hence must have large mass, because
–  fast orbits within very small region: vorbit ~ (G M / r)1/2
•  only a black hole can pack this much matter in so densely
–  the power is generated at a few Schwartzschild radii
RS = 2 G Mbh / c2 (≈ 3 km x Mbh / Msolar)
•  But without evidence for such beasts adopting this idea
was difficult
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The Galactic Centre
M BH = 3.6 ×10 6 M 
€
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Why Seyferts
I and II ?
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unified model
•  The several types may be the same kind of object seen
from different angles (Unified Model)
–  see narrower lines if a spectrum comes from gas
orbiting further out
–  fastest stuff is in an accretion disk around the black hole
narrow line region
clouds at ~0.1-1 kpc
broad line region
accretion torus
see Seyfert 2
(not to scale!)
see Seyfert 1
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can also see polarized light from
near the nucleus if scattered
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AGN Unification Antonucci, 1993
Blazar
Quasar
Seyfert 2 galaxy
Torus
Seyfert 1 galaxy
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Jet !!!
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Energetics: The Eddington limit
•  As matter spirals in it will heat
•  The accretion disc will glow and radiate.
•  This radiation pressure opposes the infall.
•  As the Eddington Limit is reached the outward pressure
balances the inward force.
•  This regulates how much matter a SMBH can consume
per year.
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FP
LOOSE (UNBOUND)
PROTON ELECTRON PAIR
σT
€
PLASMA
Fg
r
€
€
€
€
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BH
Lnuc
€
e4
σ T Galaxies
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6πεo2c 4 m e2
Electron cross-section
for Thomson scattering
GM BH m p +e
r2
σ
∂
Fp = T 2 . (mv)
4 πr ∂t
For a photon (pushing
on the electron) :
E
∂ (mv) ∂ (( c 2 )v)
=
∂t
∂t
But for photon :
∂E
v = c,
= L nuc
∂t
∴
σ L
FP = T 2 nuc 22
4 πr c
Fg =
€
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Eddington Limit
• When forces just balance the luminosity has reached the
Eddington limit, I.e.,
Lnuc = LE ,
GMm p +e
GMm p σ T LE
≈
=
,
r2
r2
4 πr 2c
4 πGMm p c
M
M
LE =
= 1.3 ×10 31 BH W ≈ 30000 BH L
σT
M
M
• Therefore if Lnuc=1011L(sol) then MBH>106M(sol)
€
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Energy Efficiency
•  Maximum energy that can be extracted from mass M
falling into a black hole is theoretically 0.42 mc2
–  in practice ~0.1 mc2 comes out as radiation
•  the remaining energy is swallowed by the black hole as
additional mass
•  C.f. stellar nucleosynthesis where 0.007Mc2 is released
•  Black hole is more efficient at releasing energy than fusion !
•  Can calculate that a 1012 Lsolar black hole is radiating
energy ~ 0.1 Msolar c2 per year, rest is swallowed...
–  must increase in mass by ≥ 1 Msolar per year
–  hence can create a 109 Msolar black hole in 109 years
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