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- Today: More Chapter 14, Active Galaxies
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- Many galaxies have extremely
bright “unresolved” star-like nuclei
- Very large energy generation
- Brightness often varies quickly
- Implies small size (changes not smeared out by light-travel time)
- High velocities often seen (> 10,000 km/s in lines)
- Emission all over the electro-magnetic spectrum
- Jets seen emerging from galaxies
- More common in colliding galaxies
- More common at large distances (redshift): Quasars!
- So more common in distant past
(look-back time)
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- Classical Doppler Effect:
- Also refer to Δλ/λ as the “redshift” or
“z”
- What if z is so large that v => c?
- v
(z + 1)2 - 1
- c (z
+ 1)2 + 1
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- Escape velocity from the surface at radius R is
- At small enough R we have VEscape=
c (speed of light)
- That R is by definition the Schwarzschild radius
- Far from the black hole gravity is the same as for any ordinary mass M
- Stars will just orbit around it like any other mass
- Gas orbiting it collides, tries to slow down, (just like reentering
satellite)
- As gas falls inward it ends up speeding up and heating up
- Gas will be moving at close to speed of light by the time it reaches RS
- Light emitted by hot gas just outside RS can still escape
- Provides a way to release about 10% of E=mc2 of energy
- Fusion releases only about 1%
- Signature of black hole:
Very high energy release, very high velocity
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- Black hole is “active” only if gas is present to spiral into
it
- Isolated stars just orbit black hole same as they would any other mass
- Gas collides, tries to slow due to friction, and so spirals in (and
heats up)
- Conservation of angular momentum causes gas to form a disk as it spirals
in
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- The torus of gas and dust can block part of our view
- Seyfert 2 galaxies: Edge on view
Only gas well above and below disk is visible
See only “slow” gas Þ narrow emission lines
- Seyfert 1 galaxies: Slightly tilted view
Hot high velocity gas close to black hole is visible
High velocities
Þ
broad emission lines
- BL Lac objects: Pole on view
Looking right down the jet at central region
Extremely bright – vary on time scales of hours
- Quasars: Very active AGN at large distances
Can barely make out the galaxy surrounding them
Were apparently more common in distant past
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- The torus of gas and dust can block part of our view
- Seyfert 2 galaxies: Edge on view
Only gas well above and below disk is visible
See only “slow” gas Þ narrow emission lines
- Seyfert 1 galaxies: Slightly tilted view
Hot high velocity gas close to black hole is visible
High velocities
Þ
broad emission lines
- BL Lac objects: Pole on view
Looking right down the jet at central region
Extremely bright – vary on time scales of hours
- Quasars: Very active AGN at large distances
Can barely make out the galaxy surrounding them
Were apparently more common in distant past
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- The torus of gas and dust can block part of our view
- Seyfert 2 galaxies: Edge on view
Only gas well above and below disk is visible
See only “slow” gas Þ narrow emission lines
- Seyfert 1 galaxies: Slightly tilted view
Hot high velocity gas close to black hole is visible
High velocities
Þ
broad emission lines
- BL Lac objects: Pole on view
Looking right down the jet at central region
Extremely bright – vary on time scales of hours
- Quasars: Very active AGN at large distances
Can barely make out the galaxy surrounding them
Were more common in distant past
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- Need a supply of gas to feed to the black hole
- (Black holes from 1 million to >1 billion solar masses!
- Scales as a few percent of galaxy bulge mass.)
- Collisions disturb regular orbits of stars and gas clouds
- Could feed more gas to the central region
- Galactic orbits were less organized as galaxies were forming, also
recall the “hierarchical” galaxy formation
- Expect more gas to flow to central region when galaxies are young =>
Quasars (“quasar epoch” around z=2 to z=3)
- Most galaxies may have massive black holes in them
- They are just less active now because gas supply is less
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- A nice website: http://www.mssl.ucl.ac.uk/www_astro/agn/agn_beginners.html
- Other links on the website will take you to movies showing quasar
structure, and discussing unified models.
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- Discovery of Active Galactic Nuclei (AGN)
- Seyfert Galaxies and Radio Sources
- The Unified Model
- Black Holes in Galaxies, disks, orientation, +
- Quasars
- Distances and Relativistic Redshifts
- Quasars as extreme AGN
- Evolution of Quasars/Galaxies
- Gravitational Lensing
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Notes
