Astr 1050     Fri., Apr 2, 2004
   Today:  Extra Credit Articles
                 Start Chapter 14, Active Galaxies

Chapter 14: Galaxies with Active Nuclei
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

The (slightly) active nucleus of our galaxy
Probable Black hole
High velocities
Large energy generation
At  a=275 AU  P=2.8 yr Þ 2.7 million solar masses
Radio image of Sgr A*
about 3 pc across, with model of surrounding disk

Active Galactic Nuclei:  AGNs
A small fraction of galaxies have extremely bright “unresolved” star-like cores (active nuclei)
Shown here is an HST image of NGC 7742, a so-called “Seyfert galaxy” after Carl Seyfert who did pioneering work in the 1940s

NGC4151 with a range of exposures

Spectra of Stars, Spectra of AGNs

Active Galactic Nuclei:  AGNs
Small fraction of 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

3C31

Many Views of Radio Galaxy Centaurus A

Active Galactic Nuclei:  AGNs
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)

Relativistic Doppler (Red) Shift
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

Relativistic Doppler (Red) Shift

Black Holes:  Review
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

Accretion Disks
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

Different Views of the Accretion Disk
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

Different Views of the Accretion Disk
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

Different Views of the Accretion Disk
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

Quasar Images 1

Nice, early Quasar Quote:

Quasar Images II

Quasar Images III: “Starburst-Quasar”

What makes an AGN active?
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

Gravitational Lensing

For more information, movies:
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.

Chapter 14: Galaxies with Active Nuclei
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