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1
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- Today: Finish Chapter 14, Active
Galaxies
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“Town Meeting” (Noon class)
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- Q1. You observe two Cepheid variable stars, A and B, which have the same
average apparent magnitude. Star A brightens and dims with a period of 5
days, star B has a period of 18 days. Which is closer to Earth?
- Faster varying variables are less luminous, therefore Star A is less
luminous than B. Therefore if Star A appears as bright from Earth as B,
it must be closer.
- Q2. Which of the following is not a characteristic of the stars of the
disk component of our galaxy?
- Q3. If all the mass in our galaxy were centrally concentrated, we'd
expect velocities to fall with increasing distance according to Kepler's
laws. This is not seen in the disks of spiral galaxies. Galactic
rotation curves appear "flat" with increasing distance. This
must be due to:
- The gravitaional influence of "dark matter" in the halo.
- Q4. If the sun is 5 billion
years old, how many times has it orbited the galaxy? Assume a circular
orbit for the sun.
- Need velocity of the sun (about 220 km/s) and the orbital circumference
(2πr), where r=8. 5kpc.
Number of orbits in 5 billion years is then 5 billion years/time
for one orbit, which is circumference/220 km/s. Converting units gives an orbital
period of 240 million years.
Therefore in 5 billion years the sun has orbited about 21 times.
- Q5. If interstellar dust makes and RR Lyrae star look 1 magnitude
fainter than it should, by how much will we overestimate its distance?
- Star looks 1 mag fainter.
Distance = 10 (m-M+5)/5 so our estimate will differ
by a factor of 101/5 which is about 1.6.
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3
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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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4
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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
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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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7
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8
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9
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10
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11
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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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12
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- Won’t show this one in class – but you can look at it on
your own if you wish. Shows
a bulge of a spiral galaxy rapidly “ignite” as a central
black hole is fuelled:
- http://imgsrc.stsci.edu/op/pubinfo/mpeg/quasar.mpg
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13
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- http://imgsrc.stsci.edu/op/pubinfo/pr/1998/14/content/centauf.mov
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14
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15
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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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16
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- The Hubble Expansion -- review
- Olber’s paradox
- The Big Bang
- Refining the Big Bang
- Details of the Big Bang
- General Relativity
- Cosmological Constant
- Origin of Structure
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Notes