| Today: Finish Chapter 14, Active Galaxies | |
| “Town Meeting” (Noon class) | |
| 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? | ||
| Randomly inclined orbits | ||
| 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. | ||
| 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 Ž narrow emission lines |
||
| Seyfert 1 galaxies: Slightly tilted
view Hot high velocity gas close to black hole is visible High velocities Ž 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 Ž narrow emission lines |
||
| Seyfert 1 galaxies: Slightly tilted
view Hot high velocity gas close to black hole is visible High velocities Ž 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 Ž narrow emission lines |
||
| Seyfert 1 galaxies: Slightly tilted
view Hot high velocity gas close to black hole is visible High velocities Ž 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 III: “Starburst-Quasar”
| 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 | ||
| 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 |
The Central Engine of Centaurus A
| http://imgsrc.stsci.edu/op/pubinfo/pr/1998/14/content/centauf.mov |
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 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 | |