| Today: Astronomy Articles | |
| Start Chapter 14, Active Galaxies | |
| “Town Meeting” (9 AM class) | |
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 |
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| 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
| Small fraction of galaxies have
extremely bright “unresolved” star-like nuclei |
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| 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 | |||
Many Views of Radio Galaxy Centaurus A
| Many galaxies have extremely bright “unresolved” star-like nuclei |
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| 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
| 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 | |||
| 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 |
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| Seyfert 1 galaxies: Slightly tilted
view Hot high velocity gas close to black hole is visible High velocities Þ broad emission lines |
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| BL Lac objects: Pole on view Looking right down the jet at central region Extremely bright – vary on time scales of hours |
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| 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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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 |
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| 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 |
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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 | ||