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Active Galaxies

0. Galaxies with extremely violent energy release in their nuclei (pl. of nucleus). Active Galaxies. → “Active Galactic Nuclei” (= AGN). Up to many thousand times more luminous than the entire Milky Way; energy released within a region approx. the size of our solar system!. 0.

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Active Galaxies

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  1. 0 Galaxies with extremely violent energy release in their nuclei (pl. of nucleus). Active Galaxies → “Active Galactic Nuclei” (= AGN) Up to many thousand times more luminous than the entire Milky Way; energy released within a region approx. the size of our solar system!

  2. 0 If you take a spectrum of a normal galaxy, what would you expect to see? 1) A pure blackbody spectrum from the continua of all the stars in the galaxy 2) A blackbody spectrum with many absorption lines from the stars in the galaxy 3) A spectrum with many emission lines from hot gases

  3. 0 Answer: 2) A blackbody spectrum with many absorption lines from the stars in the galaxy The light from the galaxy should be mostly star light, and should thus contain many absorption lines from the individual stellar spectra.

  4. 0 Seyfert Galaxies Unusual spiral galaxies: • Very bright cores • Emission line spectra. NGC 1566 • Variability: ~ 50 % in a few months Most likely power source: Accretion onto a supermassive black hole (~107 – 108 Msun) Circinus Galaxy

  5. 0 Model for Seyfert Galaxies Gas clouds Emission lines UV, X-rays Dense dust torus Supermassive black hole Accretion disk

  6. 0 The Dust Torus in NGC 4261

  7. 0 Quasars Active nuclei in elliptical galaxies with even more powerful central sources than Seyfert galaxies Also show strong variability over time scales of a few months. Also show very strong, broad emission lines in their spectra.

  8. 0 Quasar Red Shifts z = 0 z = 0.178 Quasars have been detected at the highest red shifts, up to z ~ 6 z = 0.240 z = 0.302 z = Dl/l0 z = 0.389

  9. :33 0 What can astronomers study when observing high-redshift quasars? • Large-scale structures in the universe. • The early history of the universe. • Galaxy evolution. • Dark matter. • All of the above.

  10. 0 Probing Dark Matter with High-z Quasars:Gravitational Lensing Light from a distant quasar is bent around a foreground galaxy → two images of the same quasar! Light from a quasar behind a galaxy cluster is bent by the mass in the cluster. Use to probe the distribution of matter in the cluster.

  11. 0 Quasar Host Galaxies Elliptical galaxies; often merging / interacting galaxies 0316-346 PG 0052+251 IRAS 04505-2958 IRAS 13218+0522 PHL 909 PG 1012+005

  12. 0 If quasars and other AGN are powered by accretion disks onto supermassive black holes, which other phenomenon would you expect to see in many AGNs? • Supernovae. • Pulsars. • Jets. • Gamma-Ray bursts. • Planetary Nebulae.

  13. 0 Cosmic Jets and Radio Lobes Many active galaxies show powerful radio jets Hot spots: Energy in the jets is released in interaction with surrounding material Radio image of Cygnus A Material in the jets moves with almost the speed of light (“Relativistic jets”).

  14. Other Types of AGN and AGN Unification 0 Quasar or BL Lac object (properties very similar to quasars, but no emission lines) Emission from the jet pointing towards us is enhanced (“Doppler boosting”) compared to the jet moving in the other direction (“counter jet”). Observing direction

  15. Blazars BL Lac Objects and quasars: AGN with their jets directed almost right at us. Very bright at all wavelengths, from radio to gamma-rays Over 100 gamma-ray blazars detected! Fermi Gamma-Ray Space Telescope

  16. Blazars Visible Radio Over 1000 times more powerful than an entire galaxy (~ 100 billion stars); smaller than our solar system!

  17. Faster than the speed of light?

  18. 0 Superluminal Motion Individual radio knots in quasar jets: Sometimes apparently moving faster than speed of light! Light-travel time effect: Material in the jet is almost catching up with the light it emits

  19. 0 Superluminal Motion Apparent motion at up to ~ 20 times the speed of light!

  20. 0 Other Types of AGN and AGN Unification Observing direction Cyg A (radio emission) Radio Galaxy: Powerful “radio lobes” at the end points of the jets, where power in the jets is dissipated.

  21. 0 Radio Galaxies M 87: The central galaxy of the Virgo cluster of galaxies

  22. 0 Radio Galaxies X-rays + Optical X-rays Optical Radio M 87: The central galaxy of the Virgo cluster of galaxies

  23. 0 Radio Galaxies Optical image Radio image Centaurus A (“Cen A”): the most nearby AGN.

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