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Active Galaxies

Active Galaxies. PHYS390 Astrophysics Professor Lee Carkner Lecture 22. Active Galactic Nuclei (AGN). Often associated with galaxies Caused by central black hole Need a unified model of a black hole system than can explain all types of AGNs. Seyfert Galaxies. Seyfert 1

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Active Galaxies

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  1. Active Galaxies PHYS390 Astrophysics Professor Lee Carkner Lecture 22

  2. Active Galactic Nuclei (AGN) • Often associated with galaxies • Caused by central black hole • Need a unified model of a black hole system than can explain all types of AGNs

  3. Seyfert Galaxies • Seyfert 1 • show broad lines (~3000 km/s) and narrow lines (500 km/s) • Seyfert 2 • show just narrow lines • X-rays weak and heavily absorbed

  4. Radio Galaxies • Like Seyferts, there are two kinds: • Broad-line (BLRG) • Narrow-line (NLRG) • compact core and large halo • flanking lobs and jets

  5. Lobes • Radio lobes are very large • can be larger than galaxy source • Radio energy many times greater than energy of non-AGN galaxy • Radio emission is synchrotron • From electrons in magnetic fields

  6. Jets • Each about 70 kpc long • Shocked clumps of ejecta • Some jets are bent • Galaxy moving through intercluster gas

  7. BL Lac • Point source with very rapid variability • Originally thought to be stars, but distance is extra-galactic • No radio lobes or jets • Sometimes called Blazars

  8. Quasar • Quasi-stellar radio source • L ~ 5X1029 W • ~100000 times as bright as normal galaxy • Have high redshift • Some quasars have weak radio emission • Quasars can be radio-load or radio-quiet

  9. Quasar Distance • Quasar distances often given by redshift parameter, z z = (lobs-lrest)/lrest = Dl/l • The radial speed away from us z = [(1+(vr/c))/(1-(vr/c))]½-1 d =cz/H0 • Where d is in Mpc, c is in km/s and H0 = 71 km s-1 Mpc-1 • Quasar redshifts go up to ~5.5

  10. AGN Spectra • AGNs produce emission over a wide range of frequencies • Radio is synchrotron emission • Big blue bump of thermal emission from hot accretion disk

  11. Central Engine • The power source must be small (few AU) • The fastest the change can occur is the speed of light • About 108 Msun in a radius of 2 AU • AGNs are powered by accretion on to supermassive black hole

  12. Energy • Matter falling into a strong gravitational field releases energy as, L = hMc2 • h is the efficiency • More energy generated per kg than nuclear fusion

  13. Disk • Structure of accretion disk is uncertain • Inner disk is thick and hot • ~104 K • Outer disk is thin and heated by inner disk • Similar to PSM star jets • Beyond thin disk is optically thick torus

  14. Line Production • Consists of clouds of gas heated by the disk to produce emission • T~104, n = 1015 m3 • Narrow line region must lie outside the torus • Is less dense and can show forbidden lines • May be clouds pushed out form center by wind or jets

  15. Unified Model • Edge-on • Seyfert 2, narrow-line radio galaxy • Inclined • Seyfert 1, broad-line radio galaxy, quasar • Face-on • BL Lac

  16. Why AGNs? • AGNs are fueling the black hole due to gravitational interaction or merger • Gravitational interaction causes gas to fall into the center • Quasars are from the early universe

  17. Next Time • Read 27.1, 27.3 • Homework: 27.1, 27.3, 27.9, 27.13

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