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Galactic Nuclei

Galactic Nuclei. Active Galactic Nucleus. The centers of galaxies are found to be 1. a dense stellar cluster, with composite stellar-type absorption and emission spectra 2. extremely bright and compact nucleus, sometimes brighter than the entire galaxy, with non-stellar spectra

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Galactic Nuclei

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  1. Galactic Nuclei

  2. Active Galactic Nucleus The centers of galaxies are found to be 1. a dense stellar cluster, with composite stellar-type absorption and emission spectra 2. extremely bright and compact nucleus, sometimes brighter than the entire galaxy, with non-stellar spectra The second type are the AGN, up to 10% of all galaxies, possibly an active phase of all galaxies AGN have strong, broad and rapidly variable emission lines from hot gas – the nuclei are a few light-days across !

  3. Active Galactic Nuclei and Quasars Centers of galaxies are extremely bright and active; due to super-massive black holes at the center Emit light strongly in ALL wavelength bands, from gamma rays and X-rays to Radio The spectra do NOT look like that of stars – evidence of unusual non-stellar activity Quasars: Quasi-Stellar Radio Sources, appear star-like owing to large distances and red-shifted spectra Hence QSO’s were formed early in the Universe, and are otherwise also active galactic nuclei (AGN)

  4. Quasar

  5. Redshifted H Balmer Lines

  6. Redshift vs. speed (distance)

  7. Number of Quasars vs. Age

  8. AGN and Quasars There is considerable evidence of evolution: Quasars are early (high-z) versions of AGN, before evolving into normal galaxies AGN and quasars are thought to harbor supermassive black holes that are the engines which power the highly energetic activity leading to extreme luminosities Quasars are the most luminous objects in the Universe (note that supernovae can be equally luminous but are transient, and remain that bright only for a few months at most)

  9. Quasars and host galaxies at same redshift

  10. AGN –Seyfert Galaxies Spiral galaxies with bright compact nuclei showing broad emission lines like quasars are called Seyfert galaxies (after Carl Seyfert) Broad emission lines are due to Doppler broadening of light from gas clouds moving at high velocities, about 10,000 Km/sec Radiation from AGN and Quasars indicates a plasma source with temperatures 100,000 to millions of degrees (stars have temperatures much less than 100,000 K) How does the Black Hole power AGN and quasars ?

  11. Blackbody Stellar vs. Non-thermal AGN Spectra • Stellar spectra are that of a BB with peak emission around in one wavelength region • AGN and Quasar spectra are non-thermal (non-BB), with radiation flux decreasing monotonically with frequency (energy), but remains significant at all wavelengths

  12. Solar Radiation vs. Wavelength (Black Dots): Spectral Fit to Blackbody at 5700 K (Solid Line)

  13. AGN Spectra: Constant Radiation at All Wavelengths

  14. Active Radio Galaxies • Radio loud: powerful radio sources - Low Power: Radio galaxies - High Power: Quasars • Radio Quiet: weak radio sources - Low Power: Seyfert galaxies - High Power: quasi-stellar radio sources • Radio sources have jets of relativistic particles due to magnetic acceleration, and large “lobes” • Jets of high-velocity charged particles emit radio waves called “synchrotron radiation”

  15. Quasar 3C273 and Relativistic Jet

  16. Radio lobes at endpoints of jets

  17. Geometry and Physical Properties • Observed image and spectral properties depend on orientation • Obscuring torus hides the active nucleus and black hole activity if seen edge-on, e.g. narrow-line spectra similar to H II regions or nebulae with low-velocity clouds • Face-on view reveals highly non-thermal (non-stellar) spectra with broad emission lines

  18. Supermassive Black Hole Paradigm:Structure of AGN and Quasars • Geometry of AGN/Quasars : Black Hole surrounded by an accretion disc, embedded in a torus (doughnut shaped), with jet streams perpendicular to the disc • High velocity clouds moving around, with Doppler broadened emission lines • Orientation of disc determines our view: - face on looking at the nucleus OR - edge-on view of the obscuring torus • Most extreme Quasars : BLAZARS (BL Lac objects), viewing the jets end-on, no emission or absorption lines, but surrounding elliptical galaxies seen as “fuzz”

  19. AGN Variability or Reverberation • The luminosity of AGN often varies on a time-scale of days  the emitting nucleus must be very compact size, only light-days across • Nuclear activity propagates all around within days • Size ~ 0.01 pc ~ 10 light-days ~ 1000 AU ~ size of the solar system (give or take a factor of 2) • AGN/Quasars emit light equal to an entire galaxy from a region not much bigger than a stellar system !

  20. Evolutionary Sequence of Galaxies • Quasars are nuclei of active galaxies in the distant past: ultra-luminous, high-z objects • AGN are the link between quasars and normal galaxies, activity continues to present day as they evolve • Quasar/AGN activity fueled but a super-massive black-hole (SMBH) engine • All galaxy formation may be driven by SMBH activity during some epoch

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