16-03 Active Galactic Nuclei (AGN)


  • BL Lac Objects, or Blazars (Lac, Lacerta, the Lizard)
     
    BL Lac was discovered in 1929; featureless spectrum, fast varying (doubled in a week), strong polarization.
    In early 1970 it was found to be the bright core of a faint fuzzy elliptical galaxy.


    [BL Lac]

    Later, radio observations of blazars show faint radio halos surrounding bright radio cores.
    Blazars seem to be the center of distant radio galaxies with jet towards the direction of the Earth.

    Active galaxies (radio galaxies and Seyfert galaxies) and thier nuclei
    (Active Galactic Nuclei (AGN): distant ones are blazars, quasars (QSOs))
     
  • The central-engine problem -- what powers active galaxies?
    Size < light-days
    The Eddington limit = 1038 erg/s (M/M)
    ==> Supermassive black holes!
     
  • Most galaxies have a supermassive black hole at its center:


    [M87: 450 km/sec within 60 ly, indicating 3x109 M; VLBI shows a core less than a few light weeks. EHT shows an image of a partial donut with a diameter about 40 μas, corresponding to about 0.01 ly, i.e. 1000 AU or so, at a distance of about 16 Mpc.]


    [The M31 rotation curve, which indicates that M31 has 3x107 M within 5 pc of its center.]

    Our Milky Way Galaxy also has a 4x106 M supermassive black hole at the galactic center!

     
  • A unified model for AGN


     
    The disk was found !!!


      [NGC 4261; the left is the optical galaxy superposed on the radio jets; the right is the HST image showing a disk of gas and dust with 100 pc diameter]

    Triggering of AGN activities?
    Formation of the supermassive black holes?
    Evolution of galaxies?


      [The infrared image of the central part of Cygnus A.]