Seyfert Galaxies

• In 1943, Carl Seyfert published a list of 12 otherwise normal spiral galaxies which contain anomolously bright central nuclei. These galaxies are now known as "Seyferts."

• The following figure shows successively deeper images of the Seyfert galaxy NGC4151. In short exposures, only the bright nucleus is apparent, but deeper images reveal the normal spiral galaxy around it.

• In some Seyferts, the central nucleus outshines the whole surrounding galaxy.

• The central nucleus light output varies on timescales of less than a year
  • So, the emitting region must be less than ~ a light year across, as the source cannot vary coherently on timescales shorter than this due to light travel time effects:

    

• Spectra of the nuclei in Seyferts are non-stellar. They contain:
  • Non-thermal continuum emission
  • Narrow (=> low velocity), forbidden (=> low density material) lines which do not vary detectably (=> large emitting region)
  • Broad (=> high velocity), forbidden permitted lines which vary on fairly short timescales (=> small emitting region)
  • Also, strong emission in the radio, infrared, ultraviolet, and X-ray parts of the spectrum.

• Seyfert galaxies have been classified into two basic types:
  • Seyfert 1 galaxies are bright in the optical, and have both broad and narrow lines in their spectra.
  • Seyfert 2 galaxies are fainter in the optical (but the same as Seyfert 1s in the infrared), and they only have narrow lines in their spectra.
  • Intermediate cases with weak broad emission lines also exist, and are classified as Seyfert 1.3, 1.6, etc.

• The current paradigm for Active Galactic Nuclei explains the variations between these different classes by the different inclinations from which the galaxies are viewed:

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  • In the broad-line region (BLR)
    • The Keplerian orbital speeds of the clouds around the central massive body will be large => lines are Doppler broadened.
    • Density is high => no forbidden lines are emitted

  • In the narrow-line region (NLR)
    • The Keplerian orbital speeds of the clouds will be much smaller => lines are narrow
    • Density is low => forbidden lines are emitted

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  • So, if the above Seyfert were viewed from direction (1), you would see:
    • Broad permitted lines
    • Narrow Forbidden Lines
    • Bright continuum from the central engine
    • i.e. a Seyfert 1

  • If, on the other hand, it were viewed from direction (2), you would see:
    • No broad permitted lines (obscured by dust torus)
    • Narrow Forbidden Lines
    • No bright continuum from the obscured central engine
      • except in the infrared and X-ray region, which gets through the dust
    • i.e. a Seyfert 2

• One elegant confirmation of this model comes from observing Seyfert 2 galaxies in polarized light:
  • A small faction of the light emitted by the BLR will scatter off material in the NLR into our line of sight.

    

  • Scattered light is polarized (that's why polaroid sun glasses work!)

  • So, if we look at a Seyfert 2 in polarized light, we should see the broad lines from the BLR.

  • And we do!

• Note that we already have enough information to estimate the mass of the central engine:
  • H-beta emission lines (wavelength l = 486nm) from the BLR are typically broadened to a width of Delta l ~ 2 nm. The gas must therefore have turbulent velocities of ~v, where v/c ~ Delta l/l, so v ~ 10^6 m/s

  • This emission varies on timescales of ~ a year, so the emission must come from a region whose size r ~ 1 light year ~ 10^16 m across.

  • So if the motion of the clouds are bound orbits around the central engine, we can estimate the mass from the usual dynamical formula,

    

    i.e. M ~ 10^38 kg ~ 10^8 solar masses.

    • All well within a region ~1 light year across!!