Energy Output and Active Galaxies

We have so far discussed the appearance, makeup, and dynamics of normal galaxies, but astronomers have long known of strange objects out there that do not fit into our scheme of normal galaxies.  Some are clearly galaxies, but with some peculiar characteristics.  Other objects are not galaxies themselves, but are thought to be objects at the cores of galaxies.  We will discuss these within a unified framework that focuses on the cores, or nuclei, of galaxies.  The peculiar objects are thus referred to as Active Galactic Nuclei, or AGN.  The single criterion that distinguishes AGN from normal galaxies is energy output:

• Normal galactic nuclei -- energy output ~ 10⁴² erg = 10⁹ L_o.
• Active Galactic Nuclei -- energy output > 10⁴⁴ erg = 10¹¹ L_o.

AGN come in several types:

• Seyfert Galaxies
• Bright nucleus
• Occur in spiral galaxies
• Broad emission lines (Seyfert 1) -- doppler speeds of 1000-5000 km/s, rapid variability -- Broad Line Region (BLR)
• Narrow emission lines (Seyfert 2) -- no variability -- Narrow Line Region (NLR)
• Model: small, intense photoionizing source illuminates two distinct kinds of gas, (1) inner, high-density, rapidly rotating region (BLR), and (2) outer, low-density, slowly moving region (NLR)
• BL Lac Objects
• Star-like appearance
• No (or very faint) spectral lines (cannot determine distance!)
• Rapid changes in luminosity on timescale of 10-100 days
• Radio Galaxies
• Occur in elliptical galaxies
• optical jet
• radio jet
• radio lobes
• central object of M87
• Quasars
• Quasi-Stellar Object or QSO -- looks like an ordinary star
• HST shows that they are found in host galaxies (spirals)
• Blazars
• Relatively low UV and optical luminosity
• High luminosity at TeV (!) energies
• Synchrotron source, with inverse Compton scattering of photons to higher energies

How can all of these very different appearing objects be brought under a single model?  Clearly the underlying theme is the existence of a supermassive black hole, which along with its accretion disk provide the power to run these titanic engines in the centers of galaxies.  The basic picture is exactly the same as we saw with X-ray binaries, but on a much vaster scale.  Important issues:

• Brightness of central object depends on orientation of the jet
• pointed away from us and shrouded by accretion disk -> narrow line Seyfert galaxy
• pointed away from us, but accretion disk visible -> broad line Seyfert galaxy
• pointed generally toward us -> BL Lac Object or Quasar
• aligned perfectly at us -> Blazar
• Existence and length of jet depends on amount of dust in host galaxy
• lots of dust (spiral galaxy) -> no, or weak jet (seyfert galaxies)
• little dust (elliptical galaxy) -> extended (superluminal) jets and large radio lobes
• Variability depends on how small of a region dominates the emission
• Emission dominated by gas outside accretion disk -> NLR Seyfert
• Emission dominated by accretion disk -> BLR Seyfert
• Emission dominated by synchrotron emission from jet -> BL Lac Object
• Total luminosity depends on size of supermassive black hole and rate of accretion