1 : Preliminaries   6 :   Dynamics I 11 : Star Formation  16 : Cosmology
2 : Morphology   7 :   Ellipticals 12 : Interactions 17 : Structure Growth 
3 : Surveys 8 :   Dynamics II 13 : Groups & Clusters  18 : Galaxy Formation 
4 : Lum. Functions  9 :   Gas & Dust   14 : Nuclei & BHs 19 : Reionization & IGM  
5 : Spirals 10 : Populations    15 : AGNs & Quasars 20 : Dark Matter




Under Current Construction : last update April 6 2011

(1) Introduction

(a) Operational Definition of AGN

  1. Nuclear SMBHs are not sufficient to assign an AGN classification:
    e.g. M31 and M32 both have SMBH's but neither show signs activity (accretion power).
  2. Accreting stellar mass black holes are not AGN -- we need an SMBH located in a galaxy nucleus.
  3. The activity classification explicitly excludes stars or stellar life cycles
    Star formation, HII regions, Supernovae, etc may be present, but they do not constitute an AGN.
  4. Luminosity isn't part of the definition: activity can be quite weak
    For this reason, one often qualifies the power of the AGN, for example:
    "QSOs are powerful AGN"; "M81 contains a weak AGN"

  1. Emission lines with distinct ratios (defined in 3d below)   [image].
    Often, these are strong & imply photoionizaton by a hard spectrum, rather than by a black body.
  2. Broad H wings or profile (1000 km/s) - i.e. much more than typical galactic velocities.   [image].
  3. Broad band unresolved continua, with comparable power in X-ray, UV, optical, IR
    In the optical, they often appear blue; overall, their continua are unlike starlight. [image]
  4. Variability of one or more parts of this continuum (or the emission lines)
    this points to a highly compact (1pc) source of emission   [image].
  5. Radio source, often bipolar & significantly more luminous than in normal galaxies
    this may also show jet-like morphology; steep spectrum jet & lobes; flat spectrum core   [image].

(b) Reasons for Studying AGN

  1. At their heart lie giant black holes, perhaps the most exotic astrophysical animal we know of.
    AGN give us observational access to these otherwise ellusive objects

  2. Almost by definition, physical processes near SMBHs are in the relativistic regime
    A host of interesting and extreme physical process occur.
    While often confusingly complex, they provide a laboratory for studying high energy processes.

  3. Black hole accretion is very efficient, making AGN amongst the most luminous objects known.
    e.g. for mass m in orbit at RS = 2GM/c2, we have 2KE ~ PE ~ GMm/RS = ½mc2
      typical accretion efficiencies are ~¼ mc2
    this is huge compared to either fusion (~0.7%) or chemical (~10-6%) energy sources.
    Accreting just 1 M yr-1 yields ~1046 erg s-1 4 × 1012 L 200 L galaxies.

  4. In addition to photon luminosity, AGN can provide powerful collimated mechanical luminosity.
    The creation, propagation and interaction of these jets provide yet another important area of study.
    Of course, both accretion flows and jet flows are found in many astrophysical contexts
      AGNs provide excellent windows on both these complex phenomena.

    High AGN luminosities yield other important opportunities:

  5. Luminous AGN can be seen to enormous distances (currently, z~7, lookback time ~95%)
    One can therefore study directly their cosmic evolution.
    They show strong evolution, with a "hey day" at z~2 (×1000 more common than today).
    This evolution is closely tied to early galaxy construction -- another topic of great importance.

  6. As distant beacons, their spectra provide access to the intervening IGM; for example:

  7. The accretion power ultimately emerges in essentially all spectral wavebands
      AGN present observationally very rich targets
      they feature in ~all new satellite & instrument observational programs.

  8. AGN influence 11 decades, from accreting SMBH (~AUs) to jet powered radio lobes (~Mpc)
    Of these decades, ~half are observationally unresolvable.
      AGN offer a wonderful window on a huge range of environments and physical processes.

(2) Brief Historical Sketch

(a) Early Optical Spectroscopy

(b) Early Radio Observations

(c) Quasar Discovery

(d) Redshift Controversy


(3) The AGN Paradigm

Log r
Other Region/Properties
-5 2 AU RS = 2 MBH,8 AU,   last stable orbit is 3RS   (MBH,8= MBH/108M)
-4 20 AU relativistic accretion disk (AD); Fe K line; variable X-ray emission (mins-hours)
-3 200 AU UV accretion disk; radiation supported AD;
-2 2000 AU optical AD; Broad Line Region (BLR); V~104 km s-1; ~few light days.
-1 0.1 pc compact, flat-spectrum radio core; VLBI jet; outer BLR
  0 1 pc star cusp with velocities affected by BH; dense gas may block some sight lines.
  1 10 pc Inner Narrow Line Region (NLR); V 103 km s-1; inner bulge.
  2 100 pc bulge; NLR; forbidden emission lines; warm dust; VLA jet.
  3 1 kpc inner disk; disk ISM; Extended-NLR; Bar & inflow; VLA jet.
  4 10 kpc host galaxy; distortions/merger?; ENLR; weak jets blocked.
  5 100 kpc near neighbors; tidal influences; powerful jets.
  6 1 Mpc jets terminate in IGM hotspots; may affect ICM in cluster.

  1. On large scales, galaxy disturbances/mergers are efficient at removing angular momentum from orbiting gas and this finds its way down to the nuclear regions, possibly with an associated starburst. Some of this gas may cool and form a dusty thick disk in the central 1-1000 pc. This gas can obscure our view of the central regions and, conversely, block the outgoing radiation from the AGN along certain directions.

  2. Ultimately, some of this gas finds its way into an inner accretion disk, whose Keplerian dynamics is dominated by the black hole. Here, magnetic fields create viscosity in the disk, which causes the gas to move inwards, releasing gravitational energy which heats the disk. The disk has a temperature gradient, and generates at least some of the optical/UV/X-ray continuum emission via thermal radiation at its surface. Also likely are non-thermal processes in a disk corona which add to the continuum. Ultimately, gas within a few RS emits X-rays before crossing the horizon, adding to the mass and angular momentum of the black hole.

  3. It is in this innermost region that a jet is somehow generated moving out along the disk axis, probably via magnetic fields tied to the rotating disk. It is not known why this jet is sometimes very powerful (radio loud AGN), and sometimes much weaker (radio quiet AGN). The jets are born with bulk relativistic speeds, and contain magnetic fields and particles with random relativistic speeds -- hence these jets are visible mainly via synchrotron. The jets burrow out through the surrounding galaxy ISM with varying degrees of difficulty, entraining and disturbing the ISM en-route. The most powerful jets can escape the galaxy ISM altogether and traverse a Mpc before slamming into an outward moving "working surface" of IGM material.

  4. Returning to the central engine, the accretion process also yields a luminous source of X-ray, UV, and optical photons, with roughly power-law spectral shape. This radiation floods out into the galaxy, although some directions may be blocked by the denser ISM components. Whether or not we get a clear line of sight to the nucleus is thought to explain at least some of the observational variety of AGN.

  5. The UV-X-ray radiation is an effective ionizer. It first encounters gas clouds within the central few light days/weeks and the resulting line emission reveals cloud velocities ~103 - 104 km s-1 with gas densities ~1010 cm-3. This emission defines the so-called "Broad Line Region" (BLR). The BLR velocities are probably gravitational, while the origin of the BLR material is uncertain -- perhaps arising from an accretion disk wind.

  6. At larger distances, the ISM velocities and pressures both drop. By 10pc - 1kpc the ionized gas clouds yield velocities ~few ×102 km s-1 -- roughly equal to typical bulge gravitational speeds -- and gas densities fall from ~106 to ~102 cm-3. This emission defines the so-called "Narrow Line Region" (NLR). Depending on the geometry of the inner obscuration, the NLR may show bi-conical shape. If jets are blocked in this region, the gas often exhibits significantly higher disturbed velocities.

  7. Sometimes the jets and ionized gas can be seen to much larger radii, often in association with eachother. The most powerful jets can travel far from the galaxy and dump their energy in the IGM or, if the galaxy is in a cluster, in the ICM. In that case, large cavities in the X-ray emission can be seen surrounding the expanding radio source.

(4) AGN Taxonomy

(a) Four Basic Criteria

(b) Taxonomic Table

(c) Mixed Classifications

(d) Emission Line Classification


(5) AGN Detection and Identification

(a) Radio Surveys

(b) Optical Colors

(c) Slitless Spectroscopy

(d) Emission Lines

(e) Infra-red

(f) X-ray & Gamma-ray

(g) Variability & Proper Motion


(6) AGN Demographics

(a) Frequency of AGN in the Local Galaxy Population

(b) AGN Preference for Big Bulges

(c) Higher Luminosities


(7) AGN Unified Schemes

(a) Two Different Angle-Dependent Phenomena

(b) Seyfert 2s / Seyfert 1s

(c) NLRGs / BLRGs

(d) Radio Loud: PRG-II / LD-QSR / CD-QSR

(e) RG-I / BL-Lacs : straight down the beam

(f) Intrinsic Differences: Luminosity & Radio Loudness


(8) Accretion Power

(a) The Eddington Luminosity & Growth Rate

(b) Thin Accretion Disks

(c) Accretion Efficiency: The Inner Radius

(d) Feeding the Disk

(e) Extracting Rotational Energy?

(f) Inevitability of Black Hole Formation


(9) Continuum SEDs


(10) Emission Line and Ionized Cloud Physics

Ionizing radiation from the central engine enters surrounding gas and ionizes it.
There are a number of simple phenomena related to this ionized gas worth discussing.

Recall, there are two emission regions in our standard picture: the BLR and the NLR [image]
How do we find out the properties of the gas in these regions?

(a) Simple Density Limits for the NLR and BLR

There are two kinds of emission lines:

(b) Properties of a Homogeneous Gas at Single ne and Te

  • Temperature sensitive line ratios have two upper levels with E kT
    collisional excitation favors the lower level by exp(-E / kT)
    the line from this level is relatively stronger at lower temperature.

    A good example is [OIII] 5007 / 4363     [image]

  • Density sensitive line ratios have two upper levels with E kT
    collisional excitation gives equal populations
    However the line strengths differ due to their different critical densities.

    A good example is [SII] 6717 / 6731     [image]

  • Of course, in detail, the line ratios are functions of both temperature and density [image]
    One can then use various line ratios to hunt for a single combination of ne and Te [image]
    This is only justified for a single emission region with uniform properties (not, usually, an entire NLR).

    (c) The Structure of Ionized Clouds

  • The NLR and BLR are thought to contain clouds that are photoionized by the central UV-X-ray source.
    The clouds are optically thick to the ionizing Lyman continuum.
     This leads to a layered cloud structure:
          highly ionized front; decreasing ionization into the cloud; neutral/low ionization back [image].

  • For a black body spectrum, ionization degree ~uniform up to Stromgren depth, then neutral.
    For a power-law (AGN) spectrum, wide range of ionization & extended partially ionized region.
    wide range of excitation (e.g. [OIII] and [OI] lines strong). [image]

  • There is a long history of computing the structure and emergent spectrum from photoionized gas clouds.
    The most widely used code is "CLOUDY", written by Gary Ferland   [o-link1 and o-link2]
    [Here is a nice vignette of its 1978 origins at the IoA Cambridge].

    (d) The Radiation Parameter

  • The most important parameter governing the emission from photoionized gas is the radiation parameter.
    It is defined by the ratio of ionizing photon density to electron (gas) density at the front of the cloud:


    where, more precisely, Qion = L / h   d from the Lyman limit to infinity.

  • Basically, higher U gives more highly ionized gas, for the obvious reason:
          greater photon density causes higher rates of ionization
          greater electron density causes higher rates of recombination

     Their ratio sets the equilibrium ionization degree.

  • The emitted spectrum from a single cloud changes with U as you'd expect:
          High U stronger high ionization lines (e.g. [OIII], CIV etc)
          Low U stronger low ionization lines (e.g. [OI], [SII] etc)

       Here are some calculated line strengths for a sequence of models with different U:   [image]

    (e) Calculating Simple Region Properties

    Some simple relations yield estimates for some emission region properties
    The examples used here are for a typical BLR.      

    (11) The Broad Line Region

    See Peterson Review: here

    High density and anisotropic cloud emission: o-link


    (12) The Narrow Line Region


    (13) Intrinsic Absorption Lines


    (14) AGN Radio Properties : Cores, Jets, Lobes

    Compilation of double radio sources by Leahy & Bridle is here: [o-link]

    Paper on collimation in M87 is here

    More high res (7mm VLBA) on nearby RGs is here


    (15) AGN Host Galaxies and Environment


    (16) AGN Evolution

    include source counts; V/Vmax (Krolik ch 3)