the phenomenon of active galactic nuclei: an introduction
DESCRIPTION
The Phenomenon of Active Galactic Nuclei: an Introduction. Outline. Active Galactic Nuclei (AGN): > Why are they special? > The power source > Sources of continuum emission > Emission & absorption features >Jets and radio emission > AGN classification & unification - PowerPoint PPT PresentationTRANSCRIPT
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The Phenomenon of Active Galactic Nuclei: an Introduction
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Outline
Active Galactic Nuclei (AGN):
> Why are they special?
> The power source
> Sources of continuum emission
> Emission & absorption features
>Jets and radio emission
> AGN classification & unification
> The co-evolution of black holes and galaxies
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What makes AGN Special?
• Very large luminosites are possible (up to 10,000 times a typical galaxy)
• The emission spans a huge range of photon energy (radio to gamma-rays)
• The source of energy generation is very compact (< size of the solar system)
• In some cases, there is significant energy transported in relativistic jets
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The High Luminosity of AGN
• The AGN here is several hundred times brighter than its host galaxy, just in visible light alone
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The “Broadband” emission
• Comparable power emitted across ~seven orders of magnitude in photon energy
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The Small Size
• Light travel time argument: a source that varies significantly in time t must have size R < ct
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The Building Blocks of AGN
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The Power Source: Accretion onto a Supermassive Black Hole
• Efficient, compact, and capable of producing high-energy emission and jets
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• Newton: M = v^2 R/G!• Water masers mapped in NGC 4258: M = 40 million solar masses• Orbits of stars in the Galactic Center: M = 3 million solar masses
Black Holes Masses: Newton!
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Larger samples
• Masses measured dynamically for a few dozen• Only probe larger distances (>10 pc)• Much less precise masses
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Energetics
• Conservation of energy plus the Virial Theorem: the relativistically deep potential well allows ~10% of the rest-mass energy to be radiated by accreted material
• This is ~100 times more efficient than nuclear burning in stars
• Required accretion rates: ~1 solar mass per year for typical powerful AGN
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Global Energetics
• Add up all the energy produced by AGN over the history of the universe
• Compare this to total mass in black holes today• Consistent with E ~ 0.1 M c^2
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The Eddington Limit
• The maximum luminosity is set by requirement that gravity (inward) exceeds radiation pressure (outward)
• Maximum luminosity L ~ 40,000 M when L and M are measured in solar units
• Observed AGN luminosities imply minimum black hole masses of ~million to a few billion solar masses
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EDDINGTON RATIOS
AGN obey the Eddington Limit!
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The Continuum Emission in AGN
• Optical-UV: broad feature (“Big Blue Bump”)• Hard X-rays• Infrared: broad feature
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The Accretion Disk
• Given the size (few to ten Schwarzchild radii) of the accretion disk and its luminosity, we expect thermal emission peaking in the far-ultraviolet
• The source of the “big blue bump”
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The Accretion Disk Corona
• Very hot gas responsible for the X-ray emission• X-rays irradiate the disk, which alters the X-ray spectrum
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The Infrared: Dust Emission
• Dust in the molecular torus absorbs optical/UV radiation from the accretion disk
• Dust heated to ~100 to 1000K. Emit in the IR• L_IR ~ L_UV: torus intercepts ~half the light
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Emission & Absorption Features
• Produced by the interaction of energetic photons with the surrounding gas
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The Accretion Disk
• Hard X-rays from corona illuminate the accretion disk and excite iron K-shell electrons
• Subsequent decay produces Fe K-alpha line at 6.4 keV• Broadened by relativistic effects (Doppler and
gravitational redshift)
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The Broad Emission-Line Region
• Gas clouds moving at several thousand km/sec• These appear to be orbital motions (gravity)• Gas is photoionized by radiation from the
accretion disk and its corona
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Reverberation Mapping
• Measure the time lag in response of BLR clouds to changing ionizing flux from the accretion disk
• Implied sizes range from light weeks in low power AGN to light years in powerful ones
• Size plus velocity yield black hole mass
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Broad & Narrow Absorption-Lines
• High velocity outflows (up to ~0.1c)• Sizes are uncertain: similar to BLR? (<torus)• Small sizes imply modest kinetic energy
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The Narrow Emission-Line Region
• Gas located ~kpc from the black hole• Photoionized by radiation escaping along the
polar axis of the torus
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The Narrow Emission-Line Region
• Orbits in the potential well of the galaxy bulge (velocities of hundreds of km/sec)
• Distinguished from gas excited by hot stars by its unusual ionization conditions and high T
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THE [OIII] LINE AS A PROXY FOR THE BOLOMETRIC LUMINOSITY
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Radio Sources
• A highly collimated flow of kinetic energy in twin relativistic jets that begin near the black hole and transport energy to very large scales
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Synchrotron Radiation
• Requires relativistic electrons and magnetic field
• Indicated by the high degree of linear polarization and power-law spectral energy distribution
• Total required energy can exceed 10^60 ergs in extreme cases
• Bulk KE in jet used to accelerate particles in strong collisionless shocks
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Morphology
• Lower power jets: maximum brightness nearest the nucleus. KE dissipated gradually (“FR I”)
• Very powerful jets: maximum brightness at termination point of jet (“FR II”)
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Evidence for Relativistic Velocities
• “Superluminal” velocites (v ~ 3 to 10 c)• Due to time dilation when a relativistic jet is
pointing close to the line-of-sight• “Doppler boosting”: we see only the approaching
side of the twin jet
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Radio Jets: Energetics based on cavities inflated in the hot ICM
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Classification & Unification
There are three basic factors that determine the observed properties of an AGN and its classification:
The relative rate of the kinetic energy transport in the jet compared to the radiative bolometric luminosity
The orientation of the observerThe overall luminosity
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Radio-loud vs. Radio-quiet AGN
• Two primary independent modes in the local universe• Radio-quiet AGN: high accretion rates in lower mass BH• Radio-loud AGN: low accretion rates in higher mass BH
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• Our view of the basic building blocks depends on orientation relative to the torus
• UV/Optical/soft X-rays & BLR blocked by the torus• Hard X-rays: torus can be optically thick or thin• IR from the torus and NLR emitted ~isotropically
Orientation: Radio Quiet AGN
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Example: Optical Spectra
• View “central engine” directly in “Type 1” AGN• Central engine occulted in “Type 2” AGN• Still see the NLR, but continuum is starlight
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Orientation: Radio Loud AGN
• Typical orientation: a “radio galaxy”
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Orientation: Radio Loud AGN
• Viewed close to the jet axis we see a “Blazar”• Entire SED dominated by Doppler boosted
nonthermal emission from the compact jet• Emission peaks in Gamma-rays & varies rapidly
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Luminosity
• Lower power Type 1 AGN are called Type 1 Seyfert galaxies. L_AGN < L_Gal
• High power Type 1 AGN are called quasars or QSOs (quasi-stellar objects). L_AGN > L_Gal
• No real physical difference other than luminosity
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Luminosity range in type 2 AGN
• Type 2 Seyferts: lower power AGN• Type 2 Quasars: higher power AGN
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Luminosity & Radio Galaxies
• Lower power jets: maximum brightness nearest the nucleus. KE dissipated gradually (“FR I”)
• Very powerful jets: maximum brightness at termination point of jet (“FR II”)
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Radio-Loud Quasars
• The nuclei of very strong radio sources (FR II’s) strongly resemble ordinary radio-quiet quasars
• These are the FR II’s in which we look near the polar axis of the torus
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The Lowest Luminosity AGN
• Low Ionization Nuclear Emission-Line Regions• LINERs are found in nearly all nuclei of bulge-
dominated galaxies• They appear to be “dormant” black holes
accreting at very low rates (L << L_Edd)
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THE CO-EVOLUTION OF GALAXIES & BLACK HOLES
The rate at which black holes grew via accretion (as AGN) was very much higher in the early universe
A similar trend is seen in rate at which galaxies grew via star formation
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BLACK HOLE MASS STRONGLY LINKED TO HOST PROPERTIES
Marconi & Hunt Tremaine et al.
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The Connection is to the Bulge Component of Galaxies
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The Local Galaxy “Landscape”
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Where do they live?
• They live in “hybrid” galaxies• Near the boundaries between the bimodal population• Structures/masses similar to early-type galaxies• Bulges: young stellar population
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Luminosity Dependence
• As the AGN luminosity increases the stellar population in the bulge becomes younger
• And the amount of dust/cold-gas increases
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Trigger: Morphology
• Usually ~normal early-type disk galaxies
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WHICH BLACK HOLES ARE GROWING?
• Mass resides in the more massive black holes• Growth dominated by less massive ones
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MASS-DOUBLING TIMES
• Only ~ Hubble Time for lower mass black holes• Orders-of-magnitude longer for the most
massive black holes (“dead quasars”)
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DOWNSIZING
The characteristic mass scales of the populations of rapidly growing black holes and galaxies have decreased with time in the universe. The most massive form earliest.
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Final Thoughts
AGN are important for several reasons:
> They have produced ~10% of all the luminous energy since the Big Bang
> They are unique laboratories for studying physics under extreme conditions
> They played a major role in the evolution of the baryonic component of the universe (galaxies and the inter-galactic medium)