an introduction to the agn phenomenon -...
TRANSCRIPT
![Page 1: An Introduction to the AGN Phenomenon - huji.ac.ilold.phys.huji.ac.il/~joaw/winterschool/heckman_1.pdfHard X-ray Pros and Cons • This comprises a significant fraction (~10%) of the](https://reader036.vdocuments.us/reader036/viewer/2022071406/60fb5fd04e51e515a707a702/html5/thumbnails/1.jpg)
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 lines
>Jets and radio emission >AGN Classification & Unification >Finding & characterizing AGN
> Cosmic Evolution
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1.What makes AGN Special?
• Very large luminosites are possible (up to 10,000 times the entire Milky Way)
• 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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2. 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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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 10^12 L_sun 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 (“Zoltan Argument”)
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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! They span broad range in Eddington ratio
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Characteristic Growth Time
• The “Salpeter Time” is defined as the time it would take for the black hole to double its mass if accreting at the Eddington limit:
• t = e/(1-e) x M c^2/L_Edd where e is the accretion efficiency
• t ~ 50 Myr for e = 10% • Real BH may or may not grow this rapidly
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3. 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) 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
• Compton scattering by hot gas produces X-ray emission • X-rays irradiate the disk, which alters the X-ray spectrum
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X-ray Reflection
Reynolds (1996)
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Fe K-Alpha Emission
• 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 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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4. Emission & Absorption Lines
• Produced by the interaction of energetic photons with the surrounding gas
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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 yields a black hole mass
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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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Absorption-Lines = AGN Outflows
• High velocity outflows seen in absorption (up to ~0.1c) in radio-quiet AGN
• Estimated outflow rates are small in typical AGN
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Outflows in Powerful AGN
• At very high (QSO-level) luminosities the outflow rates can be substantial (1-10% of the bolometric luminosity). Arav et al.
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• High-velocity molecular outflows seen in ULIRGs with AGN (Sturm et al. 2011)
• Galaxy-scale disturbed ionized gas in Type 2 QSOs (Greene et al. 2011)
Global Winds in Type 2 QSOs
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5. 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 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
• Bulk kinetic energy transported in the jets typically exceeds the observed synchrotron luminosity by two orders-of-magnitude
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6. 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 observer The 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: The “Unified Model”
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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 “off-axis” 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 & Nomenclature
• 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
• Higher luminosity mostly due to higher BH mass
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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 strongly resemble ordinary radio-quiet quasars
• These are the FR II’s in which we look near the polar axis of the torus rather than near the plane
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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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What are these objects?
>Radiatively Inefficient Accretion Flow (RIAF) or Advection Dominated Accretion Flow (ADAF) > t_cool >> t_flow
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7. Finding & Characterizing AGN
> For Type 1 (unobscured) AGN this is rather easy:
Hard & soft X-rays, UV/Optical (multi-color), mid-IR
> Much more challenging for Type 2 (obscured AGN)
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Proxy #1: The [OIII]5007 line
Arises outside the obscuring torus (“narrow line region”) Classification AGN vs. SF: emission-line ratios [OIII]5007 as AGN tracer Strongest AGN line and minimal contribution from SF
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[OIII]5007: Pros & Cons
• [OIII] 5007 line is easy to observe (>1 million SDSS galaxies and growing)
• An optical line is prone to dust extinction • Can correct for extinction using Balmer
decrement (implied corrections ~0 to 3 mags) • This requires excellent data • This will miss cases of very high extinction • Can be contaminated by SF contribution in
“Composite” objects
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Proxy #2: The [OIV] 25.9 micron line
Spitzer IRS: Type 2 AGN And Starburst
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[OIV] 25.9: Pros & Cons
• Like [OIII]5007 line, arises well outside the torus in NLR
• Far less affected by dust in the host galaxy • The brightest AGN-dominated line in the
mid-IR • Less potential contamination by SF regions
in composite objects • Relatively little data compared to [OIII]
(hundreds rather than millions)
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Proxy #3: The Mid-Infrared Continuum
UV & optical emission reprocessed by the dusty torus is about 25% of the bolometric luminosity in Type 1 AGN
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Mid-IR: Pros and Cons
• It is relatively easy to measure via photometry • The mid-IR emission from the torus is not
expected to be emitted in a completely isotropic fashion (better at longer wavelengths)
• Potential contamination from dust heated by stars (less contamination at shorter wavelengths)
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Proxy #3: Hard X-rays
• Produced in a hot corona surrounding the accretion disk
• Inside the torus and subject to its high column densities
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Effects of Column Density
> The 2-10 keV band is blocked by columns
~ 10^24: Chandra & XMM “hard band”
> Even emission above 10 keV is attenuated for columns > 10^25: Swift BAT band
“Compton Thick”
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Hard X-ray Pros and Cons • This comprises a significant fraction (~10%)
of the bolometric luminosity in Type 1 AGN • The hard X-rays penetrate the host galaxy
ISM • The 2-10 keV band emission is easily
detected in the Chandra and XMM surveys • Surveys at higher energies are currently very
shallow (Swift/BAT) • NuSTAR will survey the 10-40 keV band to
over 1000 times greater depth • The truly Compton-thick sources are still
affected even in the 10-40 keV band
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BLACK HOLE MASS ESTIMATES
Marconi & Hunt Gultekin et al. Assume these correlations hold generally (only
safe assumption at low-z)
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Black Hole Mass Estimates
• Use cases with black hole masses directly determined to calibrate a scaling relationship yielding BH mass from the AGN luminosity and BLR emission-line widths (Vestergaard & Peterson)
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8. COSMIC EVOLUTION OF AGN
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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Deep X-ray Surveys of AGN Evolution
Luminosity-dependent density evolution (LDDE)
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Bolometric QSO LF z=0.1
z=0.5
z=1.0
z=1.5
z=2.0
z=3.0
z=4.0
z=5.0
optical soft X hard X MIR
Hopkins et al. 2006
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Anti-Hierarchical Evolution G.H., Miyaji & Schmidt, 2005
Total emissivity follows star formation rate!
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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)