dusty agn torii - leiden observatoryrottgering/site/agn...5 dust grains and photons typical size of...
TRANSCRIPT
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Dusty AGN torii
Ionization cones: toroidalobscuration
hstcone
hstcone
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A hidden seyfert I inside a Seyfet II
Seyfert I/II statistics as a measure ofH/R
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Conclusion so far:
unification model requiresobscuration and indirect
evidence supports the idea
Astronomical dust
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Astronomical dust resembles fumesfrom cars.
Dust particles are delicate.
- If the temperature of the dust grain is greaterthan the sublimation temperature (1500K) thedust will get destroyed
- An energetic photos can take a dust grain apart- Lab experiments show that dust grain colliding
with relative velocities as low as 8m/s getdestroyed
- Around an AGN, typical dust velocities 200km/s
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Dust grains and photons Typical size of a dust grain 1nm - 1µm
Optical depth of a grain depends on it’s sizecompared to the photon’s wavelength.
Therefore dust absorbs uv, blocks optical lightand is transparent in the mid-IR (5-20 micron)
Emission from dust
• Blackbody emission + molecular lines
Resource :
code ‘DUSTY’
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Emission from dust
• Mie theory:• Being a classical theory, wavelength and grain
size enter only as their ratio : x=2πa/λ• recall a general refractive index is complex,• m = n - ik, where k tracks absorption.
Absorption/Emission from dust
• Absorption and scattering efficiences, Qabs Qsca :
• Qabs(x,M)=(absorption cross-section)/πa2
• Qscat(x,M)=(scattering cross-section)/πa2
• recall a general refractive index is complex,• m = n - ik, where k tracks absorption.
• In the limit λ>>a: Mie theory gives:• Qabs~-4X Im(M)=f(λ-1) , M=(m2-1)/(m2+2)• Qscat~8/3X4|M|= f(λ-4)• Qext=Qabs+Qscat
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Absorption/Emission from dust• In the Qabs(x,M)=(absorption cross-section)/πa2
• mit λ>>a: Mie theory gives:• Qabs~-4X Im(M)=f(λ-1) , M=(m2-1)/(m2+2)• Qscat~8/3X4|M|= f(λ-4)• Qext=Qabs+Qscat
For some absorption (m imaginary):
Qext~Qabs =f(λ-1)
Emission? Kirkhoff’s law: Good (bad) absorbers are good (bad)
emitters: Fem(λ)=Qem(λ)xB λ(T) Qem(λ)=Qabs(λ)
Since dust doet not absorb much in the IR, dustgrains will be poor IR emitters:
GREENHOUSE EFFECT
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Molecular lines
Why dusty torus?
NGC 1068
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Why dusty torus?
Circinus
So far, so good…
NGC 1068at 12micron
As seen bya 10mtelescope.
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The idea of an extended(puffed up) structure orbitingan object for a long time is
problematic
Matter orbitinga compactobject is
expected tocollapse into a
thin disk
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9.7µm feature problem first we have to remind ourselves of some basic features of emission
/absorption:
– An absorption feature is seen when viewing an optically thick object whichhas temperature increasing away from the observer.
– An emission feature is seen when an optically thick object hastemperature decreasing away from the observer, or when the object isoptically thin.
Therefore: we expect the silicate feature to appear inabsorption for type II AGNs and in emission for type I.The emission feature, however, has never beenobserved except very recently in quasars. althoughthe question still remains why is it not seen in othertype I objects
9.7 problem: example.
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9.7 feature in emission
Suggested solution to the thicknessissue:
Radiation pressure within the torus may beenough to support it
Nuclear strarburst Supernova and turbulence Mechanical heating by stars Viscous heating Warped disks
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Example: torus as a warped disk
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Open questions:• geometrical structure• Temperature structure• Chemical composition• Size• Relation to other AGN
components
Clumpy / smooth dustdistribution
Origin of dust
Example: radiative pressure
Krolik 2007
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AGN modeling
The gaussian edge models predict that Seyfert galaxies maychange types
Nenkova et al
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Direct observations of AGN toriiThe small size of the torii compels us touse interferometry : the carefulcombination of light from two telescopes
MIDI is the onlyinstrument /telescope able toresolve thesestructures
But.. MIDI is limitedin magnitude ! Andtherefore in #ofobjects
1. NGC 1068
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Circinus
Centaurus A
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Other objects
Overview dusty torii:
We discussed: Indirect evidence suggesting an opaque dusty
structure envelops the AGN The properties of dust and its interaction with
other dust particle and with light The problems with the torus and suggested
soluons Latest observational evidence for torii
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literature
http://www.astro.virginia.edu/class/whittle/astr553/Topic09/Lecture_9.html
Dust-filled doughnuts in space Nature 429, 29-30 (6 May 2004)
The central dusty torus in the active nucleus ofNGC 1068 2004Natur.429...47J
Assignment
1. Read
2. What do the authors see as the problems with the standard torusmodel
3. What alternative do they propose? Give some relevantquantitative numbers of their model.
4. What issues does this model solve and what issues does itraise?
5. Do you agree with the authors that the wind model should bepreferred over the standard model?