dusty agn torii - leiden observatoryrottgering/site/agn...5 dust grains and photons typical size of...

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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?