Dusty AGN torii. Ionization cones: toroidal obscuration

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Alice Lamb
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1 Dusty AGN torii Ionization cones: toroidal obscuration hst cone hst cone 1

2 A hidden seyfert I inside a Seyfet II Seyfert I/II statistics as a measure of H/R 2

3 Conclusion so far: unification model requires obscuration and indirect evidence supports the idea Astronomical dust 3

Astronomical dust resembles fumes from cars.

energetic photos can take a dust grain apart - Lab experiments

4 Astronomical dust resembles fumes from cars. Dust particles are delicate. - If the temperature of the dust grain is greater than the sublimation temperature (1500K) the dust 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 get destroyed - Around an AGN, typical dust velocities 200km/s 4

5 Dust grains and photons Typical size of a dust grain 1 nm - 1 µm Optical depth of a grain depends on it s size compared to the photon s wavelength. Therefore dust absorbs uv, blocks optical light and is transparent in the mid-ir (5-20 micron) Emission from dust Blackbody emission + molecular lines Resource : code DUSTY 5

6 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, Q abs Q sca : Q abs (x,m)=(absorption cross-section)/πa 2 Q scat (x,m)=(scattering cross-section)/πa 2 recall a general refractive index is complex, m = n - ik, where k tracks absorption. In the limit λ>>a: Mie theory gives: Q abs ~-4X Im(M)=f(λ -1 ), M=(m 2-1)/(m 2 +2) Q scat ~8/3X 4 M = f(λ -4 ) Q ext =Q abs +Q scat 6

7 Absorption/Emission from dust In the Q abs (x,m)=(absorption cross-section)/πa 2 mit λ>>a: Mie theory gives: Q abs ~-4X Im(M)=f(λ -1 ), M=(m 2-1)/(m 2 +2) Q scat ~8/3X 4 M = f(λ -4 ) Q ext =Q abs +Q scat For some absorption (m imaginary): Q ext ~Q abs =f(λ -1 ) Emission? Kirkhoff s law: Good (bad) absorbers are good (bad) emitters: F em (λ)=q em (λ)xb λ (T) Q em (λ)=q abs (λ) Since dust doet not absorb much in the IR, dust grains will be poor IR emitters: GREENHOUSE EFFECT 7

8 Molecular lines Why dusty torus? NGC

9 Why dusty torus? Circinus So far, so good NGC 1068 at 12micron As seen by a 10m telescope. 9

10 The idea of an extended (puffed up) structure orbiting an object for a long time is problematic Matter orbiting a compact object is expected to collapse into a thin disk 10

11 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 which has temperature increasing away from the observer. An emission feature is seen when an optically thick object has temperature decreasing away from the observer, or when the object is optically thin. Therefore: we expect the silicate feature to appear in absorption for type II AGNs and in emission for type I. The emission feature, however, has never been observed except very recently in quasars. although the question still remains why is it not seen in other type I objects 9.7 problem: example. 11

12 9.7 feature in emission Suggested solution to the thickness issue: Radiation pressure within the torus may be enough to support it Nuclear strarburst Supernova and turbulence Mechanical heating by stars Viscous heating Warped disks 12

13 Example: torus as a warped disk 13

other AGN components Clumpy / smooth dust

14 Open questions: geometrical structure Temperature structure Chemical composition Size Relation to other AGN components Clumpy / smooth dust distribution Origin of dust Example: radiative pressure Krolik

15 AGN modeling Nenkova et al The gaussian edge models predict that Seyfert galaxies may change types 15

16 Direct observations of AGN torii The small size of the torii compels us to use interferometry : the careful combination of light from two telescopes MIDI is the only instrument / telescope able to resolve these structures But.. MIDI is limited in magnitude! And therefore in #of objects 1. NGC

17 Circinus Centaurus A 17

Other objects Overview dusty torii: We discussed: Indirect evidence suggesting an opaque dusty structure envelops the AGN The properties of dust and

18 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 18

19 literature Dust-filled doughnuts in space Nature 429, (6 May 2004) The central dusty torus in the active nucleus of NGC Natur J Assignment 1. Read 2. What do the authors see as the problems with the standard torus model 3. What alternative do they propose? Give some relevant quantitative numbers of their model. 4. What issues does this model solve and what issues does it raise? 5. Do you agree with the authors that the wind model should be preferred over the standard model? 19

A Unified Model for AGN. Ryan Yamada Astro 671 March 27, 2006

A Unified Model for AGN. Ryan Yamada Astro 671 March 27, 2006 A Unified Model for AGN Ryan Yamada Astro 671 March 27, 2006 Overview Introduction to AGN Evidence for unified model Structure Radiative transfer models for dusty torus Active Galactic Nuclei Emission-line