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Outline: To Start … (I)

The Central Engine of AGNs, XiAn, 16-21 October 2006 Dust in Active Galactic Nuclei Aigen Li (University of Missouri-Columbia). Outline: To Start … (I). Dust obscuration plays an important role in the “Unified Theory of AGNs”;

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Outline: To Start … (I)

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  1. The Central Engine of AGNs, XiAn, 16-21 October 2006Dust in Active Galactic NucleiAigen Li(University of Missouri-Columbia)

  2. Outline: To Start … (I) • Dust obscuration plays an important role in the “Unified Theory of AGNs”; • orientation-dependent obscuration by dust torus  Seyfert 1 vs. Seyfert 2; • correct dust obscuration  interpret optical emission lines  probe the physical conditions; • IR emission accounts for ~10% of the bolometric luminosity of Type 1 AGNs, >50% of Type 2; • Heated dust  IR emission; • IR emission modeling  circumnuclear structure (critical to the growth of supermassive black hole);

  3. Outline: To Start … (II) • To correct for dust obscuration, to understand the observed IR emission, we need to know the nature of the dust in AGNs; • size, composition  extinction and IR emission properties; • The nature of the Milky Way interstellar dust • extinction dust size; • spectral features  composition; • IR emission  dust size, composition; • A comparative overview of the dust in AGNs and the Milky Way interstellar dust;

  4. Milky Way Interstellar Extinction: Grain Size • 2 grain populations: • a < 100 Å; • a>0.1 µm; • Characterized by RV=AV/E(B-V); • dense regions: larger RV; • larger RV larger grains; • 2175 Å bump • aromatic carbon; • small graphitic grains or PAHs;

  5. Infrared Emission: Grain Size and Composition • Classic grains • 100Å <a <3000 Å; • Td ~ 20K; • emit at λ>60 µm; • ~ 2/3 of total emitted power; • Ultrasmall grains: • PAHs (~10% C); • a<100 Å; • emit at λ<60 µm; • stochastic heating; • ~ 1/3 of total power;

  6. PAHs are ubiquitous in Astrophysical Environments also see Imanishi, Sturm’s talks

  7. PAHs are ubiquitous in space(Draine & Li 2006)

  8. PAHs in high-redshift galaxies ULIRGs (Yan et al. 2005) Luminous submm galaxy z~ 2.8 (Lutz et al. 2005)

  9. AbsorptionFeatures: Grain Composition • Silicate dust • 9.7 µm: Si-O stretching; • 18 µm: O-Si-O bending; • Amorphous;

  10. AbsorptionFeatures: Grain Composition • Ices • dense regions (Av>3 mag); • H2O 3.1, 6.0µm; • CO 4.68µm; • CO2 4.28, 15.2µm; • CH3OH 3.54, 9.75µm; • H2CO 5.81µm; • CH4 7.68µm; • NH3 2.97µm;

  11. AbsorptionFeatures: Grain Composition • Aliphatic hydrocarbon • 3.4 µm C-H stretching band; • diffuse ISM; • PPN CRL 618; • other galaxies;

  12. AGN Dust Extinction: flat/gray?  large grains? • Czerny et al. (2004): 5 SDSS composite quasar spectra flat extinction • Amorphous carbon with dn/da ~ a-3.5, 0.016≤ a ≤0.12μm; • Gaskell et al. (2004): 72 radio-loud, 1018 radio-quiet AGNs flat extinction; also see Czerny, Gaskell’s talks

  13. AGN Dust Extinction: flat/gray?  large grains? • But Willott (2005): lumino.-dependent reddening biases in quasar composite spectra! • Gaskell & Benker (2006): HST spectra of 14 AGNs  flat extinction; mean extinction (radio-quiet; Gaskell & Benker 2006)

  14. AGN Dust Extinction: Lower E(B-V)/NH and AV/NH ratios large grains? Low-lum. AGNs • Maiolino et al. (2001):E(B-V)/NH for 16 AGNs smaller than the Galactic value by a factor of 3-100grain growth? • optical/near-IR emission lines  E(B-V) • X-ray absorp.  NH 

  15. AGN Dust Extinction: Lower E(B-V)/NH and AV/NH ratios large grains? • grain growth flat extinction, and lower E(B-V)/NH and AV/NH ratios; • circumnuclear region: high density  grain growth through coagulation can occur; • But, Weingartner & Murray (2002): X-ray absorp. and optical extinction may occur in distinct media? 

  16. AGN Dust: Composition lack of 2175 Å extinction bump depletion of small graphitic grains/PAHs? Li & Draine 2001: PAHs  2175 Å bump Maiolino et al. 2001

  17. AGN Dust: Composition silicate dust • Unified scheme of AGNs expect to see silicate emission in Seyfert 1, silicate absorption in Seyfert 2; silicate absorption through cold outer regions of torus silicate emission from hot inner regions of a dusty torus

  18. AGN Dust: Composition silicate dust • Spitzer/IRS detected silicate emission in both quasars and low-luminosity AGN(Hao et al., Siebenmorgen et al, Sturm et al., Weedman et al. 2005); also see Hao, Schmeitzer’s talks Hao et al. 2005

  19. AGN Dust: Composition silicate dust NGC 1068: Jaffe et al. 2004 • silicate absorption at 9.7mm with various strength seen in Seyfert 2 (Roche et al. 1991); • spatially resolved AGNs (Circinus, NGC 1068): silicate absorption strength varies (Jaffe et al., Mason et al., Rhee & Larkin, Roche et al.)

  20. NGC 1068 along slit PA = -8 deg. Rhee & Lakin Also see Mason et al. Mason: P-iD-52 NGC 1068, 12mm image , Bock et al. 2000, ApJ, 120, 2904

  21. AGN Dust: Composition silicates differ from Milky Way ISM? large shift in the center of the 9.7mm silicate featureshift to 10.5mm in Mrk 231 Mrk 231 (Roche et al. 1983)

  22. AGN Dust: Composition silicates differ from Milky Way ISM? non-olivine MgFeSiO4 composition ? calcium aluminium silicate Ca2Al2SiO 7 ? NGC 1068 (Jaffe et al. 2004)

  23. AGN Dust: Composition silicates differ from Milky Way ISM? large shift in the center of the 9.7mm silicate featureshift to 11mm in NGC 3998; Much broader than the Galactic silicate feature;  Large grains? elongated grains? different composition? Rad.transf. effects? NGC 3998 (low-luminosity AGN) (Sturm et al. 2005)

  24. AGN Dust: Compositionaliphatic hydrocarbon dust(3.4μm C-H stretching absorption feature) NGC 1068 (Mason et al. 2005) also see Imanishi’s talk ubiquitously seen in different environments; They all look very similar; They are seen in AGN dust torus  in AGN SED modeling, hydrocarbon dust should be included;

  25. AGN Dust Composition: no ices! • Widely seen in Galactic dense molecular clouds with Av>3 mag; • Seen in most ULIRGs; • Never seen in AGNs! • Td>100K even at 100pc from the central engine; • Water ice sublimates at Td~100K; •  Water ice can not survive in AGN torus;

  26. AGN Dust Composition: no PAHs! Le Floc’h et al. 2001 • PAHs are ubiquitously seen in various Galactic, extragalactic environments; • But PAHs are not seen in AGNs PAHs are photodestroyed by hard X-ray/UV photons;

  27. PAHs are ubiquitous in galaxies! Milky Way Galaxy M82: Starburst galaxy

  28. PAHs as a tracer of AGN/starburst contributor in ULIRGs (Genzel et al. 1998)

  29. PAHs are deficient in low-metallicity galaxies SBS0335-052 1/40 solar metal. IRS spectrum (Houck et al. 2004)

  30. Photophysics of PAHs For small PAHs, if # of vibrational degrees of freedom not able to accommodate the absorbed photon energy  photodest! • Photoabsorption  vibrational excitation, photoionization, photodestruction Draine & Li 2001 Li 2004, 2006

  31. In some Seyfert 2, PAHs are detected  PAHs are from the circumnuclear star-forming region, not from AGN! NGC 1068 (Le Floc’h et al. 2001) Starburst ring (r~1.5 kpc) spatial res. 5”

  32. In some Seyfert 2, PAHs are detected  PAHs are from the circumnuclear star-forming region, not from AGN! ISOPHOT: 24” Sibenmorgen et al. 2004 TIMMI2: 3”

  33. Dust IR Emission Spectral Energy Distribution Modeling • Key parameters to be specified or determined: • dust spatial distribution (geometry of the dust torus); • dust properties (size distribution, composition) : interstellar?  constraints from extinction studies; • Pier & Krolik (1992, 1993):a uniform annular (cylindrical) ring of dust, interstellar silicate-graphite mixture with dn/da ~ a-3.5, 50Å≤ a ≤0.25μm, neglected scattering,  predicted SED too narrow. • Laor & Draine (1993): optically thick plane parallel dust slab, silicate-graphite mixture or SiC-graphite mixture  small silicate or SiC grains depleted, or grains are large (a ≤10μm)  to supress the 9.7μm silicate emission feature.

  34. Dust IR Emission Spectral Energy Distribution Modeling • Granato & Danese (1994): an extended dust torus (100pc), silicate-graphite mixture;  to supress the 9.7μm silicate emission feature  silicates destroyed by shocks in the inner ~10pc. • Rowan-Robinson (1995): a geometrically thin, optically thick spherical shell; • Efstathiou & Rowan-Robinson (1995), Stenholm (1995): tapered disk; • Manske et al. (1998): optically thick, flared dust disk, silicate-graphite mixture  to supress the 9.7μm silicate emission feature  anisotropic radiation source (large optical depth); • Nenkova et al. (2002), Elitzur (2006):clumpy torus model, with interstellar dust mixture  broad SED, no silicate emission;

  35. Dust IR Emission Spectral Energy Distribution Modeling • Models are getting more and more complicated … • van Bemmel & Dullemond (2003): varying geometry, surface density, grain opacity, size distribution… • Schartmann et al. (2005): 3D rad.trans., stratification of dust spatial distribution (size/composition) spatial distribution… • Hoenig (P-ID 246)…

  36. Summary • Dust is important for AGN studies • obscuration correction; • probing physical conditions, circumnuclear structure; • Dust extinction: flat (even “gray”)  large dust grains; • 2175Å extinction bump: no  small graphite/PAHs destroyed? • Silicate dust • 9.7, 18μm emission seen in Seyfert 1, absorption seen in Seyfert 2  consistent with the unified theory; • 9.7μm feature shifts to longer wavelength, broader  different composition? size and/or shape effects? Rad.trans. effects? • Aliphatic hydrocarbon dust: 3.4μm absorption feature closely resembles that of Milky Way; • Ices: no  AGN torus too warm so that water ice sublimates; • PAHs: no  X-ray/UV photons destroy PAHs;

  37. Summary • Dust IR emission SED modeling • dust spatial distribution; • AGN extinction  dust size distribution; • IR absorption/emission spectra  dust composition and size; • Dust destruction (by sublimation, shocks) and coagulational growth needs to be investigated  dust size distribution;

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