The Central Engine of AGNs, XiAn, 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”; 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; 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);

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; 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 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; A comparative overview of the dust in AGNs and the Milky Way interstellar dust;

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

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

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

PAHs are ubiquitous in space (Draine & Li 2006)

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

Absorption Features: Grain Composition Silicate dust Silicate dust –9.7 µm: Si-O stretching; –18 µm: O-Si- O bending; –Amorphous;

Absorption Features: Grain Composition Ices Ices –dense regions (A v >3 mag); –H 2 O 3.1, 6.0µm; –CO 4.68µm; –CO , 15.2µm; –CH 3 OH 3.54, 9.75µm; – H 2 CO 5.81µm; –CH µm; – NH µm;

Absorption Features: Grain Composition Aliphatic hydrocarbon Aliphatic hydrocarbon –3.4 µm C-H stretching band; –diffuse ISM; –PPN CRL 618; –other galaxies;

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

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

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

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

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

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

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); 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); Hao et al also see Hao, Schmeitzer’s talks

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

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

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

AGN Dust: Composition silicates differ from Milky Way ISM? non-olivine MgFeSiO 4 composition ? calcium aluminium silicate Ca 2 Al 2 SiO 7 ? NGC 1068 (Jaffe et al. 2004)

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

AGN Dust: Composition aliphatic hydrocarbon dust (3.4μm C-H stretching absorption feature) 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; NGC 1068(Mason et al. 2005) also see Imanishi’s talk

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

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

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

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

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

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

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”

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

Dust IR Emission Spectral Energy Distribution Modeling Key parameters to be specified or determined: 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. 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 ≤10 μm)  to supress the 9.7μm silicate emission feature. 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 ≤10 μm)  to supress the 9.7μm silicate emission feature.

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. 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; Rowan-Robinson (1995): a geometrically thin, optically thick spherical shell; Efstathiou & Rowan-Robinson (1995), Stenholm (1995): tapered disk; 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); 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; Nenkova et al. (2002), Elitzur (2006): clumpy torus model, with interstellar dust mixture  broad SED, no silicate emission;

Dust IR Emission Spectral Energy Distribution Modeling Models are getting more and more complicated … 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)…

Summary Dust is important for AGN studies Dust is important for AGN studies – obscuration correction; – probing physical conditions, circumnuclear structure; Dust extinction: flat (even “gray”)  large dust grains; Dust extinction: flat (even “gray”)  large dust grains; 2175Å extinction bump: no  small graphite/PAHs destroyed? 2175Å extinction bump: no  small graphite/PAHs destroyed? Silicate dust 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; Aliphatic hydrocarbon dust: 3.4μm absorption feature closely resembles that of Milky Way; Ices: no  AGN torus too warm so that water ice sublimates; Ices: no  AGN torus too warm so that water ice sublimates; PAHs: no  X-ray/UV photons destroy PAHs; PAHs: no  X-ray/UV photons destroy PAHs;

Summary Dust IR emission SED modeling 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; Dust destruction (by sublimation, shocks) and coagulational growth needs to be investigated  dust size distribution;