Interstellar Dust and Extinction

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1 University of Oxford, Astrophysics November 12, 2007

2 Outline Extinction Spectral Features Emission Scattering Polarization Grain Models & Evolution Conclusions

3 What and Why? Dust covers a range of compound molecules with widely varying absorption, emission and scattering properties. Grain sizes range from µm and composes 1% by mass of the Interstellar Medium (ISM). Dust absorbs optical and ultra-violet (UV) light, scatters optical - x-rays and emits in the infra-red (IR). IR emission provides 30% of the flux of a typical galaxy (e.g. Milky Way). Two-thirds of this is in the far infra-red (FIR) (λ 50µm) from cold, larger grains ( 20K, 0.01µm) near early-type stars. The remaining third is from smaller grains ( 0.005µm) cooling from photo-absorption. Large dust grains provide the most efficient source of cooling in the ISM and provide UV-free surfaces for chemical reactions to occur, including formation of the H 2 molecule.

4 Extinction Spectral Features Emission Scattering Polarization Grain Models & Evolution Conclusions Definitions

5 Extinction Definitions Extinction = Absorption + Scattering Extinction is commonly expressed as A(λ) A(V ) A(λ) = 2.5log( F 0 λ F λ ) E(λ V ) = A(λ) A(V ) or E(λ V ) E(B V ) where An extinction law is a measure of the variation of extinction with wavelength. These are usually defined by the parameter R v A V E(B V ) = A v (A B A V )

6 Extinction Law R v A V E(B V ) = A v (A B A V ) For large grains R v For Rayleigh scattering with A λ λ 4 R V 1.2 In the galactic diffuse ISM and Large Magellanic Cloud R V 3.1 varying along sightlines from 2.1 to 5.8 In the Small Magellanic Cloud R V 3.2, but without the 2175Å feature.

7 Extinction Law

8 Extinction Laws around the Universe (Czerny 2006) (Vote for the stupidest one!) Milky Way: R V 3.1, prominent 2175Å feature SMC: R V 3.2, no 2175Å feature Starburst Galaxies, lensing galaxies: SMC-like, but less steep at shorter λ Quasars: R V from 0.7 to 5.5 where found, some with 2175Å feature. Some show flatter and greyer curves than MW and SMC, but may be resolved to SMC accounting for bias. The lack of a 2175Å feature is associated with: low metallicity, strong radiation fields and/or large densities.

9 Measuring Extinctions The Pair Method: The most reliable method of measuring extinction. Spectrophotometry of two stars of the same spectral class are compared: one with negligible foreground dust and the other heavily reddened. This method assumes that dust extinction goes to zero at long wavelengths. This method is also used for galaxies that overlap other galaxies and gravitationally lensed QSOs. Multi-band photometry: Based on fitting a known extinction curve, or fitting a theoretical one, to photometric data.

10 Tracing the Dust Moving from optical to near-ir in the DR21 Starforming region (Spitzer)

11 2175 Å Feature and Diffuse Interstellar Bands 2175Å feature: A broad absorption feature associated with aromatic (no long-range order) Carbon, particularly some form of graphite, from oscillator-strength calculations. Seen with varying strength in this galaxy and others, but not observed in the Small Magellanic Cloud or in circumnuclear dust around AGN. Diffuse Interstellar Bands: Small finely-structured absorptions mainly seen in the optical. Sometimes correlated with the 2175Å UV band (Desert et al. 2005) and 6614Å bands show structure consistent with rotational bands in molcules (Kerr et al 1996, 1998). Possibly of PAH compostion.

12 2175 Å Feature and Diffuse Interstellar Bands DIBs (Kerr et al. 1998) and Extinction Curves (Fitzpatrick 1999)

13 Silicate and Hydrocarbon Absorption Silicates: At 9.7µm and 18µm. 9.7µm feature associated with the Si-O stretching mode, while 18µm feature associated with O-Si-O bending mode. 3.4µm Feature: Present in diffuse atomic regions and linked with the C-H stretching mode in aliphatic (chain-like) hydrocarbons. Exhibits substantial substructure allowing carbon-soups to be tested in the laboratory. Ice Features: In dense molecular clouds a 3.1µm feature is observed, due to O-H stretching in solid H 2 O. In the Taurus dark cloud complex, this was measured to be present only in regions where A V 3.3mag. There is also evidence for CO 2, NH 3, CO, CH 3 OH, CH 4 and others.

14 Silicate Absorption The 9.7µm silicate absorption in two galaxies (Roche 2007)

15 3.4µm Absorption The 3.4µm hydrocarbon absorption in two galaxies (Mason 2004)

16 Polycyclic Aromatic Hydrocarbons (PAHs) PAH observations are concentrated in 5 emission features: C-H stretching mode at 3.3µm C-C stretching mode at 6.2µm C-C stretching mode at 7.7µm C-H in-plane bending mode at 8.6µm C-H out-of-plane bending mode at: 11.3µm (no adjacent H), 12.0µm (2 contiguous H), 12.7µm (3 H), 13.55µm (4 H) PAHs are observed to be particularly strong in star forming regions: hence the now often-used correlation between PAH activity and star formation.

17 Polycyclic Aromatic Hydrocarbons (PAHs) The PAH bands in the Mid-IR (Mason 2007)

18 Dust Emission A movie comparing optical and IR emission in the Carina Nebula (around Eta Carinae) (Spitzer)

19 Dust Emission Infra-red emission: Except for the highest densities, dust heating is by starlight photons of various energies. For large grains with radii 200 Å this heating can be approximated as steady and the cooling a steady thermal process. For smaller grains the heating can be erratic and highly quantized excitation and emission. These nanoparticles account for 35% of starlight re-emission. (Witt 1999) Extended Red Emission (ERE): also known as Unidentified Infra-red Bands (UIBs). These carry at least 10% of optical/uv photons. The ERE carrier must be a very efficient photoluminescent. The best candidates are HACs (Hydrogenated Amorphous Carbon) and PAHs, but both have problems (Witt 1999).

20 Scattering Optical and UV: dependent on the grain albedo and the scattering assymetry factor g cosθ. Appears to be a decrease in the albedo from optical to UV; a rise in g from optical to UV; consistent values with carbonaceous/silicate grain models (Witt et al 1992) X-Ray: scattered through small scattering angles causing a halo effect around x-ray sources. The observation of this effect allows tight constraints to be place on grain models: grain sizes and morphology.

21 Polarization In optical and UV the Serkowski law empirically gives the degree of polarization due to aligned dust grains (Serkowski 1973,1975): p(λ) = p max exp( K(ln(λ/λ max )) 2 ) where λ max 5500Å and K 1. In IR one can observe the polarization in the absorption and emission bands for preferential alignments. This can constrain grain models that require co-existing populations of different grain types, or silicate/hydrocarbon grains composites. Evidence so far indicates that the polarization of the 9.7µm feature does not correlate with that of the 3.4µm feature (Mason 2007).

22 What is the dust made of? An artist s impression of a dusty disc around a young star (Spitzer)

23 Candidate Grain Materials Silicates: at least 95% amorphous (Li & Draine 2002), expected to be predominantly Mg or Fe compounds from cosmic abundances. Where crystalline silicates are found, they are Mg rich (Tielens 1998). Carbonaceous Materials: including diamond, graphite, amorphous/glassy carbon, hydrogenated amorphous carbon, PAH, aliphatic hydrocarbons. SiC: Found in meteorites and an 11.3µm feature in carbon stars. Line extinctions suggest that less than 5% Si in dust is in this form. Carbonates: Found in dusty discs, but estimated to contribute less than 1% of dust mass.

24 Grain Models A comprehensive grain model does not exist that can fully describe dust grains and their properties.

25 Grain Models A model of grain sizes and populations (Draine 2003)

26 Grain Evolution? Stellar Outflows: Dust is observed in stellar outflows and some dust clearly must condense from gas in the stellar envelope. Similarly this occurs around AGN and other gas-shock phenomena. ISM Processing: Studies of the destruction of dust grains suggest a residence time of order yr. Taking the mass of the ISM as 5 9 M and a star formation rate of 5M yr 1 then the mean residence time of a metal atom in the ISM is 10 9 yr. Only a fraction (0.2) of the Si atoms in the ISM would still be in the original dust particle in which they left the star. However, over 90% of Si is missing from the gas phase: there must be reprocessing in the ISM.

27 Dust: You can t ignore it! An artist s impression of a heavily obscured galaxy, moving from optical to the IR (Spitzer)

28 Conclusions Dust causes extinction in the optical and UV bands which can be modelled by a standard curve for which one only needs to measure/estimate R V = A V E(B V ) to fit. R V = 3.1, on average, in the Milky Way. The composition of the dust is estimated through absorption and emission features and atomic calculations. This can then tell us about the region the dust is in. The best candidates are silicates and carbonaceous materials. Dust is a catalyst for interstellar chemistry. Dust models need to reproduce the UV, optical and IR absorptions, the polarizations, scattering coefficients, IR and PAH emissions and fit elemental abundances to be taken seriously.