Dust Extinction in the Diffuse Interstellar Medium Karl D. Gordon Astronomer STScI, Baltimore, MD ASIAA Colloquium 1 Nov 2022 [email protected] @karllark2000 karllark@github “Have Dust – Will Study” Slides on speakerdeck
Summary ● Extinction measurements are fundamental to understanding dust ● Spectroscopic Extinction from FUV to MIR in Milky Way – Overall quite smooth – Few broad features including some new ones – Intriguing correlation, comforting non-correlations – One R(V) extinction relationship for all wavelengths ● Beyond Milky Way – Mostly like the the Milky Way, but not all ● Future w/ HST and JWST – More galaxies, more environments, larger samples
Extinction ● Critical clues to dust grain abundance, size, composition, and shape ● Clues in the continuum and features ● Straightforward measurement ● Focus on spectroscopic measurements ● Biased by my views, not comprehensive
Not Covered ● Dust scattering (e.g., albedo, scattering phase function) ● Dust emission ● Atomic composition of dust (e.g., depletions) ● Dust Polarization ● Dust grain models ● All important, but not the focus of this talk
Extinction (not Attenuation) ● Extinction – Absorption and scattering out of line-of-sight – Specific to a point source dust – Proportional to dust grain properties ● Attenuation – scattering into the line-of-sight – Varying extinction to stars – Applies to galaxies, circumstellar dust, etc.
Basic measurement is the color “excess” versus wavelength (in magnitudes as we are astronomers) Normalize so measurements with different dust columns can be compared A(V) determined by extrapolating to infinite wavelength e.g.,
Wavelength axis variations Allow full FUV-MIR extinction to be seen Wavelength scale is proportional to energy Emphasizes UV wavelengths Versus λ Versus 1/λ UV Opt NIR MIR
Diffuse vs Dense Dust ● Diffuse sightlines – Most detailed extinction studies – Generally A(V) values less than a few – No 3.0 μm H2 0 ice feature ● Dense sightlines – Generally studied in the NIR/MIR only – Have 3.0 μm H2 0 ice feature Decleir et al. (2022, ApJ, 930, 15) H20 Ice
FUV rise extinction component & H2 column Very strong correlation & goes through zero!!! Grain responsible for FUV rise and H2 co-spatial Not the case for the 2175 Å bump
Broad Optical Features (origin unknown) Long known Very Broad Structure Explained! Massa, Fitzpatrick, Gordon, et al. 2020, ApJ, 891, 67 Centers: 4370, 4870, & 6300 Å Widths: ~10% Two blue correlate w/ 2175 Å
3.0 μm ice feature measured in the diffuse average at A(ice)/A(V) = 0.0019 +/- 0.007 Not a significant detection, but intriguing at the level predicted by Potapov et al. (2021) if ice is present in shadowed pits in silicate grains
MW Extinction Summary ● Spectroscopic from FUV to MIR – FUV smooth & consistent with feature peaking ~800 Å – H2 correlates with FUV rise → co-spatial! – New optical features → unknown origin – NIR powerlaw & may contain H2 0 ice – MIR versus UV features → 2175 Å not due to silicates – R(V) dependent extinction relationship → on relationship for all wavelengths
SMC Extinction Curves Milky Way-like! (2175 Å bump) 4 similar curves are found in the star forming bar of the SMC! Ha image of the SMC Gordon & Clayton (1998, ApJ, 500, 816) Gordon et al. (2003, ApJ, 594, 279) STIS
SMC AzV 456 SMC Bar LMC2 LMC General MW Quiescent Processed Continuum of Properties Gordon et al. (2003, ApJ, 594, 279) Gordon et al. (2016, ApJ, 826, 104)
M31/M33 Extinction ● All w/ HST photometry ● Similar to MW ● Radial, metallicity variations ● 1st measurements in M33 Petia Yanchulov Merica-Jones Clayton, G et al., in prep Yanchulov M-J, P. et al., in prep
Beyond the MW Summary ● Large Magellanic Cloud (½ solar) – Most of LMC similar to MW – Differences in LMC2 Supershell (near 30 Dor) ● Small Magellanic Cloud (1/5 solar) – Most very different from MW (very steep, no bump) – Small fraction, closer to MW (flatter w/ bump) ● M31 (~solar) and M33 (½ solar) – Similar to MW (preliminary)
Summary ● Extinction measurements are fundamental to understanding dust ● Spectroscopic Extinction from FUV to MIR in Milky Way – Overall quite smooth, few broad features, DIBs (dust?) only narrow features – New broad features found in the optical – Intriguing correlations (or not) between features and gas tracers (esp. H2 ) – One R(V) extinction relationship for all wavelengths ● Beyond Milky Way – Most of LMC, small fraction of SMC, M31, & M33 → similar to Milky Way – SMC & LMC2 Supershell region → weaker/non-existent bumps, stronger UV slope ● Future w/ HST and JWST – More galaxies, more environments, larger samples
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