1. Dust Extinction - Overview
Árdís Elíasdóttir
Department of Astrophysical Sciences
Princeton University
Current Problems in Extragalactic Dust
NBI, Copenhagen Denmark
Department of Astrophysical Science, Princeton University

2. Outline Local extinction curves High-z extinction curves GRBs Lenses
SNe
Galaxies
QSOs
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3. LOCAL EXTINCTION CURVES
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4. Dusty Worlds
Dust between the stars in galaxies causes the dimming of light from background sources
Extinction curves measure this dimming as a function of wavelength
Traditionally measured by comparing two stars of the same spectral type
Important for
Galaxy formation studies
Dark energy surveys
Well determined for the Milky Way
Very little known about extragalactic dust extinction
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5. The Galactic Extinction Curve
“Reddening”
(Cardelli et al. 1989)
Empirically determined
Mean value is RV = 3.1 (blue)
Extreme values: RV = 1.8 (green) and RV = (red) (Cardelli et al. 1989, Fitzpatrick et al. 1999,Udalski 2003)
Larger RV -> larger dust grains
Bump at 2175 Å (4.6 m-1)
Unknown origin
Graphite?
PAHs?
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6. Other nearby galaxies
LMC: Smaller bump and steeper rise into the UV (Nandy et al. 1981)
SMC: No bump, well fitted by A(λ)  1/ λ (Prevót et al. 1984)
M31: Average Galactic extinction law (Bianchi et al. 1996)
Graph from Pei (1992)
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7. Why measure higher redshift extinction curves?
From the four examples we know, we see that extinction curves can vary greatly
When analysing data where extinction needs to be accounted for the galactic extinction curve is frequently assumed
Examine if dust, and hence extinction, varies with z and galaxy type
So, how do we do it?
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8. HIGH-z EXTINCTION CURVES
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9. High-z Extinction - GRBs
The intrinsic powerspectrum of the GRB is described by a powerlaw (or a broken powerlaw)
Deviation from the slope is due to absorption along the line of sight
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10. High-z Extinction - GRBs
Typically SMC extinction (e.g. Jakobsson et al. 2004, Kann et al. 2005)
A detection of the 2175 bump at z=2.45
Can we locate tracers of the bump?
Metallicity? - No.
UV radiation field - CI lines (talk by Malesani)?
Dust to gas ratio?
(Kann et al. 2005)
(ÁE et al. 2009)
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11. High-z Extinction - Lenses
Compare two images, where ideally one should suffer no extinction and the other go through the galaxy
For more than doubly imaged quasars have the possibility of getting more than one curve for the lensing galaxy
Optimized for redshifts z=0-1 - future surveys will reach z2-3
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12. High-z Extinction - Lenses
SBS
Double
zl = 0.83; zQ = 1.38
Extinction:
E(B-V) = 0.21 ± 0.02
RV = 2.1 ± 0.9
A(V)  0.44
Strong detection of the 2175 Å bump
(Motta et al. 2002)
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13. Extinction along both lines of sight
Method measures a differential extinction curve
Galactic extinction:
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14. Extinction along both lines of sight
The deviation of the real RV to the deduced RVdiff is given by , where:
(ÁE et al. 2006)
Does NOT give a large systematic bias for mean RV values!
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15. VLT Survey 10 systems, (5 doubles and 5 quads)
Broad wavelength coverage (U,B,V,R,I,z’,J,H,Ks )
3 late type, 7 early type galaxies
Lens redshift zl=
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16. High-z Extinction - LensesB
Double
Late type galaxy
zl = 0.44; zQ = 1.02
Extinction
RV = 2.1 ± 0.1
A(V) = ± 0.09
(ÁE et al. 2006)
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17. High-z Extinction - LensesMG
Quad
Early type
zl = 0.96; zQ = 2.64
Extinction (for both A2-B and A2-C):
A(V) = 0.9 ± 0.1
RV = 2.7 ± 0.2
(ÁE et al. 2006)
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18. High-z Extinction - LensesHE
Double
Late type galaxy
zl = 0.93; zQ = 1.57
Extinction
A(V) = 0.14 ± 0.04 (for MW extincton)
RV = 1.7 ± 0.4
(ÁE et al. 2006)
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19. Dust to gas ratios (Dai+Kochanek 2009) (ÁE et al. 2009)
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20. Redshift dependence?
See no strong correlation between extinction properties and redshift in our sample
Find (but beware small number statistics!):
RV=2.3 ± 0.5 (late type)
RV=3.2 ± 0.6 (early type)
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21. Future Surveys Quasar 4000 Supernova 500 SNAP LSST Galaxy Quasar
SNAP and LSST are proposed space/ground telescopes
Both missions will provide an extensive sample to study extinction and its evolution with redshift
SNAP
LSST
(Marshall et al. 2005)
Galaxy
Quasar
Deep (SNIa)
5.000
~10
Wide (WL)
50.000
Quasar
4000
Supernova
500
Compared to 10 systems in the VLT survey!
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22. Ideal LSST data z=0 z=1 z=2 Department of Astrophysical Sciences

23. Goals Measure typical reddening (E(B-V))
Gives a lower limit
Measure the steepness of the slope (RV)
Independent measurement of dust extinction
Measure the frequency of the 2175 bump
Can we find tracers for the bump?
Is the bump created or destroyed?
Find trends with galaxy type and z
Follow-up? PAH emission? Dust to gas ratio? Column densities?
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24. High-z Extinction - Sne Ia
Find low RV:
Not (yet) sensitive to the presence of the bump
Affects estimates of w, systematic error (Howell et al. 2009)
(Folatelli et al. submitted)
(Wang et al. 2008)
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25. High-z Extinction - SNe Ia
Lower RV values - SNe Ia environments systematically different? (Branch & Tammann 1992, Krisciunas et al. 2000)
Extinction estimates might be affected by circumstellar dust (Wang 2005)
“Normal” RV values for not-heavily reddened SNe Ia (Folatelli et al. submitted)
(Folatelli et al. submitted)
(Wang 2005)
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26. High-z Extinction - Galaxies I
Starburst galaxies - “local” (Calzetti et al. 1994)
No 2175 bump
Grayer dust (larger RV)
Lyman-break galaxies at 2<z<4 (Vijh et al. 2003)
SMC type of extinction
Massive starforming galaxies (Noll et al. 2007, 2009)
30% display significant 2175 bump
(Noll et al. 2009)
(Calzetti et al. 1994)
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27. High-z Extinction - Galaxies II
Dust lanes in ellipticals:
RV values slightly lower than for the MW
2.1<RV<3.3 (Goudfrooij 1994)
2.03<RV<3.46, <RV>=3.02 (Patil et al. 2007)
1.9<RV<4.1, <RV>=2.80.4 (Finkelman et al. 2008)
(Patil et al. 2007)
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28. High-z Extinction - QSO
Compare either individual or composite spectra with an unreddened mean spectra
Stacked spectra show no evidence of a 2175 bump (York et al 2006, Ménard et al. 2008) - although it could still be present in up to 30% of the lines of sight
The 2715 bump has been found in a few individual spectra (Wang et al. 2004, Srianand et al. 2008, Noterdaeme et al. 2009)
Can be confused with emission line features in the QSO itself
(Noterdaeme et al. 2009)
(York et al. 2006)
(Srianand et al. 2008)
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29. Discussion points?
Do we expect extinction properties to vary with z? How?
Will the uncertainty in extragalactic extinction estimates seriously hamper future cosmological surveys? Can we get a better handle on the possible bias?
The 2175 bump is present early on in the Universe - but what are its carriers and environment?
Is the dust around SNe different?
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