dust polarization in the microwave: challenges on the ...cosmology.lbl.gov/talks/hensley_16.pdf ·...
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Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Dust Polarization in the Microwave:Challenges on the Quest for B-Modes
Brandon HensleyJet Propulsion Laboratory,
California Institute of Technology
In collaboration withBruce Draine (Princeton)
Aaron Meisner (LBL)
c© 2016All RightsReserved
INPA SeminarJanuary 29, 2015
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
The Quest for Primordial B-Modes
Credit: ESA/Planck
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
The Microwave Sky in Intensity
Planck 2015 X
10 30 100 300 1000
Frequency (GHz)
10
-110
010
110
2
Rm
s bri
ghtn
ess
tem
per
ature
(µ
KR
J)
CMB
Thermal dust
Free-freeSynchrotron
30 44 70 100 143 217 353 545 857
Spin
nin
g d
ust
CO 1-0
Sum fg
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Why is Dust Emission Polarized?
Two ingredients required:
1 Grains must be aspherical
2 Grains must be aligned
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Grain Alignment
Grain spins about ~J~J systematically aligns with ~B
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
The Microwave Sky in Polarization
Planck 2015 X
10 30 100 300 1000
Frequency (GHz)
10
-11
00
10
11
02
Rm
s b
rig
htn
ess
tem
per
atu
re (
µK
)
CMB
Thermal dust
Synchrotron
30 44 70 100 143 217 353
Sum fg
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Key Questions
• Could the AME be polarized?(and do we really know what it is?)
• What is the frequency-dependence of the dustpolarization? Is it the same as total intensity?
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Part I
• PAHs and the AME
• Hensley, Draine, and Meisner 2015. Submitted to ApJarXiv:1505.02157
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Anomalous Microwave Emission
• Discovery of adust-correlated“bump” inemission(Kogut et al1996, Leitchet al 1997, deOliveira-Costaet al 1997)
Planck Collaboration 2011
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Spinning Dust Physics
• AME can be explained byspinning dust grains(Draine & Lazarian1998ab)
• Very small grains (e.g.PAHs) can get spun up bygas collisions, radiativetorques, and otherprocesses
• If grains have a dipolemoment, this rotationcauses them to radiate
Credit: Yacine Ali-Haïmoud
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
PAHs
• AttractiveAME carrierbecause theyare small andubiquitous
• Abundancetraced by IRemissionfeatures at 8and 12µm
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Spinning Dust Emissivity
Galactic spinning dust emissivity
jν, 30 GHz/nH = 3× 10−18 Jy cm2 sr−1 H−1
Emissivity per PAH fairly robust to environmental conditions, soassume a linear scaling with ΣPAH
IAMEν, 30 GHz = 1.0
(ΣPAH
M� kpc−2
)Jy sr−1
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Spinning Dust Theory
Warning: Reality may not be so simple
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Full-Sky Test of the Spinning PAH Hypothesis
Full-sky maps of the AME derived from component separationof the microwave sky by Planck let us test the AME-PAHconnection in detail.
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
IAMEν
Asd
0.01 0.1 1 10mKRJ @ 30 GHz
Planck 2015 X
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
τ353
Iν = τνBν (Td )τν = κνMd
Ad
0.01 0.1 1 10mKRJ @ 545 GHz
Planck 2015 X
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
R
-7
-6
-5
log10(R / W m-2 sr-1)
Planck 2013 XI
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
I12µmν
-3 -2 -1 0 1 2-3 -2 -1 0 1 2
log10(MJy/sr)
Meisner & Finkbeiner 2014
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Model Predictions
1 Linear correlation with τ353
2 Even tighter correlation with fPAHτ353
3 No strong correlation with radiation field
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Model Predictions
X Linear correlation with τ353
2 Even tighter correlation with fPAHτ353
3 No strong correlation with radiation field
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Model Predictions
X Linear correlation with τ353
X Even tighter correlation with fPAHτ353
3 No strong correlation with radiation field
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Model Predictions
X Linear correlation with τ353
X Even tighter correlation with fPAHτ353
X No strong correlation with radiation field
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
A Further Test
• Does PAH abundance explain fluctuations in AME/R?
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Correlation with fPAHR
• fPAH does NOT improve the correlation with R
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Alternate Models
• What are our next-best theories?
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Magnetic Nanoparticles
• Emissivity per unitvolume of 0.01µmgrains heated to 18K
• Emissivity in mmand sub-mm muchstronger thanamorphous silicategrains
Draine and Hensley 2013
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Some Problems...
• Not great at reproducing the shape of the SED• Emission would likely be strongly polarized, in conflict with
observations
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Spinning Non-PAHs
• Still spinning dust, just not PAHs• Not clear whether including a sufficient number of
ultrasmall grains of a different type (e.g. silicates) wouldviolate other constraints (e.g. UV extinction)
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
No Spinning PAH Emission?
• Invoking alternate explanation still requires asking why thePAHs aren’t producing significant spinning dust emission
• Electric dipole moments overestimated?
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Conclusions
• No apparent link between AME and PAHs, other carriersand other mechanisms should be (re)considered
• New data is needed to better separate AME from otheremission
• Major blind spot in our knowledge of Galactic microwaveemission– puzzle needs to be solved!
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Part II
• New Models of Interstellar Dust(with Polarization!)
• Hensley & Draine 2016. In prep.
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Big Picture
• Grains producing polarized emission in the IR are thesame grains that produce polarized extinction in theoptical
• Use multi-wavelength data to construct a physical modelof dust compatible with the observations
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Method
1 Collect latest observations on dust in diffuse ISM–extinction (total and polarized), emission (total andpolarized), and abundances
2 Identify candidate grain materials, grain shapes, and grainsizes that can reproduce the observations
3 Assess the observational consequences of differentmodels
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Composition Effects
Grains are of different composition appear to have differentpolarization properties
• Silicate Features– Polarization detected
• Carbonaceous Features– Unpolarized
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Composition Effects
“We find that fitting a single modified blackbodycomponent for the thermal dust where the “real” skyshould account for two dust components may stronglybias the estimation of the tensor-to-scalar ratio by morethan 5σ”
— Remazeilles et al 2015
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Observational Constraints
What observations does a successful dust model need toreproduce?
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Draine and Fraisse 2009
• Used Draine andLi dust materialsto makepredictions forpolarizedemission in thePlanck bands
• Model predictstoo muchextinction per unitemission (PlanckInt XXIX 2014)
Draine and Fraisse 2009
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Polarized Dust Emission
• Models with silicate and carbonaceous grains alone havedifficulty reproducing the observed decline in thepolarization fraction with increasing wavelength
• A new ingredient– magnetic nanoparticles
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Example
• 2:1 spheroidal silicate grains with 5% iron nanoparticles byvolume as inclusions
• Unaligned carbonaceous grains• PAHs
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Summary
• Inclusion of iron grains allows us to match thefrequency-dependence of the polarized emission
• We alleviate the tension between emission and extinctionin the Draine and Li 2007 model by making silicates moreemissive at long wavelengths
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
With a Model, We Can...
• Test the model against the Planck sky and learn whatdrives variations in dust properties
• Predict dust properties at all wavelengths given a model fit
• Simulate different realizations of dust properties and theimplications for component separation
Introduction PAHs and the AME New Models of Interstellar Dust Conclusions
Conclusions
• Fluctuations in AME/R are uncorrelated with fPAH, castingdoubt on the association of AME and PAHs
• Uncovering the nature of the AME is important forunderstanding microwave foregrounds– more observationsand analyses are needed!
• Our new models of interstellar dust successfully reproducethe mean properties of dust in the diffuse ISM, including inpolarization
• The new models enable future work on the properties ofGalactic dust as well as next-generation componentseparation