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 Shape

Introduction PAHs and the AME New Models of Interstellar Dust Conclusions

Grain Alignment

Introduction PAHs and the AME New Models of Interstellar Dust Conclusions

Grain Alignment

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

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

fPAH

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

Correlation with τ353

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

Correlation with fPAHτ353

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

Correlation with R

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

Emission Constraints

Introduction PAHs and the AME New Models of Interstellar Dust Conclusions

Extinction Constraints

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

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

Emission

Introduction PAHs and the AME New Models of Interstellar Dust Conclusions

Extinction

Introduction PAHs and the AME New Models of Interstellar Dust Conclusions

Polarized Extinction

Introduction PAHs and the AME New Models of Interstellar Dust Conclusions

Polarized Emission

Introduction PAHs and the AME New Models of Interstellar Dust Conclusions

Polarization Fraction

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