Cambridge CMB meeting 20th July 2009

CMB B-modes: Foregrounds

Paddy Leahy, Clive Dickinson,

Mike Preece, Mike Peel

(Manchester)

Cambridge CMB meeting 20th July 2009

Polarized Foregrounds

• rms Q,U @ 1°

• E• B,

r = 0.1• 3.4%anomalous

dust• 10%

thermal dust

QUIJOTE

Cambridge CMB meeting 20th July 2009

FG separation strategy

• Adjacent bands give little leverage on spectral parameters– No point in having bands too close together

• Widely separated bands susceptible to subtle departures from simple spectral models (power law)– No point in having bands too far from CMB

minimum

• If only the CMB had a spectral “feature”!

Cambridge CMB meeting 20th July 2009

Synchrotron spectral are smooth!

• Power law is just an approximation…

• …but a good one• The best-measured

synchrotron sources are well fit by a 2nd-order log-log polynomial over 2 decades of frequency

Cambridge CMB meeting 20th July 2009

Cosmic ray spectrum

• CR energy spectra well known to be smooth over many orders of magnitude…

• … but dominated by baryons.

• What about the electrons & positrons that produce the synchrotron radiation?

Simpson (ARNPS 1983)

Cambridge CMB meeting 20th July 2009

Fermi electron spectrum5 20 100 350 GHz (B sinθ = 2.5 µG)

Cambridge CMB meeting 20th July 2009

Fermi e−/e+ results

• Apparent curvature in spectrum suggests new feature @ E > 100 GeV, perhaps related to increasing positron fraction in PAMELA data

• But with current calibration, data consistent with pure power law, p = −3.04 (i.e. β = −3.02)

• Synchrotron emitted in CMB band (< 300GHz) dominated by E < 100 GHz.

• TBD: assess impact of apparent curvature.

Cambridge CMB meeting 20th July 2009

Synchrotron Polarization

• Synchrotron polarization varies with frequency for curved spectra (as expected in the Galaxy).

• Detail of variation depends on B-field geometry, dependence of electron energy on pitch angle.– Diagnostic of scattering

efficiency.

Degree of polarization vs. scaled frequency for “single burst” spectral ageing model

(Leahy, Black & Chan in prep.)

Cambridge CMB meeting 20th July 2009

Spectral Index 21:1.3 cm

Cambridge CMB meeting 20th July 2009

Fractional Polarization

• Minimum polarized intensity coincides with minimum 408 MHz intensity

• Typical fractional polarization at high latitude outside loop ≈ 10% = 75%/√N– Unless strongly

contaminated…free-free? anomalous dust?

• N ~ 50: much line-of-sight structure in field direction, even straight up out of plane.

• If angle can vary on LOS, so can spectral index.

Kogut et al (2007)

Cambridge CMB meeting 20th July 2009

Loop I / North Polar Spur

WMAPHaslam map

2° smoothing

Cambridge CMB meeting 20th July 2009

Local & Distant B-fields

• North polar spur supposed to be at ~140 pc.

• Synchrotron scale height ~ 1 kpc

• Projected B-field angle the same in spur and in “diffuse” emission outside it!!?

Cambridge CMB meeting 20th July 2009

Finkbeiner Davis & Schlegel (1999)

• #7: “Physical” model: – silicate grains, emissivity α = 1.5, T = 9.6 K

– carbon (?) grains, α = 2.6, T = 16.4 K

– α from lab measurements, T from fit to FIRAS data.

• #8: “Free-fit” model:– Emissivity indices allowed to

float: α = 1.67, 2.70; T = 9.4, 16.2 K

– Reduced χ2: 2.031.85

• Fits exclude |b| < 7°

• Good evidence that cold component more dominant in HI vs H2 clouds (NB composition not Tdust!)– Composition or

emission/abs properties– 15% effect; not included in

released FDS models.• FDS #7 & #8: good fits to

WMAP 94 GHz dust – outside mask– Model underpredicts by

26% (Gold et al) or 15% (my analysis).

• Grotesquely over-simplified?

Cambridge CMB meeting 20th July 2009

Point sources30 GHz 97 GHz

150 GHz 220 GHz

r = 0.1r = 0.01r = 0.001

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Cambridge CMB meeting 20th July 2009

State of the art

• CMBPol foreground subtraction report – (Dunkley et al. 2008. arXiv:0811.3915):– We got away with it for the FIR Background– We have codes ready to run to do B-mode foreground

separation. e.g:• Codes that assume spectrum of each component is uniform

over the sky (ILC, ICA)• Codes that assume each foreground component has simple

spectrum (e.g. power law) (FGFit/Miramare)– These assumptions known to be wrong:

• How wrong?• How much difference does it make?• Simulations in progress…

Cambridge CMB meeting 20th July 2009

PSM Models

• Update to CMBPol report

• Now with latest dust polarization level:– Gives 5% at high

latitude after geometric effects.

Cambridge CMB meeting 20th July 2009

Summary

• Next 3 years should define problem– Planck HFI on thermal dust spectrum &

polarization (not to mention BICEP)– Fermi + AMS on cosmic ray electron/positron

primaries.• What to minimise…

– Sensitivity? • Observe 60-150 GHz

– Foreground uncertainty? • Observe 200-350 GHz

Cambridge CMB meeting 20th July 2009

Faraday Rotation

Cambridge CMB meeting 20th July 2009

Faraday Rotation

• Away from Galactic plane, RMS Faraday rotation between λ1.3 cm and λ21 cm is 33°

– < 3° at 6 cm– < 0.2° at 1.3 cm

• Significantly less than Faraday rotation of extragalactic sources

– Diffuse synchrotron emission is mixed with ionized layer.

• PA differences between WMAP bands (22.5 – 33 GHz) suggest large Faraday rotation near Galactic Centre:

– 3°-4° at 1.3 cm, – RM ≈−700 rad m-2

• A few pixels show up to 22° rotation between 22.5-33 GHz

– Random errors (~ 5σ, but non-Gaussian)

– Change of emission mechanism (dust polarization?)

– Very large RM??