cmb b-modes: foregrounds
DESCRIPTION
CMB B-modes: Foregrounds. Paddy Leahy, Clive Dickinson, Mike Preece, Mike Peel (Manchester). rms Q,U @ 1 ° E B, r = 0.1 3.4% anomalous dust 10% thermal dust. Polarized Foregrounds. QUIJOTE. FG separation strategy. Adjacent bands give little leverage on spectral parameters - PowerPoint PPT PresentationTRANSCRIPT
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
All> 1 Jy> 0.1 Jy> 0.01 Jy
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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??