WMAP observations: Foreground Emission
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Transcript of WMAP observations: Foreground Emission
WMAP observations: Foreground Emission
Adric Riedel
http://map.gsfc.nasa.gov/m_mm.html
Overview
• What it is• The Cosmic Background• Removing the foreground• Sources of Contamination
– Free-Free emission– Synchrotron emission– Thermal Dust emission– Spinning/Magnetic Dust emission– Extragalactic sources– Sunyaev-Zeldovich effect
What it is
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ilkinsonicrowavenisotropyrobe
http://map.gsfc.nasa.gov/m_ig/990293/990293.html
• Designed to measure minute CMB variations
• Follow up to COBE
• Launched 2001
• Located at the L2 Lagrange Point
• Still in operation
The Cosmic Microwave Background
• Predicted by Gamow as a consequence of the Big Bang theory
• Predicted to be observable by Dicke• Discovered accidentally by Penzias &
Wilson at Bell Labs in 1965• Isotropic (so they thought)• Non-polarized (so they thought)• Constant (non-seasonal)
• Traces a blackbody curve (Weymann, 1967) http://www.smecc.org/microwave_oven.htm
COBE
• The Big Bang model predicted the CMB should not be isotropic
• The COBE satellite was the first to measure the anisotropy of the CMB (variations of 10-
5 out of 2.725 K)
http://lambda.gsfc.nasa.gov/product/cobe/cobe_images/cmb_fluctuations_big.gif
How the CMB is observed
• WMAP is outfitted with sensors for a variety of frequencies: 23, 33, 41, 61, 94 Ghz
• The CMB dominates all other emission between 30-150 GHz
• Other spacecraft (including COBE) have made detailed maps of the sky at various relevant frequencies
• Use other data sets to find the extent of contamination
http://www.bu.edu/iar/images/Hinshaw.ppt
Removing the foreground
• The Cosmic Microwave background is in the background, hidden behind everything else in the universe.
• To get the CMB, the foreground must be removed. Masks based on K-band (23 GHz)
A note on notation
• All the spectra are characterized as power law spectra TA~υβ , and TA is the antenna temperature.
• Spectra also characterized by flux S~υ where we assume β=-2
• So, for a given wavelength υ and varying fluxes S, we can convert flux to temperature given the value of β (or )
• The intent is to separate out just the CMB around T=2.725 K by filtering out the microwave emissions of other processes
http://antwrp.gsfc.nasa.gov/apod/ap050102.html
Sources of Contamination: Earth
• Cars, antennas, radios• WMAP is situated at Earth’s L2 point,
thus removing it from Earthbound interference
http://map.gsfc.nasa.gov/m_mm/ms_status2.html
Sources of Contamination: Galactic
• Four major processes:• Free-Free emission• Synchrotron Emission• Thermal emission• Spinning dust (Magnetic)
Sources of Contamination: Galactic
• Free-Free emission
• TA~υβ where β=-2.15 for Microwave frequencies (S~υ-0.15)
• Found in hydrogen clouds.• Not mapped in radio waves- emission
is not dominant at any radio frequency
γ
Sources of Contamination: Galactic
• Free-Free emission• Fortunately, H has been mapped• H corresponds to the same thing
(Hydrogen)• Not a perfect correspondence
especially due to dust, helium presence, rates...
Sources of Contamination: Galactic
http://heritage.stsci.edu/2000/20/big.html
• Synchrotron Emission• Produced by acceleration of electrons to
cosmic ray levels. (Type Ib and II supernovae)
• Found: SNR, diffuse• Propagate via scattering off random B
fields (diffusion) or systematic motion (convection)
• Diffuse component more common than SNR (90%)
• SNR component more powerful due to B fields
Sources of Contamination: Galactic
• Synchrotron Emission• Various ways for cosmic rays to lose
energy- Synchrotron emission, inverse Compton scattering, adiabatic loss, free-free loss.
• Most cosmic rays do not leave the galaxy, especially the most powerful- they lose energy faster.
Sources of Contamination: Galactic
• Synchrotron Emission• N(E)~E-γ, flux density =-(γ-1)/2• γ (and ) vary greatly; the resulting
flux is very frequency-dependent• For the CMB frequencies, β=-2.6
(plane) to -3.1 (halo); average is -2.7. This is common.
The sky at 408 MhZ (Synchrotron Emission )
Sources of Contamination: Galactic
• Thermal Dust• Characteristic dust emission has
been mapped in IR (IRAS, COBE) and representative temperatures
• Seems to be correlated with the synchrotron emission; probably due to the fact that both are centred around star-forming regions.
Sources of Contamination: Galactic
• Thermal Dust• β is generally 1.5 to 2; below 20 K
the slope is between 1.6 and 2.5
W band (94 GHz)
Sources of Contamination: Galactic
• Magnetic Dust• Electric dipole emission from spinning
dust• Magnetic dipole emission from
thermally fluctuating dust.• β~-2• Though predicted, there doesn’t seem
to be very much- out of 10 examined (Finkbeiner et al. 2002), 2 ‘tentative’ detections, 8 failures.
Results from other galaxies
• Klein & Emerson (1981) and a few others report that the power spectra of galaxies is synchrotron & free-free only.
• Few observations have been carried out above 10 GHz, where spinning dust is predicted to become apparent
Sources of Contamination: Extragalactic
• Point sources• The galactic removal methods
generally clean up extragalactic sources as well
• Use galaxy catalogue of sources observed at Radio and Microwave frequencies where the CMB doesn’t dominate
• 208 sources were removed, statistically five are spurious
Sources of Contamination: Extragalactic
• Sunyaev-Zeldovich Effect• Hot Gas excites CMB
photons, shifting the peak but not increasing the amplitude to match, effectively making the CMB look cooler. (http://www.mpifr-bonn.mpg.de/staff/mthierbach/sz.html)
• Most prominently due to the gas in the Coma Cluster
• Removed like the point sources
http://www.mpifr-bonn.mpg.de/staff/mthierbach/sz.html
WMAP mapping
• Combine the five frequencies linearly, properly scaled so that the foreground cancels itself out and leaves only the background
• Model the absorption by combining properly-scaled maps of the particular emission contaminants, and subtract
WMAP map error
• Note that the maximum difference 70 μK (errors confined to 5 μK, the CMB anisotropy is on the order of 200 μK)
Result
Error
http://www.bu.edu/iar/images/Hinshaw.ppt
Works Cited
• Bennett, C.L. et al. 2003, ApJS, 148, 97• Hinshaw,G. 2003, 5th Boston University Astrophysical
Conference notes.• Weymann, R.J. 1967, ASPL, 10, 81• Theirbach, M. “Sunyaev-Zeldovich Effect” 1997,
http://www.mpifr-bonn.mpg.de/staff/mthierbach/sz.html. January 28, 1997. September 27, 2006.
http://www.bu.edu/iar/images/Hinshaw.ppt