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