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National Aeronautics and Space Administration

Goddard Space Flight Center

Astrophysics Science Division | Sciences and Exploration

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Galactic Emission

Photons from the cosmic microwave background must traverse the Galaxy before reaching Earth. Understanding Galactic radio emission is an important part of the ARCADE scientific effort.
Sky map 408 MHz radio emission
Diffuse Galactic emission at centimeter wavelengths is dominated by three components, as shown in the plot below: synchrotron emission from cosmic-ray electrons (green), electron-ion bremsstrahlung (free-free emission) from diffuse ionized gas (yellow), and thermal radiation from interstellar dust (blue). A fourth emission component, from small rapidly spinning dust grains (purple) has also been proposed.

CMB and foreground emission vs frequency
Despite surveys carried out over many years, relatively little is known about the physical conditions responsible for these diffuse emissions. A limiting factor has been the large (10%) calibration uncertainties in radio surveys and the decade gap in frequency coverage between ground-based radio surveys and the millimeter-wave measurements by the COBE and WMAP satellites.

By measuring both the spectrum and angular distribution of diffuse microwave emission, ARCADE will separate Galactic from cosmic emission and provide answers to outstanding questions on physical conditions in the interstellar medium (ISM),including:

  • What is the heating mechanism in the ISM? Is the diffuse ionized gas heated by photoionization from the stellar disk, supernova shocks, Galactic fountain flows, or decaying dark matter?

  • How are cosmic rays accelerated? Is the energy spectrum of local cosmic-ray electrons representative of the Galaxy as a whole?

  • What is the shape, constitution, and size distribution of interstellar dust? Is there a distinct "cold" component in the infrared cirrus?

  • Is spinning dust a significant component of Galactic emission? Have we properly accounted for the combined microwave emission from our galaxy?
    ARCADE's observations will be particularly useful for modeling synchrotron emission. By providing absolute calibration for large regions of the sky, tied to a common full-beam standard, it will help disentangle microwave emission at millimeter wavelengths where multiple emission components are present at roughly equal amplitude.