MAP Frequency Coverage

Multiple frequency coverage is needed to reliably separate Galactic foreground signals from CMB anisotropy. MAP will observe with five frequency bands between 22 and 90 GHz.

Galactic foreground signals are distinguishable from CMB anisotropy by their differing spectra and spatial distributions.

The figure at left shows the estimated spectra of the Galactic foreground signals and a range of expected cosmological signal intensities. The three physical mechanisms that contribute to the Galactic emission are synchrotron radiation, free-free radiation, and thermal radiation from interstellar dust. Results from CMB and other measurements show that at high Galactic latitudes CMB anisotropy dominates the Galactic signals in the range ~30-150 GHz. However, the Galactic foreground will need to be measured and removed from some of the MAP data.

There are two techniques that MAP will use to evaluate and remove the Galactic foreground. The first uses existing Galactic maps at lower (radio) and higher (far-infrared) frequencies as foreground emission templates. Uncertainties in the original data and position-dependent spectral index variations introduce errors with this technique. There is no good free-free emission template because there is no frequency where it dominates the microwave emission. High resolution, large scale maps of H-alpha emission will be a template for the free-free emission when they become available.

The second technique is to form linear combinations of multi-frequency MAP observations such that signals with specified spectra are cancelled. The linear combination of multi-frequency data makes no assumptions about the foreground signal strength or spatial distribution, but requires knowledge of the spectra of the foregrounds. Both techniques were successfully employed by COBE.

Five frequency bands with comparable sensitivity are desirable to solve for the four signals (synchrotron, free-free, dust, and CMB anisotropy) and the fifth degree of freedom is used to maximize signal-to-noise. The range of frequency coverage is more important than the specific choice of frequencies within the range. The lowest frequency to survey from space should be at the 22 GHz atmospheric water line since frequencies below this can (with difficulty) be accurately measured from the ground. The highest frequency to survey should be ~100 GHz to reduce the dust contribution and minimize the number of competing foreground signals. The choice of frequencies between 22 and 100 GHz can be dictated by the practical consideration of standard waveguide bands. Based on these considerations, MAP has selected the following five frequency bands, which are indicated on the above plot:

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Last updated: Friday, 05-21-1999