Max Tegmark's library: foregrounds

Figure 1: How foreground removal gets worse when the frequency coherence decreases

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Removing real-world foregrounds from CMB maps


Max Tegmark


Most work on foreground removal has treated the case where the frequency dependence of all components is perfectly known and independent of position. In contrast, real-world foregrounds are generally not perfectly correlated between frequencies, with the spectral index varying slightly with position and (in the case of some radio sources) with time. A method incorporating this complication in presented, and illustrated with an application to the upcoming satellite missions MAP and Planck. We find that even spectral index variations as small as 0.1 can have a substantial impact on how channels should be combined and on attainable accuracy.


This is the sequel to my 1996 foreground paper with George Efstathiou, adding another element of realism: variations in the frequency dependence of the foregrounds across the sky. It's shown how this can be dealt with, and that it's something we can't ignore even if the spectral index varies by as little as 0.1. The frequencycoherence parameter xi in the figure above is shown to be roughly the inverse of the specral index dispersion.

Reference info:

M Tegmark 1998, astro-ph/971203, accepted for publication in ApJ

Online references:

This paper is superseded by a more comprehensive one. This site also contains the latest (non-astro/ph) versions of some papers that are referenced in the text; Tegmark & Efstathiou 1996 (T96), Tegmark, Taylor & Heavens 1997, Tegmark 1997.

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This page was last modified June 17, 1999.