Planck satellite measured the light received from the whole sky in the submillimeter and microwave domain. It doesn't content with telling us which quantity of light comes from a given direction, it also collect other precious information notably related to the polarization. These complementary data are essential to better understand the physics of the source of this light, but we also need to accurately identify this source.
the light arriving in the bolometers of the high frequancy instrument essentially comes from the dust in our Milky Way and from the Cosmic Microwave Background - the other photons with extragalatic origin are usually much less numerous. Is it sufficient to look in the direction of the galactic poles to get a direct access to the Cosmic Microwave Background with a galactic "pollution" totally negligible? It is the question answered by one of the latest article of Planck collaboration, and the answer is no.
This map is build by extrapolating the polarized signal measured at 353 GHz. It is at this frequency observed by Planck that the signal of the dust is the highest. The part of the Cosmic Microwave Background is there negligible. We can thus use its proprieties, defined and validated by an in-depth analysis, to compute the polarized signal of galactic origin at 150 GHz, where the Cosmic Microwave Background signal is expected.
Read the complete news on Planck website (in French).