The sky seen through Planck's nine frequency channels
These nine images show temperature maps of the whole sky as measured by Planck through its nine frequency channels, after the signal due to the Cosmic Microwave Background (CMB) has been removed.
The dominant feature in all maps is evidently the diffuse emission from the Milky Way, which is mainly due to three different physical mechanisms: thermal radiation coming from interstellar dust, synchrotron radiation, emitted by free electrons moving in the galactic magnetic field, and free-free (or bremsstrahlung) radiation, released by free electrons that are decelerated when passing by an ion.
As these three emission processes behave differently at different frequencies, the wide spectral range probed by Planck is instrumental in separating the various contributions to each map: this is, in turn, extremely important both to investigate the interstellar medium of our Galaxy in great detail and to achieve an optimal removal of the foregrounds, the latter required to study the CMB and its anisotropies.
The maps displayed in the first row correspond to the three frequency channels probed by the Low Frequency Instrument (LFI) on board Planck: from left to right, 30, 44 and 70 GHz, respectively; the maps in the second and third row, instead, correspond to the six frequency channels probed by the High Frequency Instrument (HFI) on board Planck: from left to right, and from top to bottom, 100, 143, 217, 353, 545 and 857 GHz, respectively.
In the case of the highest frequencies probed by Planck, above 100 GHz, these are the first full-sky high-resolution maps ever recorded.