M. Giard, G. Lagache

 The diffuse matter in our Galaxy is certainly to be damned by the cosmologist because it pollutes the CMB sky with the various emissions of its components : dust, ionised gas (free-free) and energetic particles (synchrotron). We detail here our knowledge, but also the unknowns, regarding these radiation sources. As we are not in a position to be able to physically escape our Galaxy, the better knowledge of these foreground components is our only hope to build a consistent model which will at least allow us a virtual escape. However, the detailed understanding of the physics of the different emissions and of the large scale distribution of the various emitters is a delight for many of us.

 Although this is a faint light that you might be able to catch by eye only on dark nights and under clear skies, our Milky-Way is the dominant source of emission at the frequencies of the CMB radiation. This is illustrated in the Fig. 1 below where we show the full sky at visible and radio wavelengths, in a projection where the Milky-Way stands in the middle of the figure. At visible wavelengths most of the hundred billions of stars of our Galaxy are actually hidden by tiny opaque dust particles which are spread among the gas between the stars, drawing dark lanes along the Milky-Way. This is not the case at CMB wavelengths in the radio range where the dust particles become transparent, allowing us to probe the full Galaxy. The bright features which cover the whole sky at these wavelengths are emissions from the interstellar matter composed of gas, dust and energetic particles confined in a thin rotating plate. However, as we ourselves stand within this plate, the full gaseous Milky-Way is shining in all directions with a sharp brighter feature along the line which is drawing the galactic plane around us in the sky.

Fig. 1 All sky projection of the celestial sphere. Upper: visible electromagnetic radiation showing the stars in the sun neiborhood and dusty dark clouds along the Milky-Way (reproduced from A. Mellinger WEB site: http://canopus.physik.uni-potsdam.de/ axm/astrophot.html). Lower: radio sky at 408 MHz from Haslam et al. 1982 showing the synchrotron emission from relativistic particle trapped in the Galactic magnetic field.