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.
