has been ignored because of its being only a small contribution in the first case and because of poorly known parameters in the latter case. The antenna temperature can be converted to the free-free flux $S_\mathrm{free-free}(\nu)$ by means of: $$S_\mathrm{free-free}(\nu) = 2\frac{\nu^2}{c^2}\cdot k_B T_\mathrm{free-free}(\mathrm{K})\mbox{.}$$

Concerning the free-free emission, there might be the possibility of an additional free-free component uncorrelated with the $H_\alpha$-emission. This hot gas, however, should emit X-ray line radiation, which has not been observed.

CO-lines from giant molecular clouds {#foreground_co}
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In a spiral galaxy such as the Milky Way, a large fraction of the interstellar medium is composed of molecular hydrogen, that resides in giant molecular clouds (GMC), objects with masses of $10^4 - 10^6 M_{\sun}$ and sizes of $50 - 200$ pc. Apart from molecular hydrogen, the GMCs contain carbon monoxide (CO) molecules in significant abundance. The rotational transitions of the CO molecule at 115 GHz and higher harmonics thereof constitute a source of contamination for all [[*Planck*]{}]{} [[*HFI*]{}]{}-channels. An extensive search for atomic and molecular transition lines was undertaken by @1994ApJ...434..587B with the [*FIRAS*]{} instrument onboard [*COBE*]{}.

The CO-contamination is modelled by employing a mosaic of CO-surveys assembled by @1996AAS...189.7004D [@2001ApJ...547..792D]. It shows the velocity-integrated 