dust and re-emission at longer wavelengths. Galaxies with ongoing star formation can have large fractions ($\sim90$%) of their total emission at infra-red wavelengths, compared to about one third in the case of local galaxies. The integrated emission from unresolved infra-red galaxies accounts for the cosmic infra-red background (CIB) , the fluctuations of which are impeding SZ-observations at frequencies above $\nu\simeq100$ GHz [@2004astro.ph..2571A].

and @2003astro.ph..8464W have estimated the number counts of unresolved infra-red galaxies at [[*Planck*]{}]{}  frequencies, which was used by @2004astro.ph..2571A in order to estimate the level of fluctuation in the [[*Planck*]{}]{}-beam. In the easiest case, the sources are uncorrelated and the fluctuations obey Poissonian statistics, but the inclusion of correlations is expected to boost the fluctuations by a factor of $\sim1.7$ [@2003ApJ...590..664S]. According to @2004astro.ph..2571A, the resulting fluctuations vary between a few $10^2~\mathrm{Jy}/\mathrm{sr}$ and $10^5~\mathrm{Jy}/\mathrm{sr}$, depending on observing channel. A proper modeling would involve a biasing scheme for populating halos, the knowledge of the star formation history and template spectra in order to determine the K-corrections.

AGNs are another extragalactic source of submillimetric emission. Here, sychrotron emission is the radiation generating mechanism. The spectra show a variety of functional behaviours, with spectral indices $\alpha$ generally ranging from -1 to 