= Solution
A low-frequency example is <Galactic synchrotron emission>: relativistic electrons spiral in the Galactic magnetic field and emit a broad nonthermal spectrum. In antenna-temperature units its approximate spectrum is $T_A\propto\nu^{-\beta_s}$ with $\beta_s$ around3, varying with the electron population and sky direction. It is a major, often dominant contamination at frequencies below roughly20–40GHz; radio point sources and <bremsstrahlung> can contribute in the same low-frequency region.
A high-frequency example is <Galactic thermal dust emission>. Grains heated by starlight have approximate modified-blackbody spectrum $I_\nu\propto\nu^{\beta_d}B_\nu(T_d)$, with grain temperatures of order tens of kelvin. Their emissivity-weighted emission rises rapidly through the microwave/submillimeter range and commonly dominates anisotropy contamination above roughly100–200GHz, especially at higher frequencies. Dusty extragalactic galaxies provide a related high-frequency foreground.
\b[Synchrotron is the low-frequency contaminant and thermal dust the high-frequency contaminant.] There is no universal crossover frequency at which either exceeds CMB anisotropy everywhere: sky masking, angular scale, polarization and source amplitude matter. Cleaner cosmological windows lie between these regimes, and multi-frequency observations distinguish the foreground spectra from a thermodynamic CMB temperature perturbation. The separation into low-frequency emission and high-frequency dust is also used in https://wiki.cosmos.esa.int/planckpla/index.php?title=CMB_and_astrophysical_component_maps[the Planck astrophysical-component products].
Back to article page
