From 353 Gigahertz Down: The Frequency Scaling of the Polarised Dust Foreground

Polarised thermal emission from aligned interstellar grains is the principal foreground for searches for primordial gravitational-wave B-modes, and its amplitude at the observing frequencies must be extrapolated from where it is measured. This paper applies the diffuse-sky modified blackbody of the Planck analyses, temperature 19.6 K and emissivity index 1.53, to scale total and polarised intensity from 353 gigahertz to 90, 150, and 217 gigahertz. The total intensity at 150 gigahertz is 0.0641 of the 353 gigahertz value and at 90 gigahertz 0.0114; at a polarisation fraction of 0.10 the polarised intensity at 150 gigahertz is 0.0064 of the 353 total, and at the maximum observed fraction of 0.22 it is 0.0141. Because the extrapolation factor is of order a few percent to tens of percent rather than orders of magnitude, uncertainties in polarisation fraction and in spatial variation of the spectral index, not the mean spectrum, dominate the foreground error budget. All values are computed, not measured.