Constraints on a stochastic background of primordial magnetic fields with WMAP and South Pole Telescope data

We constrain a stochastic background of primordial magnetic fields (PMF) by its contribution to the cosmic microwave background (CMB) anisotropy angular power spectrum with the combination of WMAP 7 year and South Pole Telescope (SPT) data. The contamination in the SPT data by unresolved point sources and by the Sunyaev-Zeldovich (SZ) effect due to galaxy clusters has been taken into account as modelled by the SPT Collaboration. With this combination of WMAP 7 year and SPT data, we constrain the amplitude Gaussian smoothed over 1 Mpc scale of a stochastic background of non-helical PMF to B1Mpc<3.5nG at 95% confidence level, improving on previous bounds. Our analysis shows that SPT data up to ℓ=3000 bring an improvement of almost a factor two with respect to results with previous CMB high-ℓ data. We then discuss the forecasted impact from unresolved points sources and SZ effect for Planck capabilities in constraining PMF.