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

Abstract 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 B 1 Mpc 3.5 nG 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.