Evidence for a Significant Blazar Contamination in CMB Anisotropy Maps

Abstract

The analysis of the recent WMAP source catalog shows that the vast majority of bright foreground extragalactic sources detected in CMB maps are Blazars. In this paper we calculate the contamination of CMB anisotropy maps by this type of flat-spectrum, strongly variable and polarized extragalactic radio sources using up-to-date results from recent deep multi-frequency surveys. We found that more than 50 known Blazars (or Blazar candidates) are included in the 90/150 GHz BOOMERANG anisotropy maps, a factor > 15 larger than previously reported. Using a recent derivation of the Blazar radio LogN-LogS we calculate that these sources induce an average sky brightness of 0.2 Jy/deg^2, corresponding to an average temperature of ~3-5 muK. Moreover, we find that the associated level of fluctuations is of the order of C_{l, Blazar}= 1.3 10^{-2} mu K^2 sr at 41 GHz. Taking into account both Blazar variability and the many steep-spectrum radio sources that flatten at high frequencies, as well as the contribution of radio-galaxies, we find that the level of residual fluctuation due to discrete extragalactic foreground sources could be factor of ~2 - 3 higher than the above estimate. We show that the Blazar induced fluctuations contaminate the CMB spectrum at the level of ~ 20-50 % at l = 500 and 50-100 % at l = 800. Careful cleaning for Blazar contamination of high sensitivity/high resolution CMB maps is therefore necessary before firm conclusions about weak features, like secondary high-l peaks of the CMB power spectrum or very weak signals like CMB polarization measurements, can be achieved.