Cross-terms and weak frequency-dependent signals in the cosmic microwave background sky

Abstract

In this paper, we study the amplification of weak frequency dependent signals in the CMB sky due to their cross correlation to intrinsic anisotropies. In particular, we center our attention on mechanisms generating some weak signal, of peculiar spectral behaviour, such as resonant scattering in ionic, atomic or molecular lines, thermal SZ effect or extragalactic foreground emissions, whose typical amplitude (denoted by ǫ) is sufficiently smaller than the intrinsic CMB fluctuations. We find that all these effects involve either the autocorrelation of anisotropies generated during recombination (zrec) or the cross-correlation of those anisotropies with fluctuations arising at redshift zi. The former case accounts for the slight blurring of original anisotropies generated in the last scattering surface, and shows up in the small angular scale (high multipole) range. The latter term describes, instead, the generation of new anisotropies, and is non-zero only if fluctuations generated at redshifts zrec, zi are correlated. The degree of this correlation can be computed under the assumption that density fluctuations were generated as standard inflationary models dictate and that they evolved in time according to linear theory. In case that the weak signal is frequency dependent, (i.e., the spectral dependence of the secondary anisotropies is distinct from that of the CMB), we show that, by substracting power spectra at different frequencies, it is possible to avoid the limit associated to Cosmic Variance and unveil weaker terms linear in ǫ. We find that the correlation term shows a different spectral dependence than the squared (/ ǫ 2 ) term considered usually, making its extraction particularly straightforward for the thermal SZ effect. Furthermore, we find that in most cases the correlation terms are particularly relevant at low multipoles due to the ISW effect and must be taken into account when characterising the power spectrum associated to weak signals in the large angular scales.