North-South non-Gaussian asymmetry in Planck CMB maps

Abstract

We report the results of a statistical analysis performed with the four foregroundcleaned Planck maps by means of a suitably defined local-variance estimator. Our analysis shows a clear dipolar structure in Planck's variance map pointing in the direction (l, b) (220 • , −32 • ), thus consistent with the North-South asymmetry phenomenon. Surprisingly, and contrary to previous findings, removing the CMB quadrupole and octopole makes the asymmetry stronger. Our results show a maximal statistical significance, of 98.1% CL, in the scales ranging from = 4 to = 500. Additionally, through exhaustive analyses of the four foreground-cleaned and individual frequency Planck maps, we find unlikely that residual foregrounds could be causing this dipole variance asymmetry. Moreover, we find that the dipole gets lower amplitudes for larger masks, evidencing that most of the contribution to the variance dipole comes from a region near the galactic plane. Finally, our results are robust against different foreground cleaning procedures, different Planck masks, pixelization parameters, and the addition of inhomogeneous real noise. IntroductionThe temperature fluctuations of the Cosmic Microwave Background radiation (CMB), recently released by the Planck collaboration [1], confirmed with outstanding precision the concordance cosmological model, ΛCDM [2][3][4]. Such exquisite set of cosmological information allows us to test two fundamental properties of the universe expected after the standard inflationary phase [5][6][7][8], namely that the CMB field is, at large-angles, nearly Gaussian and statistically isotropic (see, e.g., [9, 10] and refs. therein).Previous studies using WMAP data indicate significant departure from either gaussianity or statistical isotropy at the largest angular scales -an unexpected result in the ΛCDM model , though possibly disputable [58]. These phenomena, also called anomalies, have been now confirmed with similar high confidence levels, ∼ 3σ, by the Planck collaboration with CMB foreground-cleaned maps [10]. On the other side, only small magnitude Gaussian deviations from primordial origin have been detected in Planck data [59,60]. However, there are more potential sources of non-Gaussianity (NG) in the CMB data than just primordial NG [61][62][63][64][65][66]. These include galactic foregrounds remnants and secondary anisotropies coming from processes after the last scattering surface [60,[67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82]. In particular, Gaussian analyses for large angular scales are delicate because galactic foregrounds contaminations are not completely understood and, as a consequence, galactic cut-sky masks are still necessary in CMB data analyses [60]. Monteserín et al. (2008) [83] reported an anomalously low variance distribution in WMAP3 maps at 98.7% CL. Cruz et al. (2011)[84] confirmed this result in WMAP5 and WMAP7 data, also pointing that some regions near the galactic plane present an anomalously high variance (95.6% CL) in the south ecliptic hemisphere. Their ...

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