The first and third year data releases from the WMAP provide evidence of an anomalous Cold Spot (CS) at galactic latitude b=-57deg and longitude l=209deg. We have examined the properties of the CS in some detail in order to assess its cosmological significance. We have performed a cluster analysis of the local extrema in the CMB signal to show that the CS is actually associated with a large group of extrema rather than just one. In the light of this we have re-examined the properties of the WMAP ILC and co-added "cleaned" WCM maps, which have previously been used for the analysis of the properties of the signal in the vicinity of the CS. These two maps have remarkably similar properties on equal latitude rings for |b|>30deg, as well as in the vicinity of the CS. We have also checked the idea that the CMB signal has a non-Gaussian tail, localized in the low multipole components of the signal. For each ring we apply a linear filter with characteristic scale R, dividing the CMB signal in two parts: the filtered part, with characteristic scale above that of the filter R, and the difference between the initial and filtered signal. Using the filter scale as a variable, we can maximize the skewness and kurtosis of the smoothed signal and minimize these statistics for the difference between initial and filtered signal. We have discovered that the shape of the CS is formed primarily by the components of the CMB signal represented by multipoles between 10<=L<=20, with a corresponding angular scale about 5-10 degs. This signal leads to modulation of the whole CMB sky, clearly seen at |b|>30deg in both the ILC and WCM maps, rather than a single localized feature. After subtraction of this modulation, the remaining part of the CMB signal appears to be consistent with statistical homogeneity and Gaussianity.
Deep Dive into Understanding the WMAP Cold Spot mystery.
The first and third year data releases from the WMAP provide evidence of an anomalous Cold Spot (CS) at galactic latitude b=-57deg and longitude l=209deg. We have examined the properties of the CS in some detail in order to assess its cosmological significance. We have performed a cluster analysis of the local extrema in the CMB signal to show that the CS is actually associated with a large group of extrema rather than just one. In the light of this we have re-examined the properties of the WMAP ILC and co-added “cleaned” WCM maps, which have previously been used for the analysis of the properties of the signal in the vicinity of the CS. These two maps have remarkably similar properties on equal latitude rings for |b|>30deg, as well as in the vicinity of the CS. We have also checked the idea that the CMB signal has a non-Gaussian tail, localized in the low multipole components of the signal. For each ring we apply a linear filter with characteristic scale R, dividing the CMB signal in
The first and third year data releases from the Wilkinson Microwave Anisotropy Probe (WMAP) provide evidence of an anomalous Cold Spot (CS) at galactic latitude b = -57 • and longitude l = 209 • . We have examined the properties of the CS in some detail in order to assess its cosmological significance. We have performed a cluster analysis of the local extrema in the CMB signal to show that the CS is actually associated with a large group of extrema rather than just one. We have also checked the idea that the CMB signal has a non-Gaussian tail. For each ring we apply a linear filter with characteristic scale R, dividing the CMB signal in two parts: the filtered part, with characteristic scale above that of the filter R, and the difference between the initial and filtered signal. Using the filter scale as a variable, we can maximize the skewness and kurtosis of the smoothed signal and minimize these statistics for the difference between initial and filtered signal. We find that, unlike its Northern counterpart, the Southern Galactic hemisphere of the CMB map is characterized by significant departure from Gaussianity of which the CS is not the only manifestation:
we have located a ring, on which there are “cold” as “hot” spots with almost the same properties as the CS. Exploiting the similarity of the WCM and the ILC maps, and using the latter as a guide map, we have discovered that the shape of the CS is formed primarily by the components of the CMB signal represented by multipoles between 10 ≤ ℓ ≤ 20, with a corresponding angular scale about 5 -10 • . This signal leads to modulation of the whole CMB sky, clearly seen at |b| > 30 • in both the a 20-45% dip in the smoothed NVSS source counts which can be interpreted, they argue, as a manifestation of the integrated Sachs-Wolfe effect, seen for a single region of the CMB sky. Jaffe et al. [14,15], Cayon et al. [16] and McEwen et al. [17,18] have investigated the Bianchi VII h anisotropic cosmological model as a possible explanation of the CS and other features of the WMAP low multipoles. Recently, Cruz et al. [5,19] have pointed out that the CS could be produced by a cosmic texture, assuming that the CMB signal is a combination of the Gaussian and non-Gaussian parts. The present status of the problem of existence of the CS remains uncertain, despite the presence of a vast collection of theoretical suggestions. Nevertheless, if we believe that one particular part of the WMAP CMB signal contains non-Gaussian features, it would be necessary to seek corroborating evidence of non-Gaussianity elsewhere in order to understand their properties more fully. In this Paper we therefore present a detailed investigation of the properties of the CS, focusing attention on the following topics.
First, in Section 2, we show how the CS can be easily detected in the pixels domain not only in the derived CMB signal, but even in the WMAP maps for K-W bands before separation of the signal into CMB and foreground components.
Second, we will demonstrate that the CS belongs to a cluster of local minima, the spatial distribution of which is modulated by the large-angle modes of the CMB signal outside the Galactic plane. For that we use the Internal Linear Combination (ILC) III map and the co-added WCM map with N side = 512 in the HEALPix format, converted to GLESP format [20], where each iso-latitude ring has the same number of pixels N φ = 2048 in azimuthal direction φ in polar coordinates. After that we perform a cluster analysis [21] of the positive and negative peaks for selected rings in the area outside the Kp0 mask, mostly concentrating our attention on the ring crossing the CS at its extrema b = -57 • and -180
Taking into consideration the signal for each ring with the latitude b, we can investigate the morphology of the CMB signal at each latitude for the whole range of φ. This approach allows to connect the morphology of the CS to the signal outside the CS for the same latitude b = -57 • . We will show that the cluster contained the CS, is not a unique feature of the b = -57 • iso-latitude ring. For example, close to the CS there are two significant clusters of maxima, but these peaks have lower amplitude that the CS.
Next, since the origin of large clusters of extrema is related to the angular modulation of the signal on large scales [21], we split the CMB signal into two parts. To do that we use the skewness and the kurtosis of the signal for selected rings, including the b = -57 • ring. Then by using a simple linear smoothing filter with characteristic scale R we separate the signal into a smoothed component and to a difference between initial signal and the smoothed component. For the smoothed signal we define the skewness S(R) and the kurtosis K(R) as a functions of R and find the scale of filtering which maximize both these characteristics S(R opt ), K(R opt ) → max. By using this scale R max we separate the initial CMB signal in two parts, one of them (the smoothed
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