Five-year Wilkinson Microwave Anisotropy Probe Research Articles

Constraints on the unified model of dark matter and dark energy

Using recently observed data: the Constitution dataset of type supernovae Ia (SNIa), the observational Hubble data (OHD), the measurement results of baryon acoustic oscillation (BAO) from the Sloan Digital Sky Survey (SDSS) and the Two Degree Field Galaxy Redshift Survey (2dFGRS), and the current cosmic microwave background (CMB) data from the five-year Wilkinson Microwave Anisotropy Probe (WMAP), we apply the Markov Chain Monte Carlo method to investigate the observational constraints on the generalized Chaplygin gas (GCG) model as the unification of dark matter and dark energy. For this unified model, the constraints on GCG mixture are discussed by considering the different expressions of current matter density or considering constraints as being independent of the matter quantity Ωm.

WMAP extragalactic point sources as potential Space VLBI calibrators

The point source list of the Wilkinson Microwave Anisotropy Probe (WMAP) is a uniform, all-sky catalogue of bright sources with flux density measurements at high (up to 94 GHz) radio frequencies. We investigated the five-year WMAP list to compile a new catalogue of bright and compact extragalactic radio sources to be potentially studied with Very Long Baseline Interferometry at millimeter wavelengths (mm-VLBI) and Space VLBI (SVLBI). After comparing the WMAP data with the existing mm-VLBI catalogues, we sorted out the yet unexplored sources. Using the 41, 61 and 94 GHz WMAP flux densities, we calculated the spectral indices. By collecting optical identifications, lower-frequency radio flux densities and VLBI images from the literature, we created a list of objects which have not been investigated with VLBI at 86 GHz before. With total flux density at least 1 Jy and declination above −40°, we found 37 suitable new targets. It is a nearly 25% addition to the known mm-VLBI sources. Such objects are also potentially useful as phase-reference calibrators for the future Japanese SVLBI mission ASTRO-G at its highest observing frequency (43 GHz). The phase-referencing capability of ASTRO-G would allow long integrations and hence better sensitivity for observing faint target sources close to suitable phase calibrators in the sky.

Neutrino masses from clustering of red and blue galaxies: a test of astrophysical uncertainties

Combining measurements of the galaxy power spectrum and the cosmic microwave background (CMB) is a powerful means of constraining the summed mass of neutrino species sum(m_nu), but is subject to systematic uncertainties due to non-linear structure formation, redshift-space distortions and galaxy bias. We empirically test the robustness of neutrino mass results to these effects by separately analyzing power spectra of red and blue galaxies from the Sloan Digital Sky Survey (SDSS-II) Data Release 7 (DR7), combined with the CMB five-year Wilkinson Microwave Anisotropy Probe (WMAP5) data. We consider fitting for a range of maximum wavenumber k using twelve different galaxy bias models. For example, using a new model based on perturbation theory and including redshift space distortions (Saito et al. 2009), the all-galaxy power spectrum combined with WMAP5 for a wavenumber range of k<0.2 Mpc/h yields 95% CL sum(m_nu)<0.46 eV. The red and blue galaxy power spectra give 0.41 and 0.63 eV respectively for this model. Using mock catalogues, we find the expected difference in these limits assuming a true neutrino mass of zero is 0.10 + or - 0.14 eV. Thus the difference of 0.22 eV between upper limits on neutrino mass for red and blue galaxies is approximately 1 sigma from the expected value. We find similar results for the other models and k ranges tested. This indicates good agreement for current data but hints at possible issues for next-generation surveys. Being able to perform such systematic tests is advantageous, and future surveys would benefit by including broad galaxy populations and luminosities that enable such a decomposition.

Improved cosmological constraints on the curvature and equation of state of dark energy

We apply the Constitution compilation of 397 supernova Ia, the baryon acoustic oscillation measurements including the A parameter, the distance ratio and the radial data, the five-year Wilkinson microwave anisotropy probe and the Hubble parameter data to study the geometry of the Universe and the property of dark energy by using the popular Chevallier–Polarski–Linder and Jassal–Bagla–Padmanabhan parameterizations. We compare the simple χ2 method of joined contour estimation and the Monte Carlo Markov chain method, and find that it is necessary to make the marginalized analysis on the error estimation. The probabilities of Ωk and wa in the Chevallier–Polarski–Linder model are skew distributions, and the marginalized 1σ errors are Ωm = 0.279+0.015− 0.008, Ωk = 0.005+0.006− 0.011, w0 = −1.05+0.23− 0.06 and wa = 0.5+0.3− 1.5. For the Jassal–Bagla–Padmanabhan model, the marginalized 1σ errors are Ωm = 0.281+0.015− 0.01, Ωk = 0.000+0.007− 0.006, w0 = −0.96+0.25− 0.18 and wa = −0.6+1.9− 1.6. The equation of state parameter w(z) of dark energy is negative in the redshift range 0 ⩽ z ⩽ 2 at more than 3σ level. The flat ΛCDM model is consistent with the current observational data at the 1σ level.

Probing CPT violation with CMB polarization measurements

The electrodynamics modified by the Chern–Simons term Lcs∼pμAνF˜μν with a non-vanishing pμ violates the Charge-Parity-Time Reversal symmetry (CPT) and rotates the linear polarizations of the propagating Cosmic Microwave Background (CMB) photons. In this Letter we measure the rotation angle Δα by performing a global analysis on the current CMB polarization measurements from the five-year Wilkinson Microwave Anisotropy Probe (WMAP5), BOOMERanG 2003 (B03), BICEP and QUaD using a Markov Chain Monte Carlo method. Neglecting the systematic errors of these experiments, we find that the results from WMAP5, B03 and BICEP all are consistent and their combination gives Δα=−2.62±0.87deg (68% C.L.), indicating a 3σ detection of the CPT violation. The QUaD data alone gives Δα=0.59±0.42deg (68% C.L.) which has an opposite sign for the central value and smaller error bar compared to that obtained from WMAP5, B03 and BICEP. When combining all the polarization data together, we find Δα=0.09±0.36deg (68% C.L.) which significantly improves the previous constraint on Δα and test the validity of the fundamental CPT symmetry at a higher level.

Frequentist comparison of CMB local extrema statistics in the five-yearWMAPdata with two anisotropic cosmological models

We present local extrema studies of two models that introduce a preferred direction into the observed cosmic microwave background (CMB) temperature field. In particular, we make a frequentist comparison of the one- and two-point statistics for the dipole modulation and ACW models with data from the five-year Wilkinson Microwave Anisotropy Probe (WMAP). This analysis is motivated by previously revealed anomalies in the WMAP data, and particularly the difference in the statistical nature of the temperature anisotropies when analysed in hemispherical partitions. The analysis of the one-point statistics indicates that the previously determined hemispherical variance difficulties can be apparently overcome by a dipole modulation field, but new inconsistencies arise if the mean and the l-dependence of the statistics are considered. The two-point correlation functions of the local extrema, the temperature pair product and the point-point spatial pair-count, demonstrate that the impact of such a modulation is to over-`asymmetrise' the temperature field on smaller scales than the wave-length of the dipole or quadrupole, and this is disfavored by the observed data.The results from the ACW model predictions, however, are consistent with the standard isotropic hypothesis. The two-point analysis confirms that the impact of this type of violation of isotropy on the temperature extrema statistics is relatively weak. From this work, we conclude that a model with more spatial structure than the dipole modulated or rotational-invariance breaking models are required to fully explain the observed large-scale anomalies in the WMAP data.

Probing interaction and spatial curvature in the holographic dark energy model

In this paper we place observational constraints on the interactionand spatial curvature in the holographic dark energy model. Weconsider three kinds of phenomenological interactions betweenholographic dark energy and matter, i.e., the interaction term Qis proportional to the energy densities of dark energy(ρΛ), matter (ρm), and matter plus dark energy(ρm+ρΛ). For probing the interaction and spatialcurvature in the holographic dark energy model, we use the latestobservational data including the type Ia supernovae (SNIa)Constitution data, the shift parameter of the cosmic microwavebackground (CMB) given by the five-year Wilkinson MicrowaveAnisotropy Probe (WMAP5) observations, and the baryon acousticoscillation (BAO) measurement from the Sloan Digital Sky Survey(SDSS). Our results show that the interaction and spatial curvaturein the holographic dark energy model are both rather small. Besides,it is interesting to find that there exists significant degeneracybetween the phenomenological interaction and the spatial curvaturein the holographic dark energy model.

Brute force reconstruction of the primordial fluctuation spectrum from five-year Wilkinson Microwave Anisotropy Probe observations

The primordial fluctuation spectrum is reconstructed from the five-year Wilkinson Microwave Anisotropy Probe data of the cosmic microwave background anisotropy. We divide the wave number space in the range of $1.23\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}\ensuremath{\lesssim}k\ensuremath{\lesssim}2.76\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$ into about 50 bins, and derive probability distributions of fluctuation amplitudes on the respective scales using Monte Carlo simulations. Although the reconstructed spectrum is basically consistent with a power-law spectrum, we find a hint of fine structure at $k\ensuremath{\approx}0.002\text{ }\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$ and $0.009\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$. The former is observed only in the temperature anisotropy, while the latter is both in the temperature and polarization anisotropies. The significance of these features are discussed, and it is shown that the deviation from a power-law spectrum at $k\ensuremath{\approx}0.009\text{ }\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$ is at $2.8\ensuremath{\sigma}$ level.

BAYESIAN ANALYSIS OF WHITE NOISE LEVELS IN THE FIVE-YEAR WMAP DATA

We develop a new Bayesian method for estimating white noise levels in CMB sky maps, and apply this algorithm to the five-year Wilkinson Microwave Anisotropy Probe (WMAP) data. We assume that the amplitude of the noise rms is scaled by a constant value, α, relative to a pre-specified noise level. We then derive the corresponding conditional density, P(α | s, C l, d), which is subsequently integrated into a general CMB Gibbs sampler. We first verify our code by analyzing simulated data sets, and then apply the framework to the WMAP data. For the foreground-reduced five-year WMAP sky maps and the nominal noise levels initially provided in the five-year data release, we find that the posterior means typically range between α = 1.005 ± 0.001 and α = 1.010 ± 0.001 depending on differencing assembly, indicating that the noise level of these maps are biased low by 0.5%-1.0%. The same problem is not observed for the uncorrected WMAP sky maps. After the preprint version of this letter appeared on astro-ph., the WMAP team has corrected the values presented on their web page, noting that the initially provided values were in fact estimates from the three-year data release, not from the five-year estimates. However, internally in their five-year analysis the correct noise values were used, and no cosmological results are therefore compromised by this error. Thus, our method has already been demonstrated in practice to be both useful and accurate.

THE CONTRIBUTION OF THE KINEMATIC SUNYAEV-ZEL'DOVICH EFFECT FROM THE WARM-HOT INTERGALACTIC MEDIUM TO THE FIVE-YEARWILKINSON MICROWAVE ANISOTROPY PROBEDATA

We study the contribution of the kinematic Sunyaev-Zel'dovich (kSZ) effect, generated by the warm-hot intergalactic medium, to the cosmic microwave background temperature anisotropies in the five-year Wilkinson Microwave Anisotropy Probe (WMAP) data. We explore the concordance {lambda}CDM cosmological model, with and without this kSZ contribution, using a Markov chain Monte Carlo algorithm. Our model requires a single extra parameter to describe this new component. Our results show that the inclusion of the kSZ signal improves the fit to the data without significantly altering the best-fit cosmological parameters except {omega}{sub b} h {sup 2}. The improvement is localized at the l {approx}> 500 multipoles. For the best-fit model, this extra component peaks at l {approx} 450 with an amplitude of 129 {mu}K{sup 2}, and represents 3.1% of the total power measured by WMAP. Nevertheless, at the 2{sigma} level a null kSZ contribution is still compatible with the data. Part of the detected signal could arise from unmasked point sources and/or Poissonianly distributed foreground residuals. A statistically more significant detection requires the wider frequency coverage and angular resolution of the forthcoming Planck mission.

Holographic dark energy models: a comparison from the latest observational data

The holographic principle of quantum gravity theory has been appliedto the dark energy (DE) problem, and so far three holographic DEmodels have been proposed: the original holographic dark energy(HDE) model, the agegraphic dark energy (ADE) model, and theholographic Ricci dark energy (RDE) model. In this work, we performthe best-fit analysis on these three models, by using the latestobservational data including the Union+CFA3 sample of 397 Type Iasupernovae (SNIa), the shift parameter of the cosmic microwavebackground (CMB) given by the five-year Wilkinson MicrowaveAnisotropy Probe (WMAP5) observations, and the baryon acousticoscillation (BAO) measurement from the Sloan Digital Sky Survey(SDSS). The analysis shows that for HDE, χmin2 = 465.912;for RDE, χmin2 = 483.130; for ADE,χmin2 = 481.694. Among these models, HDE model can give thesmallest χ2min. Besides, we also use the Bayesian evidence(BE) as a model selection criterion to make a comparison. It isfound that for HDE, ADE, and RDE, Δln BE = −0.86,−5.17, and −8.14, respectively. So, it seems that the HDE modelis more favored by the observational data.

Holographic Ricci dark energy: Current observational constraints, quintom feature, and the reconstruction of scalar-field dark energy

In this work, we consider the cosmological constraints on the holographic Ricci dark energy proposed by Gao et al. [Phys. Rev. D 79, 043511 (2009)], by using the observational data currently available. The main characteristic of holographic Ricci dark energy is governed by a positive numerical parameter $\ensuremath{\alpha}$ in the model. When $\ensuremath{\alpha}l1/2$, the holographic Ricci dark energy will exhibit a quintomlike behavior; i.e., its equation of state will evolve across the cosmological-constant boundary $w=\ensuremath{-}1$. The parameter $\ensuremath{\alpha}$ can be determined only by observations. Thus, in order to characterize the evolving feature of dark energy and to predict the fate of the Universe, it is of extraordinary importance to constrain the parameter $\ensuremath{\alpha}$ by using the observational data. In this paper, we derive constraints on the holographic Ricci dark energy model from the latest observational data including the Union sample of 307 type Ia supernovae, the shift parameter of the cosmic microwave background given by the five-year Wilkinson Microwave Anisotropy Probe observations, and the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey. The joint analysis gives the best-fit results (with $1\ensuremath{\sigma}$ uncertainty): $\ensuremath{\alpha}={0.359}_{\ensuremath{-}0.025}^{+0.024}$ and ${\ensuremath{\Omega}}_{\mathrm{m}0}={0.318}_{\ensuremath{-}0.024}^{+0.026}$. That is to say, according to the observations, the holographic Ricci dark energy takes on the quintom feature. Finally, in light of the results of the cosmological constraints, we discuss the issue of the scalar-field dark energy reconstruction, based on the scenario of the holographic Ricci vacuum energy.

Band-power reconstruction of the primordial fluctuation spectrum by the maximum likelihood reconstruction method

The primordial curvature fluctuation spectrum is reconstructed by the maximum likelihood reconstruction method using the five-year Wilkinson Microwave Anisotropy Probe data of the cosmic microwave background temperature anisotropy. We apply the covariance matrix analysis and decompose the reconstructed spectrum into statistically independent band powers. The prominent peak off a simple power-law spectrum found in our previous analysis turn out to be a $3.3\ensuremath{\sigma}$ deviation. From the statistics of primordial spectra reconstructed from mock observations, the probability that a primordial spectrum including such excess is realized in a power-law model is estimated to be about 2%.

FIVE-YEARWILKINSON MICROWAVE ANISOTROPY PROBEOBSERVATIONS: LIKELIHOODS AND PARAMETERS FROM THEWMAPDATA

The Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, has mapped out the Cosmic Microwave Background with unprecedented accuracy over the whole sky. Its observations have led to the establishment of a simple concordance cosmological model for the contents and evolution of the universe, consistent with virtually all other astronomical measurements. The WMAP first-year and three-year data have allowed us to place strong constraints on the parameters describing the ACDM model. a flat universe filled with baryons, cold dark matter, neutrinos. and a cosmological constant. with initial fluctuations described by nearly scale-invariant power law fluctuations, as well as placing limits on extensions to this simple model (Spergel et al. 2003. 2007). With all-sky measurements of the polarization anisotropy (Kogut et al. 2003; Page et al. 2007), two orders of magnitude smaller than the intensity fluctuations. WMAP has not only given us an additional picture of the universe as it transitioned from ionized to neutral at redshift z approx.1100. but also an observation of the later reionization of the universe by the first stars. In this paper we present cosmological constraints from WMAP alone. for both the ACDM model and a set of possible extensions. We also consider tlle consistency of WMAP constraints with other recent astronomical observations. This is one of seven five-year WMAP papers. Hinshaw et al. (2008) describe the data processing and basic results. Hill et al. (2008) present new beam models arid window functions, Gold et al. (2008) describe the emission from Galactic foregrounds, and Wright et al. (2008) the emission from extra-Galactic point sources. The angular power spectra are described in Nolta et al. (2008), and Komatsu et al. (2008) present and interpret cosmological constraints based on combining WMAP with other data. WMAP observations are used to produce full-sky maps of the CMB in five frequency bands centered at 23, 33, 41, 61, and 94 GHz (Hinshaw et al. 2008). With five years of data, we are now able to place better limits on the ACDM model. as well as to move beyond it to test the composition of the universe. details of reionization. sub-dominant components, characteristics of inflation, and primordial fluctuations. We have more than doubled the amount of polarized data used for cosmological analysis. allowing a better measure of the large-scale E-mode signal (Nolta et al. 2008). To this end we describe an alternative way to remove Galactic foregrounds from low resolution polarization maps in which Galactic emission is marginalized over, providing a cross-check of our results. With longer integration we also better probe the second and third acoustic peaks in the temperature angular power spectrum, and have many more year-to-year difference maps available for cross-checking systematic effects (Hinshaw et al. 2008).

FIVE-YEAR WILKINSON MICROWAVE ANISOTROPY PROBE OBSERVATIONS: DATA PROCESSING, SKY MAPS, AND BASIC RESULTS

We present new full-sky temperature and polarization maps in five frequency bands from 23 to 94 GHz, based on data from the first five years of the Wilkinson Microwave Anisotropy Probe (WMAP) sky survey. The new maps are consistent with previous maps and are more sensitive. The five-year maps incorporate several improvements in data processing made possible by the additional years of data and by a more complete analysis of the instrument calibration and in-flight beam response. We present several new tests for systematic errors in the polarization data and conclude that W-band polarization data is not yet suitable for cosmological studies, but we suggest directions for further study. We do find that Ka-band data is suitable for use; in conjunction with the additional years of data, the addition of Ka band to the previously used Q- and V-band channels significantly reduces the uncertainty in the optical depth parameter, τ. Further scientific results from the five-year data analysis are presented in six companion papers and are summarized in Section 7 of this paper. With the five-year WMAP data, we detect no convincing deviations from the minimal six-parameter ΛCDM model: a flat universe dominated by a cosmological constant, with adiabatic and nearly scale-invariant Gaussian fluctuations. Using WMAP data combined with measurements of Type Ia supernovae and Baryon Acoustic Oscillations in the galaxy distribution, we find (68% CL uncertainties): Ωbh2 = 0.02267+0.00058−0.00059, Ωch2 = 0.1131 ± 0.0034, ΩΛ = 0.726 ± 0.015, ns = 0.960 ± 0.013, τ = 0.084 ± 0.016, and at k = 0.002 Mpc-1. From these we derive σ8 = 0.812 ± 0.026, H0 = 70.5 ± 1.3 km s-1 Mpc−1, Ωb = 0.0456 ± 0.0015, Ωc = 0.228 ± 0.013, Ωmh2 = 0.1358+0.0037−0.0036, zreion = 10.9 ± 1.4, and t0 = 13.72 ± 0.12 Gyr. The new limit on the tensor-to-scalar ratio is r < 0.22(95%CL), while the evidence for a running spectral index is insignificant, dns/dln k = −0.028 ± 0.020 (68% CL). We obtain tight, simultaneous limits on the (constant) dark energy equation of state and the spatial curvature of the universe: −0.14 < 1 + w < 0.12(95%CL) and −0.0179 < Ωk < 0.0081(95%CL). The number of relativistic degrees of freedom, expressed in units of the effective number of neutrino species, is found to be Neff = 4.4 ± 1.5 (68% CL), consistent with the standard value of 3.04. Models with Neff = 0 are disfavored at >99.5% confidence. Finally, new limits on physically motivated primordial non-Gaussianity parameters are −9 < flocalNL < 111 (95% CL) and −151 < fequilNL < 253 (95% CL) for the local and equilateral models, respectively.

Bayesian Limits on Primordial Isotropy Breaking

It is often assumed that primordial perturbations are statistically isotropic, which implies, among other properties, that their power spectrum is invariant under rotations. In this article, we test this assumption by placing bounds on deviations from rotational invariance of the primordial spectrum. Using five-year Wilkinson Microwave Anisotropy Probe cosmic microwave anisotropy maps, we set limits on the overall norm and the amplitude of individual components of the primordial spectrum quadrupole and hexadecapole. We find that there is no significant evidence for primordial isotropy breaking, and constrain the relative contribution of the quadrupole and hexadecapole to be less than, respectively, 23% and 34% at 95% confidence level.

Reconstruction of the primordial fluctuation spectrum from the five-year WMAP data by the cosmic inversion method with band-power decorrelation analysis

The primordial curvature fluctuation spectrum is reconstructed by the cosmic inversion method using the five-year Wilkinson Microwave Anisotropy Probe data of the cosmic microwave background temperature anisotropy. We apply the covariance matrix analysis and decompose the reconstructed spectrum into statistically independent band-powers. The statistically significant deviation from a simple power-law spectrum suggested by the analysis of the first-year data is not found in the five-year data except possibly at one point near the border of the wave number domain where accurate reconstruction is possible.

High-energy particle acceleration and production of ultra-high-energy cosmic rays in the giant lobes of Centaurus A

The nearby radio galaxy Centaurus A is poorly studied at high frequencies with conventional radio telescopes because of its very large angular size, but is one of a very few extragalactic objects to be detected and resolved by the Wilkinson Microwave Anisotropy Probe (WMAP). We have used the five-year WMAP data for Cen A to constrain the high-frequency radio spectra of the 10° giant lobes and to search for spectral changes as a function of position along the lobes. We show that the high-frequency radio spectra of the northern and southern giant lobes are significantly different: the spectrum of the southern lobe steepens monotonically (and is steeper further from the active nucleus) whereas the spectrum of the northern lobe remains consistent with a power law. The inferred differences in the northern and southern giant lobes may be the result of real differences in their high-energy particle acceleration histories, perhaps due to the influence of the northern middle lobe, an intermediate-scale feature which has no detectable southern counterpart. In light of these results, we discuss the prospects for Fermi Gamma-ray Space Telescope detections of inverse-Compton emission from the giant lobes and the lobes' possible role in the production of the ultra-high-energy cosmic rays (UHECR) detected by the Pierre Auger Observatory. We show that the possibility of a Fermi detection depends sensitively on the physical conditions in the giant lobes, with the northern lobe more likely to be detected, and that any emission observed by Fermi is likely to be dominated by photons at the soft end of the Fermi energy band. On the other hand, we argue that the estimated conditions in the giant lobes imply that UHECRs can be accelerated there, with a potentially detectable γ-ray signature at TeV energies.

Anomalous CMB north-south asymmetry

Several accurate analyses have revealed a statistically significant north-south ecliptic asymmetry in the large-angle correlations strength of the cosmic microwave background (CMB) radiation temperature field data from the Wilkinson Microwave Anisotropy Probe (WMAP). This asymmetry is inconsistent with the statistical isotropy expected in the concordance cosmological model $\ensuremath{\Lambda}\mathrm{CDM}$. It has been suggested that a possible cause-effect relationship exists between this large-angle anisotropy and the anomalous CMB quadrupole-octopole planes alignment. In turn, this later phenomenon (or both) would be a consequence of one or more of the following undesired effects in CMB data: a systematic error in the data processing or in the instrument characterization, residual foregrounds, and large-angle correlations induced by the incomplete sky CMB data (cut-sky masks are needed to reject galactic foregrounds). Here, it is proved that the north-south asymmetry is unrelated to the quadrupole ($\ensuremath{\ell}=2$) and the octopole ($\ensuremath{\ell}=3$) properties because we find, at high confidence levels, such large-angle anisotropy in three- and five-year WMAP CMB maps containing only the multipole components $4\ensuremath{\le}\ensuremath{\ell}\ensuremath{\le}10$. The statistical significance depends on both, the CMB map analyzed as well as the cut-sky mask applied to exclude foregrounds. In general, we obtain that the significance level of the north-south asymmetry is less in five-year WMAP data with KQ75 ($\ensuremath{\gtrsim}90%\text{ }\text{ }\mathrm{CL}$) than it is in three-year data with Kp0 ($\ensuremath{\gtrsim}96%\text{ }\text{ }\mathrm{CL}$). For instance, in the WMAP internal linear combination (ILC)-five-year map with the KQ75 mask (a sky cut of 28.4%) this phenomenon is observed at 92.7% CL, whereas for the WMAP ILC-three-year map with the Kp0 mask (a sky cut of 23.5%) this phenomenon appears at 96.5% CL. Moreover, it is also shown that this hemispherical asymmetry is unlikely due to systematics or foreground contaminants, because it is present in single-frequency, multifrequency, and cleaned ILC-type CMB maps. Additionally, robustness tests show that the statistical significance of the north-south asymmetry is affected by the use of the KQ85 and KQ75 masks in five-year single-frequency WMAP CMB maps, whereas it is insensitive with respect to application of the Kp2 and Kp0 masks in three-year single-frequency WMAP CMB maps. The confidence levels were obtained using sets of Monte Carlo CMB maps produced according to the $\ensuremath{\Lambda}\mathrm{CDM}$ model.