Context. The planck satellite will map the full sky at nine frequencies from 30 to 857 GHz. The CMB intensity and polarization that are its prime targets are contaminated by foreground emission. ...Aims. The goal of this paper is to compare proposed methods for separating CMB from foregrounds based on their different spectral and spatial characteristics, and to separate the foregrounds into “components” with different physical origins (Galactic synchrotron, free-free and dust emissions; extra-galactic and far-IR point sources; Sunyaev-Zeldovich effect, etc.). Methods. A component separation challenge has been organised, based on a set of realistically complex simulations of sky emission. Several methods including those based on internal template subtraction, maximum entropy method, parametric method, spatial and harmonic cross correlation methods, and independent component analysis have been tested. Results. Different methods proved to be effective in cleaning the CMB maps of foreground contamination, in reconstructing maps of diffuse Galactic emissions, and in detecting point sources and thermal Sunyaev-Zeldovich signals. The power spectrum of the residuals is, on the largest scales, four orders of magnitude lower than the input Galaxy power spectrum at the foreground minimum. The CMB power spectrum was accurately recovered up to the sixth acoustic peak. The point source detection limit reaches 100 mJy, and about 2300 clusters are detected via the thermal SZ effect on two thirds of the sky. We have found that no single method performs best for all scientific objectives. Conclusions. We foresee that the final component separation pipeline for planck will involve a combination of methods and iterations between processing steps targeted at different objectives such as diffuse component separation, spectral estimation, and compact source extraction.
We investigate the performance of the parametric maximum likelihood component separation method in the context of the cosmic microwave background (CMB) B-mode signal detection and its ...characterization by small-scale CMB suborbital experiments. We consider high-resolution (FWHM = 8′) balloon-borne and ground-based observatories mapping low dust-contrast sky areas of 400 and 1000 square degrees, in three frequency channels, 150, 250, 410 GHz, and 90, 150, 220 GHz, with sensitivity of order 1 to 10 μK per beam-size pixel. These are chosen to be representative of some of the proposed, next-generation, bolometric experiments. We study the residual foreground contributions left in the recovered CMB maps in the pixel and harmonic domain and discuss their impact on a determination of the tensor-to-scalar ratio, r. In particular, we find that the residuals derived from the simulated data of the considered balloon-borne observatories are sufficiently low not to be relevant for the B-mode science. However, the ground-based observatories are in need of some external information to permit satisfactory cleaning. We find that if such information is indeed available in the latter case, both the ground-based and balloon-borne experiments can detect the values of r as low as ∼0.04 at 95 per cent confidence level. The contribution of the foreground residuals to these limits is found to be then subdominant and these are driven by the statistical uncertainty due to CMB, including E-to-B leakage, and noise. We emphasize that reaching such levels will require a sufficient control of the level of systematic effects present in the data.
Correlated Component Analysis (CCA) allows us to estimate how the different diffuse emissions mix in observations by cosmic microwave background (CMB) experiments, also taking into account ...complementary information from other surveys. It is especially useful for dealing with possible additional components for which little or no prior information exists. An application of CCA to the Wilkinson Microwave Anisotropy Probe (WMAP) maps assuming that only the canonical Galactic emissions (synchrotron, free–free and thermal dust) are present highlights the widespread presence of a spectrally flat ‘synchrotron’ component, largely uncorrelated with the synchrotron template, suggesting that an additional foreground is indeed required. We have tested various spectral shapes for such a component, namely a power law as is expected if it is flat synchrotron, and two spectral shapes that may fit the spinning dust emission: a parabola in the log S− log ν plane and a grey body. If the spatial distribution of the additional (‘anomalous’) component is not constrained a priori, it is found to be always tightly correlated with thermal dust, but the correlation is not perfect. Quality tests applied to the reconstructed CMB maps clearly disfavour two of the models. The CMB power spectra, estimated from CMB maps reconstructed exploiting the three surviving foreground models, are generally consistent with those obtained by the WMAP team, although at least one of the models gives a significantly higher quadrupole moment than found by WMAP. Taking foreground modelling uncertainties into account, we find that the mean quadrupole amplitude for the three ‘good’ models is less than 1σ below the expectation from the standard Λcold dark matter (ΛCDM) model. Also, the other reported deviations from model predictions are found not to be statistically significant, except for the excess power at l≃ 40. We confirm the evidence for a marked north–south asymmetry in the large-scale (l < 20) CMB anisotropies, which is stable with respect to the foreground parametrization we adopted. We also present a first, albeit preliminary, all-sky template of the ‘anomalous emission’.
We present an application of the fast Independent Component Analysis (fastica) to the Wilkinson Microwave Anisotropy Probe (WMAP) three-year data with the goal of extracting the cosmic microwave ...background (CMB) signal. We evaluate the confidence of our results by means of Monte Carlo simulations including the CMB, foreground contaminations and instrumental noise specific to each WMAP frequency band. We perform a complete analysis involving all or a subset of the WMAP channels in order to select the optimal combination for CMB extraction, using the frequency scaling of the reconstructed component as a figure of merit. We find that the combination KQVW provides the best CMB frequency scaling, indicating that the low-frequency foreground contamination in Q, V and W bands is better traced by the emission in the K band. The CMB angular power spectrum is recovered up to the degree scale; it is consistent within errors for all WMAP channel combinations considered, and in close agreement with the WMAP three-year results. A power spectrum analysis is made of the sky map divided into two hemispheres that have been previously reported as showing evidence of an asymmetric ratio of power on large angular scales. We then confirm the findings of several previous works with independent techniques.
We discuss an approach to the component separation of microwave, multifrequency sky maps as those typically produced from cosmic microwave background (CMB) anisotropy data sets. The algorithm is ...based on the two-step, parametric, likelihood-based technique recently elaborated on by Eriksen et al., where the foreground spectral parameters are estimated prior to the actual separation of the components. In contrast with the previous approaches, we accomplish the former task with help of an analytically derived likelihood function for the spectral parameters, which, we show, yields estimates equal to the maximum likelihood values of the full multidimensional data problem. We then use these estimates to perform the second step via the standard, generalized-least-squares-like procedure. We demonstrate that the proposed approach is equivalent to a direct maximization of the full data likelihood, which is recast in a computationally tractable form. We use the corresponding curvature matrices to characterize statistical properties of the recovered parameters. We incorporate in the formalism some of the essential features of the CMB data sets, such as inhomogeneous pixel domain noise, unknown map offsets as well as calibration errors and study their consequences for the separation. We find that the calibration is likely to have a dominant effect on the precision of the spectral parameter determination for a realistic CMB experiment. We apply the algorithm to simulated data and discuss the results. Our focus is on partial sky, total intensity and polarization, CMB experiments such as planned balloon-borne and ground-based efforts, however, the techniques presented here should be also applicable to the full-sky data as for instance, those produced by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite and anticipated from the Planck mission.
We describe the processing of data from the Low Frequency Instrument (LFI) used in production of the Planck Early Release Compact Source Catalogue (ERCSC). In particular, we discuss the steps ...involved in reducing the data from telemetry packets to cleaned, calibrated, time-ordered data (TOD) and frequency maps. Data are continuously calibrated using the modulation of the temperature of the cosmic microwave background radiation induced by the motion of the spacecraft. Noise properties are estimated from TOD from which the sky signal has been removed using a generalized least square map-making algorithm. Measured 1/f noise knee-frequencies range from ~100 mHz at 30 GHz to a few tens of mHz at 70GHz. A destriping code (Madam) is employed to combine radiometric data and pointing information into sky maps, minimizing the variance of correlated noise. Noise covariance matrices required to compute statistical uncertainties on LFI and Planck products are also produced. Main beams are estimated down to the ≈−10dB level using Jupiter transits, which are also used for geometrical calibration of the focal plane.
This paper provides an overview of the Low Frequency Instrument (LFI) programme within the ESA Planck mission. The LFI instrument has been developed to produce high precision maps of the microwave ...sky at frequencies in the range 27–77 GHz, below the peak of the cosmic microwave background (CMB) radiation spectrum. The scientific goals are described, ranging from fundamental cosmology to Galactic and extragalactic astrophysics. The instrument design and development are outlined, together with the model philosophy and testing strategy. The instrument is presented in the context of the Planck mission. The LFI approach to ground and inflight calibration is described. We also describe the LFI ground segment. We present the results of a number of tests demonstrating the capability of the LFI data processing centre (DPC) to properly reduce and analyse LFI flight data, from telemetry information to calibrated and cleaned time ordered data, sky maps at each frequency (in temperature and polarization), component emission maps (CMB and diffuse foregrounds), catalogs for various classes of sources (the Early Release Compact Source Catalogue and the Final Compact Source Catalogue). The organization of the LFI consortium is briefly presented as well as the role of the core team in data analysis and scientific exploitation. All tests carried out on the LFI flight model demonstrate the excellent performance of the instrument and its various subunits. The data analysis pipeline has been tested and its main steps verified. In the first three months after launch, the commissioning, calibration, performance, and verification phases will be completed, after which Planck will begin its operational life, in which LFI will have an integral part.
The detection and characterization of the B mode of cosmic microwave background (CMB) polarization anisotropies will not be possible without a high-precision removal of the foreground contamination ...present in the microwave band. In this work, we study the relevance of the component-separation technique based on the Independent Component Analysis (ICA) for this purpose and investigate its performance in the context of a limited sky coverage observation and from the viewpoint of our ability to differentiate between cosmological models with different primordial B-mode content.
We focus on the low Galactic emission sky patch centred at 40° in right ascension and −45 in declination, corresponding to the target of several operating and planned CMB experiments and which, in many respects, adequately represents a typical 'clean' high-latitude sky. We consider two fiducial observations, one operating at low (40, 90 GHz) frequencies and one at high (150, 350 GHz) frequencies and thus dominated by the synchrotron and thermal dust emission, respectively. We use foreground templates simulated in accordance with the existing observations in the radio and infrared bands, as well as the Wilkinson Microwave Anisotropy Probe (WMAP) and Archeops data and model the CMB emission adopting the current best-fitting cosmological model, with an amplitude of primordial gravitational waves set to either zero or 10 per cent. We use a parallel version of the FastICA code to explore a substantial parameter space including Gaussian pixel noise level, observed sky area and the amplitude of the foreground emission and employ large Monte Carlo simulations to quantify errors and biases pertinent to the reconstruction for different choices of the parameter values. We identify a large subspace of the parameter space for which the quality of the CMB reconstruction is excellent, i.e. where the errors and biases introduced by the separation are found to be comparable or lower than the uncertainty due to the cosmic variance and instrumental noise. For both the cosmological models, with and without the primordial gravitational waves, we find that FastICA performs extremely well even in the cases when the B-mode CMB signal is up to a few times weaker than the foreground contamination and the noise amplitude is comparable with the total CMB polarized emission. In addition, we discuss limiting cases of the noise and foreground amplitudes, for which the ICA approach fails.
Although our conclusions are limited by the absence of systematics in the simulated data, these results indicate that these component-separation techniques could play a crucial role in the forthcoming experiments aiming at the detection of B modes in the CMB polarization.
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