Component separation for cosmic microwave background studies

The detection of primordial B-mode polarization in the cosmic microwave background (CMB) remains the major outstanding goal of CMB cosmology. However, current upper limits on the strength of the primordial signal mean that it will be sub-dominant to astrophysical foreground emission at all frequencies, over the entire sky. Detecting primordial B-modes therefore becomes a problem in component separation. In this thesis I address this problem from two angles. First, I present my work analysing data from the C-Band All-Sky Survey (C-BASS), a 4.76 GHz survey covering the whole sky in total intensity and polarization. I describe the point-source detection algorithm developed for C-BASS, and the northern sky point-source catalogue that has been produced using it. This catalogue has allowed us to confirm the accuracy of the C-BASS pointing and flux-density scale, and will form the basis of any C-BASS point source masks. I also present an analysis of the synchrotron power spectra, using C-BASS, WMAP and Planck data. From this it is found the minimum synchrotron contamination to CMB B-modes corresponds to a tensor-to-scalar ratio of r~0.001, with a typical contamination at the level of r~0.01. Alongside the analysis of C-BASS data, I present a novel implementation of Bayesian parametric component separation. This uses the No-U-Turn Sampler (NUTS) to explore the posterior distribution. NUTS is a gradient-based sampling algorithm with excellent scaling to high dimensions, and also contains important self-diagnostics of potential failures in geometric ergodicity. This is particularly important for the hierarchical foreground model introduced here, which can exhibit highly complex posterior geometries. The hierarchical foreground model fits for hyper-distributions over large sky regions, from which foreground spectral parameters are drawn. This is compared to a complete pooling model, where foreground spectral parameters are assumed to be constant in each sky region. The hierarchical model is able to remove artefacts from the recovered CMB maps without inflating parameter uncertainties, translating through to reduced biases on cosmological parameters. </p

The C-Band All-Sky Survey (C-BASS): Simulated parametric fitting in single pixels in total intensity and polarization

The cosmic microwave background (CMB) B-mode signal is potentially weaker than the diffuse Galactic foregrounds over most of the sky at any frequency. A common method of separating the CMB from these foregrounds is via pixel-based parametric-model fitting. There are not currently enough all-sky maps to fit anything more than the most simple models of the sky. By simulating the emission in seven representative pixels, we demonstrate that the inclusion of a 5 GHz data point allows for more complex models of low-frequency foregrounds to be fitted than at present. It is shown that the inclusion of the C-BASS data will significantly reduce the uncertainties in a number of key parameters in the modelling of both the galactic foregrounds and the CMB. The extra data allow estimates of the synchrotron spectral index to be constrained much more strongly than is presently possible, with corresponding improvements in the accuracy of the recovery of the CMB amplitude. However, we show that to place good limits on models of the synchrotron spectral curvature will require additional low-frequency data

The C-Band All-Sky Survey (C-BASS): total intensity point source detection over the northern sky

We present a point source detection algorithm that employs the second order Spherical Mexican Hat Wavelet filter (SMHW2), and use it on C-BASS northern intensity data to produce a catalogue of point sources. The SMHW2 allows us to filter the entire sky at once, avoiding complications from edge effects arising when filtering small sky patches. The algorithm is validated against a set of Monte Carlo simulations, consisting of diffuse emission, instrumental noise, and various point source populations. The simulated source populations are successfully recovered. The SMHW2 detection algorithm is used to produce a $4.76\,\mathrm{GHz}$ northern sky source catalogue in total intensity, containing 1729 sources and covering declinations $\delta\geq-10^{\circ}$. The C-BASS catalogue is matched with the GB6 and PMN catalogues over their common declinations. From this we estimate the $90\%$ completeness level to be approximately $630\,\mathrm{mJy}$, with a corresponding reliability of $95\%$, when applying a Galactic mask covering $20\%$ of the sky. We find the C-BASS and GB6/PMN flux density scales to be consistent with one another to within $3\%$. The absolute positional offsets of C-BASS sources from matched GB6/PMN sources peak at approximately $3.5\,\mathrm{arcmin}$

The C-Band All-Sky Survey (C-BASS): constraining diffuse Galactic radio emission in the North Celestial Pole region

The C-Band All-Sky Survey (C-BASS) is a high sensitivity all-sky radio survey at an angular resolution of 45 arcmin and a frequency of 4.7 GHz. We present a total intensity map of the North Celestial Pole (NCP) region of sky, above declination &gt;+80°, which is limited by source confusion at a level of ≈0.6 mK rms. We apply the template-fitting (cross-correlation) technique to WMAP and Planck data, using the C-BASS map as the synchrotron template, to investigate the contribution of diffuse foreground emission at frequencies ∼20–40 GHz. We quantify the anomalous microwave emission (AME) that is correlated with far-infrared dust emission. The AME amplitude does not change significantly (⁠&lt;10 per cent⁠) when using the higher frequency C-BASS 4.7 GHz template instead of the traditional Haslam 408 MHz map as a tracer of synchrotron radiation. We measure template coefficients of 9.93 ± 0.35 and 9.52±0.34 K per unit τ353 when using the Haslam and C-BASS synchrotron templates, respectively. The AME contributes 55±2μK rms at 22.8 GHz and accounts for ≈60 per cent of the total foreground emission. Our results show that a harder (flatter spectrum) component of synchrotron emission is not dominant at frequencies ≳5 GHz; the best-fitting synchrotron temperature spectral index is β = −2.91 ± 0.04 from 4.7 to 22.8 GHz and β = −2.85 ± 0.14 from 22.8 to 44.1 GHz. Free–free emission is weak, contributing ≈7μK rms (⁠≈7 per cent⁠) at 22.8 GHz. The best explanation for the AME is still electric dipole emission from small spinning dust grains

The C-Band All-Sky Survey (C-BASS): constraining diffuse Galactic radio emission in the North Celestial Pole region

The C-Band All-Sky Survey (C-BASS) is a high sensitivity all-sky radio survey at an angular resolution of 45 arcmin and a frequency of 4.7 GHz. We present a total intensity map of the North Celestial Pole (NCP) region of sky, above declination >+80°, which is limited by source confusion at a level of ≈0.6 mK rms. We apply the template-fitting (cross-correlation) technique to WMAP and Planck data, using the C-BASS map as the synchrotron template, to investigate the contribution of diffuse foreground emission at frequencies ∼20–40 GHz. We quantify the anomalous microwave emission (AME) that is correlated with far-infrared dust emission. The AME amplitude does not change significantly (⁠<10 per cent⁠) when using the higher frequency C-BASS 4.7 GHz template instead of the traditional Haslam 408 MHz map as a tracer of synchrotron radiation. We measure template coefficients of 9.93 ± 0.35 and 9.52±0.34 K per unit τ353 when using the Haslam and C-BASS synchrotron templates, respectively. The AME contributes 55±2μK rms at 22.8 GHz and accounts for ≈60 per cent of the total foreground emission. Our results show that a harder (flatter spectrum) component of synchrotron emission is not dominant at frequencies ≳5 GHz; the best-fitting synchrotron temperature spectral index is β = −2.91 ± 0.04 from 4.7 to 22.8 GHz and β = −2.85 ± 0.14 from 22.8 to 44.1 GHz. Free–free emission is weak, contributing ≈7μK rms (⁠≈7 per cent⁠) at 22.8 GHz. The best explanation for the AME is still electric dipole emission from small spinning dust grains

The C-Band All-Sky Survey (C-BASS): design and capabilities

The C-Band All-Sky Survey (C-BASS) is an all-sky full-polarization survey at a frequency of 5 GHz, designed to provide complementary data to the all-sky surveys of WMAP and Planck, and future CMB B-mode polarization imaging surveys. The observing frequency has been chosen to provide a signal that is dominated by Galactic synchrotron emission, but suffers little from Faraday rotation, so that the measured polarization directions provide a good template for higher frequency observations, and carry direct information about the Galactic magnetic field. Telescopes in both northern and southern hemispheres with matched optical performance are used to provide all-sky coverage from a ground-based experiment. A continuous-comparison radiometer and a correlation polarimeter on each telescope provide stable imaging properties such that all angular scales from the instrument resolution of 45 arcmin up to full sky are accurately measured. The northern instrument has completed its survey and the southern instrument has started observing. We expect that C-BASS data will significantly improve the component separation analysis of Planck and other CMB data, and will provide important constraints on the properties of anomalous Galactic dust and the Galactic magnetic field