Impact on the tensor-to-scalar ratio of incorrect Galactic foreground modelling

Abstract

A key goal of many Cosmic Microwave Background experiments is the detection of gravitational waves, through their B-mode polarization signal at large scales. To extract such a signal requires modelling contamination from the Galaxy. Using the Planck experiment as an example, we investigate the impact of incorrectly modelling foregrounds on estimates of the polarized CMB, quantified by the bias in tensor-to-scalar ratio r, and optical depth tau. We use a Bayesian parameter estimation method to estimate the CMB, synchrotron, and thermal dust components from simulated observations spanning 30-353 GHz, starting from a model that fits the simulated data, returning r<0.03 at 95% confidence for an r=0 model, and r=0.09+-0.03 for an r=0.1 model. We then introduce a set of mismatches between the simulated data and assumed model. Including a curvature of the synchrotron spectral index with frequency, but assuming a power-law model, can bias r high by ~1-sigma (delta r ~ 0.03). A similar bias is seen for thermal dust with a modified black-body frequency dependence, incorrectly modelled as a power-law. If too much freedom is allowed in the model, for example fitting for spectral indices in 3 degree pixels over the sky with physically reasonable priors, we find r can be biased up to ~3-sigma high by effectively setting the indices to the wrong values. Increasing the signal-to-noise ratio by reducing parameters, or adding additional foreground data, reduces the bias. We also find that neglecting a 1% polarized free-free or spinning dust component has a negligible effect on r. These tests highlight the importance of modelling the foregrounds in a way that allows for sufficient complexity, while minimizing the number of free parameters.

11 pages, 7 figures, submitted to MNRAS

References in corpus (14)

• Three Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Polarization Analysis
• Planck Early Results: The Planck mission
• Extended Anomalous Foreground Emission in the WMAP 3-Year Data
• Planck Early Results XXIV: Dust in the diffuse interstellar medium and the Galactic halo
• Separation of anomalous and synchrotron emissions using WMAP polarization data
• Planck early results. XX. New light on anomalous microwave emission from spinning dust grains
• Planck early results. VI. The High Frequency Instrument data processing
• Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Foreground Polarization
• Planck Early Results. V. The Low Frequency Instrument data processing
• Constraints on the Polarization of the Anomalous Microwave Emission in the Perseus Molecular Complex from 7-year WMAP data
• Bayesian component separation and CMB estimation for the 5-year WMAP temperature data
• A Millimeter-Wave Galactic Plane Survey With The BICEP Polarimeter
• Large-Scale Polarized Foreground Component Separation for Planck
• Bayesian foreground analysis with CMB data

Cited by in corpus (20)

• The State-of-Play of Anomalous Microwave Emission (AME) Research
• SPIDER: Probing the Early Universe with a Suborbital Polarimeter
• Sensitivity and foreground modelling for large-scale CMB B-mode polarization satellite missions
• Exploring Cosmic Origins with CORE: B-mode Component Separation
• Planck intermediate results. L. Evidence for spatial variation of the polarized thermal dust spectral energy distribution and implications for CMB $B$-mode analysis
• Mitigating Complex Dust Foregrounds in Future CMB Polarization Experiments
• The Simons Observatory: pipeline comparison and validation for large-scale B-modes
• On the Detection of CMB B-modes from Ground at Low Frequency
• The C-Band All-Sky Survey (C-BASS): Constraining diffuse Galactic radio emission in the North Celestial Pole region
• A new model of the microwave polarized sky for CMB experiments
• Recovering Cosmic Microwave Background Polarization Signals with Machine Learning
• Impact of modelling foreground uncertainties on future CMB polarization satellite experiments
• Impact of Galactic dust non-Gaussianity on searches for B-modes from inflation
• Joint Bayesian estimation of tensor and lensing B-modes in the power spectrum of CMB polarization data
• Foreground removal requirements for measuring large-scale CMB B-modes in light of BICEP2
• Mitigating Bias in CMB B-modes from Foreground Cleaning Using a Moment Expansion
• Constraining the Anomalous Microwave Emission Mechanism in the S140 Star Forming Region with Spectroscopic Observations Between 4 and 8 GHz at the Green Bank Telescope
• A Bayesian ILC method for CMB B-mode posterior estimation and reconstruction of primordial gravity wave signal
• Galactic foreground constraints on primordial $B$-mode detection for ground-based experiments
• B-mode polarization forecasts for GreenPol