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

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.52). 03/2012; 424(3). DOI: 10.1111/j.1365-2966.2012.21314.x
Source: arXiv

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

  • [Show abstract] [Hide abstract]
    ABSTRACT: This clear, straightforward, fundamental introduction to radiative processes in astrophysics is designed to present - from a physicist's viewpoint - radiation processes and their applications to astrophysical phenomena and space science. The book covers such topics as radiative transfer theory, relativistic covariance and kinematics, bremsstrahlung radiation, Compton scattering, some plasma effects, and radiative transitions in atoms. The discussion begins with first principles, physically motivating and deriving all results rather than merely presenting finished formulas. Much of the prerequisite material is provided by brief reviews, making the book a self-contained reference tool. Also included are about 75 problems with solutions, illustrating applications of the material and methods for calculating results
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a new estimate of foreground emission in the Wilkinson Microwave Anisotropy Probe (WMAP) data, using a Markov chain Monte Carlo method. The new technique delivers maps of each foreground component for a variety of foreground models with estimates of the uncertainty of each foreground component, and it provides an overall goodness-of-fit estimate. The resulting foreground maps are in broad agreement with those from previous techniques used both within the collaboration and by other authors. We find that for WMAP data, a simple model with power-law synchrotron, free-free, and thermal dust components fits 90% of the sky with a reduced χ2 ν of 1.14. However, the model does not work well inside the Galactic plane. The addition of either synchrotron steepening or a modified spinning dust model improves the fit. This component may account for up to 14% of the total flux at the Ka band (33 GHz). We find no evidence for foreground contamination of the cosmic microwave background temperature map in the 85% of the sky used for cosmological analysis.
    The Astrophysical Journal Supplement Series 02/2009; 180(2):265. · 16.24 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A rigorous first order solution with respect to anisotropy is obtained for the equation of polarized radiation transfer in a homogeneous anisotropic universe with a flat co-moving space. The degree of polarization of the background radiation is shown to be very sensitive to the recombination dynamics and to the secondary reheating epoch. Provided that a quadrupole anisotropy of the background radiation is established, the measurements of its polarization degree enable one to set severe limitations on the conditions of secondary ionization.
    Monthly Notices of the Royal Astronomical Society 03/1980; 191:207-215. · 5.52 Impact Factor

Full-text (2 Sources)

Available from
Jun 6, 2014