Searcharxiv⌕ Search

arXiv subjects

Simon Prunet

Publications and source records attributed to Simon Prunet.

61 records · Page 4Linked to original sources
arXiv2004

Fast estimation of polarization power spectra using correlation functions

We present a fast method for estimating the cosmic microwave background polarization power spectra using unbiased estimates of heuristically-weighted correlation functions. This extends the O(N_pix^(3/2)) method of Szapudi et al. (2001) to polarized data. If the sky coverage allows the correlation functions to be estimated over the full range of angular separations, they can be inverted directly with integral transforms and clean separation of the electric (E) and magnetic (B) modes of polarization is obtained exactly in the mean. We assess the level of E-B mixing that arises from apodized integral transforms when the correlation function can only be estimated for a subset of angular scales, and show that it is significant for small-area observations. We introduce new estimators to deal with this case on the spherical sky that preserve E-B separation; their construction requires an additional integration of the correlation functions but the computational cost is negligible. We illustrate our methods with application to a large-area survey with parameters similar to PLANCK, and the small-area Background Imaging of Cosmic Extragalactic Polarization experiment. In both cases we show that the errors on the recovered power spectra are close to theoretical expectations.

astro-ph↗
arXiv2001

Dust Polarization From Starlight Data

We present a statistical analysis of the interstellar medium (ISM) polarization from the largest compilation available of starlight data, which comprises ~ 5500 stars. The measured correlation between the mean polarization degree and extinction indicates that ISM dust grains are not fully aligned with the uniform component of the large-scale Galactic magnetic field. Moreover, we estimate the ratio of the uniform to the random plane-of-the-sky components of the magnetic field to be B_u/B_r = 0.8. From the analysis of starlight polarization degree and position angle we find that the magnetic field broadly follows Galactic structures on large-scales. On the other hand, the angular power spectrum C_l of the polarization degree for Galactic plane data is found to be consistent with a power-law, C_l ~ l^{-1.5} (where l = 180 deg/θis the multipole order), for angular scales θ> 10 arcmin. We argue that this data set can be used to estimate diffuse polarized emission at microwave frequencies.

astro-ph↗
arXiv2001

Statistical Properties of Galactic Starlight Polarization

We present a statistical analysis of Galactic interstellar polarization from the largest compilation available of starlight data. The data comprises ~ 9300 stars of which we have selected ~ 5500 for our analysis. We find a nearly linear growth of mean polarization degree with extinction. The amplitude of this correlation shows that interstellar grains are not fully aligned with the Galactic magnetic field, which can be interpreted as the effect of a large random component of the field. In agreement with earlier studies of more limited scope, we estimate the ratio of the uniform to the random plane-of-the-sky components of the magnetic field to be B_u/B_r = 0.8. Moreover, a clear correlation exists between polarization degree and polarization angle what provides evidence that the magnetic field geometry follows Galactic structures on large-scales. The angular power spectrum C_l of the starlight polarization degree for Galactic plane data (|b| < 10 deg) is consistent with a power-law, C_l ~ l^{-1.5} (where l ~ 180 deg/θis the multipole order), for all angular scales θ> 10 arcmin. An investigation of sparse and inhomogeneous sampling of the data shows that the starlight data analyzed traces an underlying polarized continuum that has the same power spectrum slope, C_l ~ l^{-1.5}. Our findings suggest that starlight data can be safely used for the modeling of Galactic polarized continuum emission at other wavelengths.

astro-ph↗
arXiv2000

Fast CMB Analyses via Correlation Functions

We propose and implement a fast, universally applicable method for extracting the angular power spectrum C_l from CMB temperature maps by first estimating the correlation function ξ(θ). Our procedure recovers the C_l's using N^2 (but potentially N logN), operations, where N is the number of pixels. This is in contrast with standard maximum likelihood techniques which require N^3 operations. Our method makes no special assumptions about the map, unlike present fast techniques which rely on symmetries of the underlying noise matrix, sky coverage, scanning strategy, and geometry. This enables for the first time the analysis of megapixel maps without symmetries. The key element of our technique is the accurate multipole decomposition of ξ(θ). The C_l error bars and cross-correlations are found by a Monte-Carlo approach. We applied our technique to a large number of simulated maps with Boomerang sky coverage in 81000 pixels. We used a diagonal noise matrix, with approximately the same amplitude as Boomerang. These studies demonstrate that our technique provides an unbiased estimator of the C_l's. Even though our method is approximate, the error bars obtained are nearly optimal, and converged only after few tens of Monte-Carlo realizations. Our method is directly applicable for the non-diagonal noise matrix. This, and other generalizations, such as minimum variance weighting schemes, polarization, and higher order statistics are also discussed.

astro-ph↗
arXiv1998

Galactic dust polarized emission at high latitudes and CMB polarization

We estimate the dust polarized emission in our galaxy at high galactic latitudes, which is the dominant foreground for measuring CMB polarization using the high frequency instrument (HFI) aboard Planck surveyor. We compare it with the level of CMB polarization and conclude that, for angular scales $\le 1^{\circ}$, the scalar-induced CMB polarization and temperature-polarization cross-correlation are much larger than the foreground level at $ν\simeq 100 GHz$. The tensor-induced signals seem to be at best comparable to the foreground level.}

astro-ph↗
arXiv1997

The IGM at high redshift and galaxy formation

The conditions for structure formation which ultimately lead to galaxies request further ingredients behind the simple collapse criteria. The Jean's criteria and the cooling criteria are those which are currently used. However in such a simple scheme, a fundamental problem occurs in hierarchical pictures, namely the OVERCOOLING: the predicted fraction of primordial gas expected to have cooled in the history of structure formation is for too large. The solution to this problem is likely to be a substantial re-heating phase. Here, we discussed one possible solution: the warm IGM picture. If the feedback of galaxy formation is able to heat the IGM up to temperatures of the order of $10^5-10^6$ K, galaxy formation is inhibited on small mass scale. This leads to an inverse hierarchical picture: most of the large galaxies form at redshifts in the range 3 to 5, while small galaxies form at two different epoch: at an early phase at redshift greater than five and at a late phase, between redshift 3 and 0. Such a scheme may reproduce quite well the amount of HI gas versus redshift.

astro-ph↗