arXiv subjects

V. Guillet

Publications and source records attributed to V. Guillet.

32 records · Page 2Linked to original sources

arXiv2015

Planck intermediate results. XXXIII. Signature of the magnetic field geometry of interstellar filaments in dust polarization maps

Planck observations at 353GHz provide the first fully-sampled maps of the polarized dust emission towards interstellar filaments and their backgrounds. The polarization data provide insight on the structure of their magnetic field (B). We present the polarization maps of three nearby star forming filament of moderate column density (NH~10^22cm^-2): Musca, B211, and L1506. We use the spatial information to separate Stokes I, Q, and U of the filaments from those of their backgrounds, an essential step to measure the intrinsic polarization fraction (p) and angle (psi) of each emission component. We find that the polarization angles in the three filaments (psi_fil) are coherent along their lengths and not the same as in their backgrounds (psi_bg). The differences between psi_fil and psi_bg are 12deg, 6deg, and 54deg for Musca, B211, and L1506, respectively. These differences for Musca and L1506 are larger than the dispersions of psi, both along the filaments and in their backgrounds. The observed changes of psi are direct evidence for variations of the orientation of the plane of the sky (POS) projection of the B-field. As in previous studies, we find a decrease of several percent of p with NH. We show that the drop in p cannot be explained by random fluctuations of the orientation of B within the filaments because they are too small (sigma_psi<10deg). We recognize the degeneracy between dust alignment efficiency and the structure of B in causing variations in p, but we argue that the decrease of p from the backgrounds to the filaments results in part from depolarization associated with the 3D structure of B: both its orientation in the POS and with respect to the POS. We do not resolve the inner structure of the filaments, but at the smallest scales accessible with Planck (~0.2pc), the observed changes of psi and p hold information on the B-field structure within filaments.

astro-ph.GA↗

arXiv2015

Planck intermediate results. XXII. Frequency dependence of thermal emission from Galactic dust in intensity and polarization

Planck has mapped the intensity and polarization of the sky at microwave frequencies with unprecedented sensitivity. We make use of the Planck 353 GHz I, Q, and U Stokes maps as dust templates, and cross-correlate them with the Planck and WMAP data at 12 frequencies from 23 to 353 GHz, over circular patches with 10 degree radius. The cross-correlation analysis is performed for both intensity and polarization data in a consistent manner. We use a mask that focuses our analysis on the diffuse interstellar medium at intermediate Galactic latitudes. We determine the spectral indices of dust emission in intensity and polarization between 100 and 353 GHz, for each sky-patch. The mean values, $1.59\pm0.02$ for polarization and $1.51\pm0.01$ for intensity, for a mean dust temperature of 19.6 K, are close, but significantly different ($3.6\,σ$). We determine the mean spectral energy distribution (SED) of the microwave emission, correlated with the 353 GHz dust templates, by averaging the results of the correlation over all sky-patches. We find that the mean SED increases for decreasing frequencies at $ν< 60$ GHz, for both intensity and polarization. The rise of the polarization SED towards low frequencies may be accounted for by a synchrotron component correlated with dust, with no need for any polarization of the anomalous microwave emission. We use a spectral model to separate the synchrotron and dust polarization and to characterize the spectral dependence of the dust polarization fraction. The polarization fraction ($p$) of the dust emission decreases by $(21\pm6)$ % from 353 to 70 GHz. The decrease of $p$ could indicate differences in polarization efficiency among components of interstellar dust (e.g., carbon versus silicate grains). Our observational results provide inputs to quantify and optimize the separation between Galactic and cosmological polarization.

astro-ph.GA↗

arXiv2015

Planck intermediate results. XXXII. The relative orientation between the magnetic field and structures traced by interstellar dust

The role of the magnetic field in the formation of the filamentary structures observed in the interstellar medium (ISM) is a debated topic. The Planck all-sky maps of linearly polarized emission from dust at 353GHz provide the required combination of imaging and statistics to study the correlation between the structures of the Galactic magnetic field and of interstellar matter, both in the diffuse ISM and in molecular clouds. The data reveal structures, or ridges, in the intensity map with counterparts in the Stokes Q and/or U maps. We focus on structures at intermediate and high Galactic latitudes with column density from $10^{20}$ to $10^{22}$ cm$^{-2}$. We measure the magnetic field orientation on the plane of the sky from the polarization data, and present an algorithm to estimate the orientation of the ridges from the dust intensity map. We use analytical models to account for projection effects. Comparing polarization angles on and off the structures, we estimate the mean ratio between the strengths of the turbulent and mean components of the magnetic field to be between 0.6 and 1.0, with a preferred value of 0.8. We find that the ridges are preferentially aligned with the magnetic field measured on the structures. This trend becomes more striking for increasing polarization fraction and decreasing column density. We interpret the increase of alignment with polarization fraction as a consequence of projections effects. The decrease of alignment for high column density is not due to a loss of correlation between the structures and the geometry of the magnetic field. In molecular complexes, we observe structures perpendicular to the magnetic field, which cannot be accounted for by projection effects. We discuss our results in the context of models and MHD simulations, which describe the formation of structures in the magnetized ISM.

astro-ph.GA↗

arXiv2015

Planck intermediate results. XXI. Comparison of polarized thermal emission from Galactic dust at 353 GHz with interstellar polarization in the visible

The Planck survey provides unprecedented full-sky coverage of the submillimetre polarized emission from Galactic dust, bringing new constraints on the properties of dust. The dust grains that emit the radiation seen by Planck in the submillimetre also extinguish and polarize starlight in the visible. Comparison of the polarization of the emission and of the interstellar polarization on selected lines of sight probed by stars provides unique new diagnostics of the emission and light scattering properties of dust. Using ancillary catalogues of interstellar polarization and extinction of starlight, we obtain the degree of polarization, $p_V$ , and the optical depth in the V band to the star, $τ_V$. Toward these stars we measure the submillimetre polarized intensity, $P_S$, and total intensity, $I_S$, in the Planck 353 GHz channel. For those lines of sight through the diffuse interstellar medium with comparable values of the estimated column density and polarization directions close to orthogonal, we correlate properties in the submillimetre and visible to find two ratios, $R_{S/V} = (P_S/I_S)/(p_V/τ_V)$ and $R_{P/p} = P_S/p_V$ , the latter focusing directly on the polarization properties of the aligned grain population alone. We find $R_{S/V}$ = 4.2, with statistical and systematic uncertainties 0.2 and 0.3, respectively, and $R_{P/p}$ = 5.4 MJy sr$^{-1}$, with uncertainties 0.2 and 0.3 MJy sr$^{-1}$, respectively. Our estimate of $R_{S/V}$ is compatible with predictions based on a range of polarizing dust models that have been developed for the diffuse interstellar medium. However, our estimate of $R_{P/p}$ is not compatible with predictions, which are too low by a factor of about 2.5. This more discriminating diagnostic, $R_{P/p}$, indicates that changes to the optical properties in the models of the aligned grain population are required.

astro-ph.GA↗

arXiv2014

Planck intermediate results. XXX. The angular power spectrum of polarized dust emission at intermediate and high Galactic latitudes

The polarized thermal emission from Galactic dust is the main foreground present in measurements of the polarization of the cosmic microwave background (CMB) at frequencies above 100GHz. We exploit the Planck HFI polarization data from 100 to 353GHz to measure the dust angular power spectra $C_\ell^{EE,BB}$ over the range $40<\ell<600$ well away from the Galactic plane. These will bring new insights into interstellar dust physics and a precise determination of the level of contamination for CMB polarization experiments. We show that statistical properties of the emission can be characterized over large fractions of the sky using $C_\ell$. For the dust, they are well described by power laws in $\ell$ with exponents $α^{EE,BB}=-2.42\pm0.02$. The amplitudes of the polarization $C_\ell$ vary with the average brightness in a way similar to the intensity ones. The dust polarization frequency dependence is consistent with modified blackbody emission with $β_d=1.59$ and $T_d=19.6$K. We find a systematic ratio between the amplitudes of the Galactic $B$- and $E$-modes of 0.5. We show that even in the faintest dust-emitting regions there are no "clean" windows where primordial CMB $B$-mode polarization could be measured without subtraction of dust emission. Finally, we investigate the level of dust polarization in the BICEP2 experiment field. Extrapolation of the Planck 353GHz data to 150GHz gives a dust power $\ell(\ell+1)C_\ell^{BB}/(2π)$ of $1.32\times10^{-2}μ$K$_{CMB}^2$ over the $40<\ell<120$ range; the statistical uncertainty is $\pm0.29$ and there is an additional uncertainty (+0.28,-0.24) from the extrapolation, both in the same units. This is the same magnitude as reported by BICEP2 over this $\ell$ range, which highlights the need for assessment of the polarized dust signal even in the cleanest windows of the sky.

astro-ph.CO↗

arXiv2014

The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point

Context. The evolution of amorphous hydrocarbon materials, a-C(:H), principally resulting from ultraviolet (UV) photon absorption- induced processing, are likely at the heart of the variations in the observed properties of dust in the interstellar medium. Aims. The consequences of the size-dependent and compositional variations in a-C(:H), from aliphatic-rich a-C:H to aromatic-rich a-C, are studied within the context of the interstellar dust extinction and emission. Methods. Newly-derived optical property data for a-C(:H) materials, combined with that for an amorphous forsterite-type silicate with iron nano-particle inclusions, a-SilFe, are used to explore dust evolution in the interstellar medium. Results. We present a new dust model that consists of a power-law distribution of small a-C grains and log-normal distributions of large a-SilFe and a-C(:H) grains. The model, which is firmly anchored by laboratory-data, is shown to quite naturally explain the variations in the infrared (IR) to far-ultraviolet (FUV) extinction, the 217 nm UV bump, the IR absorption and emission bands and the IR-mm dust emission. Conclusions. The major strengths of the new model are its inherent simplicity and built-in capacity to follow dust evolution in interstellar media. We show that mantle accretion in molecular clouds and UV photo-processing in photo-dominated regions are likely the major drivers of dust evolution.

astro-ph.GA↗

arXiv2014

Planck intermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence

Polarized emission observed by Planck HFI at 353 GHz towards a sample of nearby fields is presented, focusing on the statistics of polarization fractions $p$ and angles $ψ$. The polarization fractions and column densities in these nearby fields are representative of the range of values obtained over the whole sky. We find that: (i) the largest polarization fractions are reached in the most diffuse fields; (ii) the maximum polarization fraction $p_\mathrm{max}$ decreases with column density $N_\mathrm{H}$ in the more opaque fields with $N_\mathrm{H} > 10^{21}\,\mathrm{cm}^{-2}$; and (iii) the polarization fraction along a given line of sight is correlated with the local spatial coherence of the polarization angle. These observations are compared to polarized emission maps computed in simulations of anisotropic magnetohydrodynamical (MHD) turbulence in which we assume a uniform intrinsic polarization fraction of the dust grains. We find that an estimate of this parameter may be recovered from the maximum polarization fraction $p_\mathrm{max}$ in diffuse regions where the magnetic field is ordered on large scales and perpendicular to the line of sight. This emphasizes the impact of anisotropies of the magnetic field on the emerging polarization signal. The decrease of the polarization fraction with column density in nearby molecular clouds is well reproduced in the simulations, indicating that it is essentially due to the turbulent structure of the magnetic field: an accumulation of variously polarized structures along the line of sight leads to such an anti-correlation. In the simulations, polarization fractions are also found to anti-correlate with the angle dispersion function $\mathcal{S}$. [abridged]

astro-ph.GA↗

arXiv2014

Planck 2013 results. XI. All-sky model of thermal dust emission

This paper presents an all-sky model of dust emission from the Planck 857, 545 and 353 GHz, and IRAS 100 micron data. Using a modified black-body fit to the data we present all-sky maps of the dust optical depth, temperature, and spectral index over the 353-3000 GHz range. This model is a tight representation of the data at 5 arc min. It shows variations of the order of 30 % compared with the widely-used model of Finkbeiner, Davis, and Schlegel. The Planck data allow us to estimate the dust temperature uniformly over the whole sky, providing an improved estimate of the dust optical depth compared to previous all-sky dust model, especially in high-contrast molecular regions. An increase of the dust opacity at 353 GHz, tau_353/N_H, from the diffuse to the denser interstellar medium (ISM) is reported. It is associated with a decrease in the observed dust temperature, T_obs, that could be due at least in part to the increased dust opacity. We also report an excess of dust emission at HI column densities lower than 10^20 cm^-2 that could be the signature of dust in the warm ionized medium. In the diffuse ISM at high Galactic latitude, we report an anti-correlation between tau_353/N_H and T_obs while the dust specific luminosity, i.e., the total dust emission integrated over frequency (the radiance) per hydrogen atom, stays about constant. The implication is that in the diffuse high-latitude ISM tau_353 is not as reliable a tracer of dust column density as we conclude it is in molecular clouds where the correlation of tau_353 with dust extinction estimated using colour excess measurements on stars is strong. To estimate Galactic E(B-V) in extragalactic fields at high latitude we develop a new method based on the thermal dust radiance, instead of the dust optical depth, calibrated to E(B-V) using reddening measurements of quasars deduced from Sloan Digital Sky Survey data.

astro-ph.GA↗

arXiv2014

Shocks in dense clouds. IV. Effects of grain-grain processing on molecular line emission

Grain-grain processing has been shown to be an indispensable ingredient of shock modelling in high density environments. For densities higher than \sim10^5 cm-3, shattering becomes a self-enhanced process that imposes severe chemical and dynamical consequences on the shock characteristics. Shattering is accompanied by the vaporization of grains, which can directly release SiO to the gas phase. Given that SiO rotational line radiation is used as a major tracer of shocks in dense clouds, it is crucial to understand the influence of vaporization on SiO line emission. We have developed a recipe for implementing the effects of shattering and vaporization into a 2-fluid shock model, resulting in a reduction of computation time by a factor \sim100 compared to a multi-fluid modelling approach. This implementation was combined with an LVG-based modelling of molecular line radiation transport. Using this model we calculated grids of shock models to explore the consequences of different dust-processing scenarios. Grain-grain processing is shown to have a strong influence on C-type shocks for a broad range of magnetic fields: they become hotter and thinner. The reduction in column density of shocked gas lowers the intensity of molecular lines, at the same time as higher peak temperatures increase the intensity of highly excited transitions compared to shocks without grain-grain processing. For OH the net effect is an increase in line intensities, while for CO and H2O it is the contrary. The intensity of H2 emission is decreased in low transitions and increased for highly excited lines. For all molecules, the highly excited lines become sensitive to the value of the magnetic field. Although vaporization increases the intensity of SiO rotational lines, this effect is weakened by the reduced shock width. The release of SiO early in the hot shock changes the excitation characteristics of SiO radiation.

astro-ph.GA↗

arXiv2014

Polarization measurements analysis II. Best estimators of polarization fraction and angle

With the forthcoming release of high precision polarization measurements, such as from the Planck satellite, it becomes critical to evaluate the performance of estimators for the polarization fraction and angle. These two physical quantities suffer from a well-known bias in the presence of measurement noise, as has been described in part I of this series. In this paper, part II of the series, we explore the extent to which various estimators may correct the bias. Traditional frequentist estimators of the polarization fraction are compared with two recent estimators: one inspired by a Bayesian analysis and a second following an asymptotic method. We investigate the sensitivity of these estimators to the asymmetry of the covariance matrix which may vary over large datasets. We present for the first time a comparison among polarization angle estimators, and evaluate the statistical bias on the angle that appears when the covariance matrix exhibits effective ellipticity. We also address the question of the accuracy of the polarization fraction and angle uncertainty estimators. The methods linked to the credible intervals and to the variance estimates are tested against the robust confidence interval method. From this pool of estimators, we build recipes adapted to different use-cases: build a mask, compute large maps, and deal with low S/N data. More generally, we show that the traditional estimators suffer from discontinuous distributions at low S/N, while the asymptotic and Bayesian methods do not. Attention is given to the shape of the output distribution of the estimators, and is compared with a Gaussian. In this regard, the new asymptotic method presents the best performance, while the Bayesian output distribution is shown to be strongly asymmetric with a sharp cut at low S/N.Finally, we present an optimization of the estimator derived from the Bayesian analysis using adapted priors.

astro-ph.IM↗

arXiv2014

Planck intermediate results. XVII. Emission of dust in the diffuse interstellar medium from the far-infrared to microwave frequencies

The dust-HI correlation is used to characterize the emission properties of dust in the diffuse interstellar medium. We cross-correlate sky maps from Planck, WMAP, and DIRBE, at 17 frequencies from 23 to 3000 GHz, with the Parkes survey of the 21-cm line emission of neutral atomic hydrogen, over a contiguous area of 7500 deg$^2$ centred on the southern Galactic pole. Our analysis yields four specific results. (1) The dust temperature is observed to be anti-correlated with the dust emissivity and opacity. We interpret this result as evidence for dust evolution within the diffuse ISM. The mean dust opacity is measured to be $(7.1 \pm 0.6) 10^{-27} cm^2/H \times (ν/353\, GHz)^{1.53\pm0.03}$ for $100 < ν<353$GHz. (2) We map the spectral index of dust emission at millimetre wavelengths, which is remarkably constant at $β_{mm} = 1.51\pm 0.13$. We compare it with the far infrared spectral index beta_FIR derived from greybody fits at higher frequencies, and find a systematic difference, $β_{mm}-β_{FIR} = -0.15$, which suggests that the dust SED flattens at $ν< 353\,$GHz. (3) We present spectral fits of the microwave emission correlated with HI from 23 to 353 GHz, which separate dust and anomalous microwave emission. The flattening of the dust SED can be accounted for with an additional component with a blackbody spectrum, which accounts for $(26 \pm 6)$% of the dust emission at 100 GHz and could represent magnetic dipole emission. Alternatively, it could account for an increasing contribution of carbon dust, or a flattening of the emissivity of amorphous silicates, at millimetre wavelengths. These interpretations make different predictions for the dust polarization SED. (4) We identify a Galactic contribution to the residuals of the dust-HI correlation, which we model with variations of the dust emissivity on angular scales smaller than that of our correlation analysis.

astro-ph.GA↗

arXiv2014

Planck intermediate results. XIX. An overview of the polarized thermal emission from Galactic dust

This paper presents the large-scale polarized sky as seen by Planck HFI at 353 GHz, which is the most sensitive Planck channel for dust polarization. We construct and analyse large-scale maps of dust polarization fraction and polarization direction, while taking account of noise bias and possible systematic effects. We find that the maximum observed dust polarization fraction is high (pmax > 18%), in particular in some of the intermediate dust column density (AV < 1mag) regions. There is a systematic decrease in the dust polarization fraction with increasing dust column density, and we interpret the features of this correlation in light of both radiative grain alignment predictions and fluctuations in the magnetic field orientation. We also characterize the spatial structure of the polarization angle using the angle dispersion function and find that, in nearby fields at intermediate latitudes, the polarization angle is ordered over extended areas that are separated by filamentary structures, which appear as interfaces where the magnetic field sky projection rotates abruptly without apparent variations in the dust column density. The polarization fraction is found to be anti-correlated with the dispersion of the polarization angle, implying that the variations are likely due to fluctuations in the 3D magnetic field orientation along the line of sight sampling the diffuse interstellar medium.We also compare the dust emission with the polarized synchrotron emission measured with the Planck LFI, with low-frequency radio data, and with Faraday rotation measurements of extragalactic sources. The two polarized components are globally similar in structure along the plane and notably in the Fan and North Polar Spur regions. A detailed comparison of these three tracers shows, however, that dust and cosmic rays generally sample different parts of the line of sight and confirms that much of the variation observed in the Planck data is due to the 3D structure of the magnetic field.

astro-ph.GA↗

arXiv2012

Modelling the dust emission from dense interstellar clouds: disentangling the effects of radiative transfer and dust properties

With Planck and Herschel, we now have the spectral coverage and angular resolution required to observe dense and cold molecular clouds. As these clouds are optically thick at short wavelength but optically thin at long wavelength, it is tricky to conclude anything about dust properties without a proper treatment of the radiative transfer (RT). Our aim is to disentangle the effects of RT and of dust properties on the variations in the dust emission to provide observers with keys to analyse the emission arising from dense clouds. We model cylindrical clouds, illuminated by the ISRF, and carry out full RT calculations. Dust temperatures are solved using DustEM for amorphous carbons and silicates, grains coated with carbon mantles, and mixed aggregates of carbon and silicate. We allow variations of the grain optical properties with wavelength and temperature. We determine observed colour temperatures, T, and emissivity spectral indices, beta, by fitting the dust emission with modified blackbodies, to compare our models with observations. RT effects can neither explain the low T nor the increased submm emissivity measured at the centre of dense clouds, nor the observed beta-T anti-correlation. Adding noise to the modelled data, we show that it is not likely to be the unique explanation for the beta-T anti-correlation observed in starless clouds. It may be explained by intrinsic variations in the grain optical properties with temperature. As for the increased submm emissivity and the low T, they have to originate in variations in the grain optical properties, probably caused by their growth to form porous aggregates. We find it difficult to track back the nature of grains from the spectral variations in their emission. Finally, the column density is underestimated when determined with blackbody fitting because of the discrepancy between T and the true dust temperature at the cloud centre.

astro-ph.GA↗

arXiv2011

Planck Early Results: Thermal dust in Nearby Molecular Clouds

Planck allows unbiased mapping of Galactic sub-millimetre and millimetre emission from the most diffuse regions to the densest parts of molecular clouds. We present an early analysis of the Taurus molecular complex, on line-of-sight-averaged data and without component separation. The emission spectrum measured by Planck and IRAS can be fitted pixel by pixel using a single modified blackbody. Some systematic residuals are detected at 353 GHz and 143 GHz, with amplitudes around -7 % and +13 %, respectively, indicating that the measured spectra are likely more complex than a simple modified blackbody. Significant positive residuals are also detected in the molecular regions and in the 217 GHz and 100 GHz bands, mainly caused by to the contribution of the J=2-1 and J=1-0 12CO and 13CO emission lines. We derive maps of the dust temperature T, the dust spectral emissivity index beta, and the dust optical depth at 250 microns tau. The temperature map illustrates the cooling of the dust particles in thermal equilibrium with the incident radiation field, from 16-17 K in the diffuse regions to 13-14 K in the dense parts. The distribution of spectral indices is centred at 1.78, with a standard deviation of 0.08 and a systematic error of 0.07. We detect a significant T-beta anti-correlation. The dust optical depth map reveals the spatial distribution of the column density of the molecular complex from the densest molecular regions to the faint diffuse regions. We use near-infrared extinction and HI data at 21-cm to perform a quantitative analysis of the spatial variations of the measured dust optical depth at 250 microns per hydrogen atom tau/NH. We report an increase of tau/NH by a factor of about 2 between the atomic phase and the molecular phase, which has a strong impact on the equilibrium temperature of the dust particles.

astro-ph.GA↗