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

The Galactic dust as a foreground to Cosmic Microwave Background maps

We present results obtained with the PRONAOS balloon-borne experiment on interstellar dust. In particular, the submillimeter / millimeter spectral index is found to vary between roughly 1 and 2.5 on small scales (3.5' resolution). This could have implications for component separation in Cosmic Microwave Background maps.Comment: 4 pages, 1 figure, proceeding of the Multi-Wavelength Cosmology conference held in Mykonos, Greece, June 2003, ed. Kluwe

Toward a better understanding of the mid-infrared emission in the LMC

In this paper we aim to constrain for the first time the dust emission in the mid-to-far infrared domain, in the LMC, with the use of the Spitzer IRS and MIPS SED data, combined with Herschel data. We also consider UV extinction predictions derived from modeling. We selected 10 regions observed as part of the SAGE-Spec program, to probe dust properties in various environments (diffuse, molecular and ionized regions). All data were smoothed to the 40arcsec angular resolution. The SEDs were modeled with DustEM models, using the standard Mathis RF, as well as three additional RFs, with stellar clusters ages ranging from 4 Myr to 600 Myr. Standard dust models used to reproduce the Galactic diffuse medium are clearly not able to reproduce the dust emission in the MIR wavelength domain. This analysis evidences the need of adjusting parameters describing the dust size distribution and shows a clear distinct behavior according to the type of environments. In addition, whereas the small grain emission always seems to be negligible at long wavelengths in our Galaxy, the contribution of this small dust component could be more important than expected, in the submm-mm range, in the LMC averaged SED. Properties of the small dust component of the LMC are clearly different from those of our Galaxy. Its abundance, significantly enhanced, could be the result of large grains shattering due to strong shocks or turbulence. In addition, this grain component in the LMC systematically shows smaller grain size in the ionized regions compared to the diffuse medium. Predictions of extinction curves show significantly distinct behaviors depending on the dust models but also from one region to another. Comparison of model predictions with the LMC mean extinction curve shows that no model gives satisfactory agreement using the Mathis radiation field while using a harder radiation field tends to improve the agreementComment: Accepted for publication in A&

A CO survey on a sample of Herschel cold clumps

Context. The physical state of cold cloud clumps has a great impact on the process and efficiency of star formation and the masses of the forming stars inside these objects. The sub-millimetre survey of the Planck space observatory and the far-infrared follow-up mapping of the Herschel space telescope provide an unbiased, large sample of these cold objects. Aims. We have observed (CO)-C-12(1-0) and (CO)-C-13(1-0) emission in 35 high-density clumps in 26 Herschel fields sampling different environments in the Galaxy. Here, we aim to derive the physical properties of the objects and estimate their gravitational stability. Methods. The densities and temperatures of the clumps were calculated from both the dust continuum and the molecular line data. Kinematic distances were derived using (CO)-C-13(1-0) line velocities to verify previous distance estimates and the sizes and masses of the objects were calculated by fitting 2D Gaussian functions to their optical depth distribution maps on 250 mu m. The masses and virial masses were estimated assuming an upper and lower limit on the kinetic temperatures and considering uncertainties due to distance limitations. Results. The derived excitation temperatures are between 8.5-19.5 K, and for most clumps between 10 15 K, while the Herschel-derived dust colour temperatures are more uniform, between 12 16 K. The sizes (0.1-3 pc), (CO)-C-13 column densities (0.5-44 x 10(15) cm(-2)) and masses (from less than 0.1 M-circle dot to more than 1500 M-circle dot) of the objects all span broad ranges. We provide new kinematic distance estimates, identify gravitationally bound or unbound structures and discuss their nature. Conclusions. The sample contains objects on a wide scale of temperatures, densities and sizes. Eleven gravitationally unbound clumps were found, many of them smaller than 0.3 pc, but large, parsec-scale clouds with a few hundred solar masses appear as well. Colder clumps have generally high column densities but warmer objects appear at both low and higher column densities. The clump column densities derived from the line and dust observations correlate well, but are heavily affected by uncertainties of the dust properties, varying molecular abundances and optical depth effects.Peer reviewe

Variations of the spectral index of dust emissivity from Hi-GAL observations of the Galactic plane

Original article can be found at: http://www.aanda.org/ Copyright The European Southern ObservatoryContext. Variations in the dust emissivity are critical for gas mass determinations derived from far-infrared observations, but also for separating dust foreground emission from the Cosmic Microwave Background (CMB). Hi-GAL observations allow us for the first time to study the dust emissivity variations in the inner regions of the Galactic plane at resolution below 1°. Aims. We present maps of the emissivity spectral index derived from the combined Herschel PACS 160 μm, SPIRE 250 μm, 350 μm, and 500 μm data, and the IRIS 100 μm data, and we analyze the spatial variations of the spectral index as a function of dust temperature and wavelength in the two science demonstration phase Hi-GAL fields, centered at l = 30° and l = 59°. Methods. Applying two different methods, we determine both dust temperature and emissivity spectral index between 100 and 500 μm, at an angular resolution (θ) of 4'. Results. Combining both fields, the results show variations of the emissivity spectral index in the range 1.8–2.6 for temperatures between 14 and 23 K. The median values of the spectral index are similar in both fields, i.e. 2.3 in the range 100–500 μm, while the median dust temperatures are equal to 19.1 K and 16.0 K in the l = 30° and l = 59° field, respectively. Statistically, we do not see any significant deviations in the spectra from a power law emissivity between 100 and 500 μm. We confirm the existence of an inverse correlation between the emissivity spectral index and dust temperature, found in previous analyses.Peer reviewe

The complete submillimetre spectrum of NGC 891

Submillimetre maps of NGC 891 have been obtained with the PRONAOS balloon-borne telescope and with the ISOPHOT instrument on board the ISO satellite. In this article, we also gather data from IRAS and SCUBA to present the complete submillimetre spectrum of this nearby edge-on spiral galaxy. We derive submillimetre emission profiles along the major axis. The modified blackbody fits, assuming a single dust component, lead to temperatures of 19-24 K toward the centre and 18-20 K toward the edges, with possible variations of the dust spectral index from 1.4 to 2. The two-component fits lead to a warm component temperature of 29 K all along the galaxy with a cold component at 16 K. The interstellar medium masses derived by these two methods are quite different: 4.6 x 10^9 Mo in the case of the one-component model and 12 x 10^9 Mo in the case of the two-component one. This two-component fit indicates that the cold dust to warm dust ratio is 20 to 40, the highest values being in the wings of this galaxy. Comparing to dust mass estimates, both estimations of the ISM mass are consistent with a gas to dust mass ratio of 240, which is close to the Milky Way value. Our results illustrate the importance of accurate submillimetre spectra to derive masses of the interstellar medium in galaxies.Comment: 5 pages, 2 figures, accepted May 2003 in the MNRA

Revisiting the dust properties in the molecular clouds of the Large Magellanic Cloud

Context. Some Galactic molecular clouds show signs of dust evolution as compared to the diffuse interstellar medium, most of the time through indirect evidence such as color ratios, increased dust emissivity, or scattering (coreshine). These signs are not a feature of all Galactic clouds. Moreover, molecular clouds in the Large Magellanic Cloud (LMC) have been analyzed in a previous study based on Spitzer and IRIS data, at 4' angular resolution, with the use of one single dust model, and did not show any signs of dust evolution. Aims. In this present analysis we investigate the dust properties associated with the different gas phases (including the ionized phase this time) of the LMC molecular clouds at 1' angular resolution (four times greater than the previous analysis) and with a larger spectral coverage range thanks to Herschel data. We also ensure the robustness of our results in the framework of various dust models. Methods. We performed a decomposition of the dust emission in the infrared (from 3.6 to 500 mu m) associated with the atomic, molecular, and ionized gas phases in the molecular clouds of the LMC. The resulting spectral energy distributions were fitted with four distinct dust models. We then analyzed the model parameters such as the intensity of the radiation field and the relative dust abundances, as well as the slope of the emission spectra at long wavelengths. Results. This work allows dust models to be compared with infrared data in various environments for the first time, which reveals important differences between the models at short wavelengths in terms of data fitting (mainly in the polycyclic aromatic hydrocarbon bands). In addition, this analysis points out distinct results according to the gas phases, such as dust composition directly affecting the dust temperature and the dust emissivity in the submillimeter and different dust emission in the near-infrared (NIR). Conclusions. We observe direct evidence of dust property evolution from the diffuse to the dense medium in a large sample of molecular clouds in the LMC. In addition, the differences in the dust component abundances between the gas phases could indicate different origins of grain formation. We also point out the presence of a NIR-continuum in all gas phases, with an enhancement in the ionized gas. We favor the hypothesis of an additional dust component as the carrier of this continuum.Peer reviewe

Multiwavelength study of the high-latitude cloud L1642: chain of star formation

L1642 is one of the two high galactic latitude (|b| > 30deg) clouds confirmed to have active star formation. We examine the properties of this cloud, especially the large-scale structure, dust properties, and compact sources in different stages of star formation. We present high-resolution far-infrared and submm observations with the Herschel and AKARI satellites and mm observations with the AzTEC/ASTE telescope, which we combined with archive data from near- and mid-infrared (2MASS, WISE) to mm observations (Planck). The Herschel observations, combined with other data, show a sequence of objects from a cold clump to young stellar objects at different evolutionary stages. Source B-3 (2MASS J04351455-1414468) appears to be a YSO forming inside the L1642 cloud, instead of a foreground brown dwarf, as previously classified. Herschel data reveal striation in the diffuse dust emission around L1642. The western region shows striation towards NE and has a steeper column density gradient on its southern side. The densest central region has a bow-shock like structure showing compression from the west and a filamentary tail extending towards east. The differences suggest that these may be spatially distinct structures, aligned only in projection. We derive values of the dust emission cross-section per H nucleon for different regions of the cloud. Modified black-body fits to the spectral energy distribution of Herschel and Planck data give emissivity spectral index beta values 1.8-2.0 for the different regions. The compact sources have lower beta values and show an anticorrelation between T and beta. Markov chain Monte Carlo calculations demonstrate the strong anticorrelation between beta and T errors and the importance of mm Planck data in constraining the estimates. L1642 reveals a more complex structure and sequence of star formation than previously known.Comment: 22 pages, 18 figures, accepted to Astronomy & Astrophysics; abstract shortened and figures reduced for astrop