Polarimétrie aux longueurs d'onde millimétriques avec les instruments NIKA et NIKA2

The role played by magnetic fields in the star formation process is an outstanding question of modern astrophysics. Herschel satellite observations have unveiled filamentary structures as the preferential sites of star formation. Complementary low resolution observations of dust polarization by the Planck satellite have demonstrated that these filamentary structures are associated to well organized magnetic fields, which should play a major role in this process. A better understanding of this process requires detailed observations of galactic dust polarization on scales of 0.01 pc to 0.1 pc. Such high resolution polarization observations can be carried out at the IRAM 30 meter telescope using the recently installed New IRAM Kid Array (NIKA2) camera, which features two frequency bands at 260 (polarized) and 150 (non polarized) GHz for a total of 3300 detectors, 12 and 18 arcsec FWHM resolution and Field of View (FoV) of 6.5 arcminutes diameter. The NIKA camera, which consists of two arrays of 132 and 224 LEKIDs (Lumped Element Kinetic Inductance Detectors) covering a FoV of ∼1.8 arcminutes at 1.15 (260) and 2.05 (150) mm (GHz), was installed at the IRAM 30 telescope from 2012 to 2015.The purpose of this thesis was the characterization of the NIKA polarization system performance. The system consisted of a rotating multi-mesh half wave plate and a grid polarizer. The rapidity of the LEKID detectors combined to the modulation of the HWP permits the simultaneous measurement of the three stokes parameters I,Q,U, components of the linear polarization. Then the signal is extracted with a lock-in procedure by isolating the amplitude of the mechanical rotation fourth harmonic.In the first part of the thesis the instrumental efficiency characterization of the system is presented. Later, the dedicated polarization data analysis is described. It has been specifically developed for the NIKA instrument and has allowed to map polarisation observations of compact and extended sources.The data reduction of unpolarized sources observations showed an instrumental polarization systematic effect. In order to correct for this effect an algorithm has been developed allowing to reduce an observed instrumental polarization of the order of ~ 3 % to below ~ 1%. In addition, polarized sources observations corrected for the systematic effect confirmed the potentiality of the NIKA polarimeter to measure the polarization. This opens the way to forthcoming observations with NIKA2 that will undoubtedly provide advances in the field of Galactic emission and interactions with the magnetic field.Le rôle des champs magnétiques dans le processus de formation stellaire reste une question ouverte de l'astrophysique moderne.Récemment, les observations du satellite Herschel ont dévoilé que certaines structures filamenteuses seraient propices à ces processus de formation d’étoile. Ces structures filamenteuses ont été associées à des régions de champ magnétique bien organisé grâce aux observations polarimétriques du satellite Planck. Une meilleure compréhension du rôle des champs magnétiques lors de la formation stellaire requiert néanmoins des observations polarimétriques détaillées de la poussière galactique aux échelles de 0.01 - 0.1 pc.Des observations polarimétriques à hautes résolutions peuvent être effectuées au télescope de 30 mètres de l’IRAM en utilisant la caméra NIKA2.Le but de cette thèse a été de valider le concept et de caractériser les performances du polarimètre monté sur la caméra NIKA2. Ces tests et caractérisations ont été réalisés sur NIKA, le démonstrateur technologique de NIKA2, installée à l’IRAM de 2012 à 2015.NIKA consiste en deux matrices de 132 et 224 LEKIDS (Lumped Element Kinetic Inductance Detectors) qui couvrent un champ de vue (FoV) de ~1.8 arcminutes à 1.15 (260) et 2.05 (150) mm (GHz).Le système de polarisation est constitué d’une lame demi-onde tournante à mesh métalliques(HWP) et d’une grille métallique (polariseur). La rapidité des détecteurs LEKIDs combinée à la modulation de la lame permettent la mesure simultanée des trois paramètres de Stokes, I, Q, U, caractérisant la polarisation linéaire. Le signal est extrait en utilisant une procédure de “démodulation”, qui est équivalente à un lock-in autour de la quatrième harmonique de la fréquence de rotation de la lame.Dans la première partie de la thèse je présente la caractérisation instrumentale de l’efficacité du système développé. Ensuite, la chaîne d’analyse des données dédiée à la polarisation est présentée. Cette dernière a été spécifiquement développée pour l'instrument NIKA et a permis de reconstruire le signal observé en polarisation et de le projeter sur des cartes.La réduction des données observationnelles de sources non polarisées a révélé la présence d’un effet systématique. Celui-ci a été interprété comme résultant d’une polarisation instrumentale due à une perte d’intensité I vers les canaux de polarisation Q et U.Afin de corriger cet effet, un algorithme a été développé. Celui-ci permet de ramener la polarisation instrumentale, initialement de l'ordre de ~3 %, en-dessous du pour-cent.Enfin, les observations de sources polarisées corrigées de l'effet systématique ont confirmé le potentiel du polarimètre NIKA afin de mesurer la polarisation du ciel.Ces résultats ouvrent la voie à de prochaines observations polarimétriques avec la caméra NIKA2, installée en octobre 2015.Cette caméra dispose de deux bandes de fréquences à 260 (polarisé) et 150 (non polarisé) GHz pour un total de 3300 détecteurs, 12 et 18 arcsec FWHM de résolution et FoV de 6.5 diamètre arcminutes.Cette caméra fournira sans aucun doute une meilleure compréhension des interactions du champ magnétique dans le milieu interstellaire de notre Galaxie

Polarimetry at millimeter wavelenghts with the NIKA and NIKA2 instruments

Le rôle des champs magnétiques dans le processus de formation stellaire reste une question ouverte de l'astrophysique moderne.Récemment, les observations du satellite Herschel ont dévoilé que certaines structures filamenteuses seraient propices à ces processus de formation d’étoile. Ces structures filamenteuses ont été associées à des régions de champ magnétique bien organisé grâce aux observations polarimétriques du satellite Planck. Une meilleure compréhension du rôle des champs magnétiques lors de la formation stellaire requiert néanmoins des observations polarimétriques détaillées de la poussière galactique aux échelles de 0.01 - 0.1 pc.Des observations polarimétriques à hautes résolutions peuvent être effectuées au télescope de 30 mètres de l’IRAM en utilisant la caméra NIKA2.Le but de cette thèse a été de valider le concept et de caractériser les performances du polarimètre monté sur la caméra NIKA2. Ces tests et caractérisations ont été réalisés sur NIKA, le démonstrateur technologique de NIKA2, installée à l’IRAM de 2012 à 2015.NIKA consiste en deux matrices de 132 et 224 LEKIDS (Lumped Element Kinetic Inductance Detectors) qui couvrent un champ de vue (FoV) de ~1.8 arcminutes à 1.15 (260) et 2.05 (150) mm (GHz).Le système de polarisation est constitué d’une lame demi-onde tournante à mesh métalliques(HWP) et d’une grille métallique (polariseur). La rapidité des détecteurs LEKIDs combinée à la modulation de la lame permettent la mesure simultanée des trois paramètres de Stokes, I, Q, U, caractérisant la polarisation linéaire. Le signal est extrait en utilisant une procédure de “démodulation”, qui est équivalente à un lock-in autour de la quatrième harmonique de la fréquence de rotation de la lame.Dans la première partie de la thèse je présente la caractérisation instrumentale de l’efficacité du système développé. Ensuite, la chaîne d’analyse des données dédiée à la polarisation est présentée. Cette dernière a été spécifiquement développée pour l'instrument NIKA et a permis de reconstruire le signal observé en polarisation et de le projeter sur des cartes.La réduction des données observationnelles de sources non polarisées a révélé la présence d’un effet systématique. Celui-ci a été interprété comme résultant d’une polarisation instrumentale due à une perte d’intensité I vers les canaux de polarisation Q et U.Afin de corriger cet effet, un algorithme a été développé. Celui-ci permet de ramener la polarisation instrumentale, initialement de l'ordre de ~3 %, en-dessous du pour-cent.Enfin, les observations de sources polarisées corrigées de l'effet systématique ont confirmé le potentiel du polarimètre NIKA afin de mesurer la polarisation du ciel.Ces résultats ouvrent la voie à de prochaines observations polarimétriques avec la caméra NIKA2, installée en octobre 2015.Cette caméra dispose de deux bandes de fréquences à 260 (polarisé) et 150 (non polarisé) GHz pour un total de 3300 détecteurs, 12 et 18 arcsec FWHM de résolution et FoV de 6.5 diamètre arcminutes.Cette caméra fournira sans aucun doute une meilleure compréhension des interactions du champ magnétique dans le milieu interstellaire de notre Galaxie.The role played by magnetic fields in the star formation process is an outstanding question of modern astrophysics. Herschel satellite observations have unveiled filamentary structures as the preferential sites of star formation. Complementary low resolution observations of dust polarization by the Planck satellite have demonstrated that these filamentary structures are associated to well organized magnetic fields, which should play a major role in this process. A better understanding of this process requires detailed observations of galactic dust polarization on scales of 0.01 pc to 0.1 pc. Such high resolution polarization observations can be carried out at the IRAM 30 meter telescope using the recently installed New IRAM Kid Array (NIKA2) camera, which features two frequency bands at 260 (polarized) and 150 (non polarized) GHz for a total of 3300 detectors, 12 and 18 arcsec FWHM resolution and Field of View (FoV) of 6.5 arcminutes diameter. The NIKA camera, which consists of two arrays of 132 and 224 LEKIDs (Lumped Element Kinetic Inductance Detectors) covering a FoV of ∼1.8 arcminutes at 1.15 (260) and 2.05 (150) mm (GHz), was installed at the IRAM 30 telescope from 2012 to 2015.The purpose of this thesis was the characterization of the NIKA polarization system performance. The system consisted of a rotating multi-mesh half wave plate and a grid polarizer. The rapidity of the LEKID detectors combined to the modulation of the HWP permits the simultaneous measurement of the three stokes parameters I,Q,U, components of the linear polarization. Then the signal is extracted with a lock-in procedure by isolating the amplitude of the mechanical rotation fourth harmonic.In the first part of the thesis the instrumental efficiency characterization of the system is presented. Later, the dedicated polarization data analysis is described. It has been specifically developed for the NIKA instrument and has allowed to map polarisation observations of compact and extended sources.The data reduction of unpolarized sources observations showed an instrumental polarization systematic effect. In order to correct for this effect an algorithm has been developed allowing to reduce an observed instrumental polarization of the order of ~ 3 % to below ~ 1%. In addition, polarized sources observations corrected for the systematic effect confirmed the potentiality of the NIKA polarimeter to measure the polarization. This opens the way to forthcoming observations with NIKA2 that will undoubtedly provide advances in the field of Galactic emission and interactions with the magnetic field

NIKA2 observations around LBV stars Emission from stars and circumstellar material

Luminous Blue Variable (LBV) stars are evolved massive objects, previous to core-collapse supernova. LBVs are characterized by photometric and spectroscopic variability, produced by strong and dense winds, mass-loss events and very intense UV radiation. LBVs strongly disturb their surroundings by heating and shocking, and produce important amounts of dust. The study of the circumstellar material is therefore crucial to understand how these massive stars evolve, and also to characterize their effects onto the interstellar medium. The versatility of NIKA2 is a key in providing simultaneous observations of both the stellar continuum and the extended, circumstellar contribution. The NIKA2 frequencies (150 and 260 GHz) are in the range where thermal dust and free-free emission compete, and hence NIKA2 has the capacity to provide key information about the spatial distribution of circumstellar ionized gas, warm dust and nearby dark clouds; non-thermal emission is also possible even at these high frequencies. We show the results of the first NIKA2 survey towards five LBVs. We detected emission from four stars, three of them immersed in tenuous circumstellar material. The spectral indices show a complex distribution and allowed us to separate and characterize different components. We also found nearby dark clouds, with spectral indices typical of thermal emission from dust. Spectral indices of the detected stars are negative and hard to be explained only by free-free processes. In one of the sources, G79.29+0.46, we also found a strong correlation of the 1mm and 2mm continuum emission with respect to nested molecular shells at ≈1 pc from the LBV. The spectral index in this region clearly separates four components: the LBV star, a bubble characterized by free-free emission, and a shell interacting with a nearby infrared dark cloud

NIKA2 observations around LBV stars Emission from stars and circumstellar material

Luminous Blue Variable (LBV) stars are evolved massive objects, previous to core-collapse supernova. LBVs are characterized by photometric and spectroscopic variability, produced by strong and dense winds, mass-loss events and very intense UV radiation. LBVs strongly disturb their surroundings by heating and shocking, and produce important amounts of dust. The study of the circumstellar material is therefore crucial to understand how these massive stars evolve, and also to characterize their effects onto the interstellar medium. The versatility of NIKA2 is a key in providing simultaneous observations of both the stellar continuum and the extended, circumstellar contribution. The NIKA2 frequencies (150 and 260 GHz) are in the range where thermal dust and free-free emission compete, and hence NIKA2 has the capacity to provide key information about the spatial distribution of circumstellar ionized gas, warm dust and nearby dark clouds; non-thermal emission is also possible even at these high frequencies. We show the results of the first NIKA2 survey towards five LBVs. We detected emission from four stars, three of them immersed in tenuous circumstellar material. The spectral indices show a complex distribution and allowed us to separate and characterize different components. We also found nearby dark clouds, with spectral indices typical of thermal emission from dust. Spectral indices of the detected stars are negative and hard to be explained only by free-free processes. In one of the sources, G79.29+0.46, we also found a strong correlation of the 1mm and 2mm continuum emission with respect to nested molecular shells at ≈1 pc from the LBV. The spectral index in this region clearly separates four components: the LBV star, a bubble characterized by free-free emission, and a shell interacting with a nearby infrared dark cloud

Frequency dependence of the thermal dust E/BE/B ratio and EBEB correlation: insights from the spin-moment expansion

The change of physical conditions across the turbulent and magnetized interstellar medium (ISM) induces a 3D spatial variation of the properties of Galactic polarized emission. The observed signal results from the averaging of different spectral energy distributions (SED) and polarization angles, along and between lines of sight. As a consequence, the total Stokes parameters $Q$ and $U$ will have different distorted SEDs, so that the polarization angle becomes frequency dependent. In the present work, we show how this phenomenon similarly induces a different distorted SED for the three polarized angular power spectra $EE$, $BB$ and $EB$, implying a variation of the $EE/BB$ ratio with frequency. We demonstrate how the previously introduced spin-moment formalism provides a natural framework to grasp these effects, allowing us to derive analytical predictions for the spectral behaviors of the polarized spectra, focusing here on the example of thermal dust polarized emission. After a quantitative discussion based on a model combining emission from a filament with its background, we further reveal that the spectral complexity implemented in the dust models commonly used by the cosmic microwave background (CMB) community produce such effects. This new understanding is crucial for CMB component separation, in which an extreme accuracy is required in the modeling of the dust signal to allow for the search of the primordial imprints of inflation or cosmic birefringence. For the latter, as long as the dust $EB$ signal is not measured accurately, great caution is required about the assumptions made to model its spectral behavior, as it may not simply follow from the other dust angular power spectra

Frequency dependence of the thermal dust E/BE/B ratio and EBEB correlation: insights from the spin-moment expansion

The change of physical conditions across the turbulent and magnetized interstellar medium (ISM) induces a 3D spatial variation of the properties of Galactic polarized emission. The observed signal results from the averaging of different spectral energy distributions (SED) and polarization angles, along and between lines of sight. As a consequence, the total Stokes parameters $Q$ and $U$ will have different distorted SEDs, so that the polarization angle becomes frequency dependent. In the present work, we show how this phenomenon similarly induces a different distorted SED for the three polarized angular power spectra $EE$, $BB$ and $EB$, implying a variation of the $EE/BB$ ratio with frequency. We demonstrate how the previously introduced spin-moment formalism provides a natural framework to grasp these effects, allowing us to derive analytical predictions for the spectral behaviors of the polarized spectra, focusing here on the example of thermal dust polarized emission. After a quantitative discussion based on a model combining emission from a filament with its background, we further reveal that the spectral complexity implemented in the dust models commonly used by the cosmic microwave background (CMB) community produce such effects. This new understanding is crucial for CMB component separation, in which an extreme accuracy is required in the modeling of the dust signal to allow for the search of the primordial imprints of inflation or cosmic birefringence. For the latter, as long as the dust $EB$ signal is not measured accurately, great caution is required about the assumptions made to model its spectral behavior, as it may not simply follow from the other dust angular power spectra

Dust polarization spectral dependence from Planck HFI data. Turning point on CMB polarization foregrounds modelling

The search for the primordial B-modes of the cosmic microwave background (CMB) relies on the separation from the brighter foreground dust signal. In this context, the characterisation of the spectral energy distribution (SED) of thermal dust in polarization has become a critical subject of study. We present a power-spectra analysis of Planck data, which improves on previous studies by using the newly released SRoll2 maps that correct residual data systematics, and by extending the analysis to regions near the Galactic plane. Our analysis focuses on the lowest multipoles between l=4 and 32, and three sky areas with sky fractions of fsky = 80%, 90%, and 97%. The mean dust SED for polarization and the 353 GHz Q and U maps are used to compute residual maps at 100, 143 and 217 GHz, highlighting spatial variations of the dust polarization SED. Residuals are detected at the three frequencies for the three sky areas. We show that models based on total intensity data are underestimating by a significant factor the complexity of dust polarized CMB foreground. Our analysis emphasizes the need to include variations of polarization angles of the dust polarized CMB foreground. The frequency dependence of the EE and BB power spectra of the residual maps yields further insight. We find that the moments expansion to the first order of the modified black-body (MBB) spectrum provides a good fit to the EE power- spectra. This result suggests that the residuals could follow mainly from variations of dust MBB spectral parameters. However, this conclusion is challenged by cross-spectra showing that the residuals maps at the three frequencies are not fully correlated, and the fact that the BB power-spectra do not match the first order moment expansion of a MBB SED. This work sets new requirements for simulations of the dust polarized foreground and component separation methods (abridged

Dust polarization spectral dependence from Planck HFI data. Turning point on CMB polarization foregrounds modelling

The search for the primordial B-modes of the cosmic microwave background (CMB) relies on the separation from the brighter foreground dust signal. In this context, the characterisation of the spectral energy distribution (SED) of thermal dust in polarization has become a critical subject of study. We present a power-spectra analysis of Planck data, which improves on previous studies by using the newly released SRoll2 maps that correct residual data systematics, and by extending the analysis to regions near the Galactic plane. Our analysis focuses on the lowest multipoles between l=4 and 32, and three sky areas with sky fractions of fsky = 80%, 90%, and 97%. The mean dust SED for polarization and the 353 GHz Q and U maps are used to compute residual maps at 100, 143 and 217 GHz, highlighting spatial variations of the dust polarization SED. Residuals are detected at the three frequencies for the three sky areas. We show that models based on total intensity data are underestimating by a significant factor the complexity of dust polarized CMB foreground. Our analysis emphasizes the need to include variations of polarization angles of the dust polarized CMB foreground. The frequency dependence of the EE and BB power spectra of the residual maps yields further insight. We find that the moments expansion to the first order of the modified black-body (MBB) spectrum provides a good fit to the EE power- spectra. This result suggests that the residuals could follow mainly from variations of dust MBB spectral parameters. However, this conclusion is challenged by cross-spectra showing that the residuals maps at the three frequencies are not fully correlated, and the fact that the BB power-spectra do not match the first order moment expansion of a MBB SED. This work sets new requirements for simulations of the dust polarized foreground and component separation methods (abridged

Dust polarization spectral dependence from Planck HFI data. Turning point on CMB polarization foregrounds modelling

The search for the primordial B-modes of the cosmic microwave background (CMB) relies on the separation from the brighter foreground dust signal. In this context, the characterisation of the spectral energy distribution (SED) of thermal dust in polarization has become a critical subject of study. We present a power-spectra analysis of Planck data, which improves on previous studies by using the newly released SRoll2 maps that correct residual data systematics, and by extending the analysis to regions near the Galactic plane. Our analysis focuses on the lowest multipoles between l=4 and 32, and three sky areas with sky fractions of fsky = 80%, 90%, and 97%. The mean dust SED for polarization and the 353 GHz Q and U maps are used to compute residual maps at 100, 143 and 217 GHz, highlighting spatial variations of the dust polarization SED. Residuals are detected at the three frequencies for the three sky areas. We show that models based on total intensity data are underestimating by a significant factor the complexity of dust polarized CMB foreground. Our analysis emphasizes the need to include variations of polarization angles of the dust polarized CMB foreground. The frequency dependence of the EE and BB power spectra of the residual maps yields further insight. We find that the moments expansion to the first order of the modified black-body (MBB) spectrum provides a good fit to the EE power- spectra. This result suggests that the residuals could follow mainly from variations of dust MBB spectral parameters. However, this conclusion is challenged by cross-spectra showing that the residuals maps at the three frequencies are not fully correlated, and the fact that the BB power-spectra do not match the first order moment expansion of a MBB SED. This work sets new requirements for simulations of the dust polarized foreground and component separation methods (abridged