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Convex Analysisの二,三の進展について

The stellar masses, mean ages, metallicities, and star formation histories of galaxies are now commonly estimated via stellar population synthesis (SPS) techniques. SPS relies on stellar evolution calculations from the main sequence to stellar death, stellar spectral libraries, phenomenological dust models, and stellar initial mass functions (IMFs) to translate the evolution of a multimetallicity, multi-age set of stars into a prediction for the time-evolution of the integrated light from that set of stars. Each of these necessary inputs carries significant uncertainties that have until now received little systematic attention. The present work is the first in a series that explores the impact of uncertainties in key phases of stellar evolution and the IMF on the derived physical properties of galaxies and the expected luminosity evolution for a passively evolving set of stars. A Monte Carlo Markov Chain approach is taken to fit near-UV through near-IR photometry of a representative sample of low- and high-redshift galaxies with this new SPS model. Significant results include the following. (1) Including uncertainties in stellar evolution, stellar masses at z ~ 0 carry errors of ~0.3 dex at 95% CL with little dependence on luminosity or color, while at z ~ 2, the masses of bright red galaxies are uncertain at the ~0.6 dex level. (2) Either current stellar evolution models, current observational stellar libraries, or both, do not adequately characterize the metallicity-dependence of the thermally pulsating AGB phase. (3) Conservative estimates on the uncertainty of the slope of the IMF in the solar neighborhood imply that luminosity evolution per unit redshift is uncertain at the ~0.4 mag level in the K band, which is a substantial source of uncertainty for interpreting the evolution of galaxy populations across time. Any possible evolution in the IMF, as suggested by several independent lines of evidence, will only exacerbate this problem. (4) Assuming a distribution of stellar metallicities within a galaxy, rather than a fixed value as is usually assumed, can yield important differences when considering bands blueward of V, but is not a concern for redder bands. Spectroscopic information may alleviate some of these concerns, though uncertainties in the stellar spectral libraries and the importance of nonsolar abundance ratios have not yet been systematically investigated in the SPS context.

The propagation of uncertainties in stellar population synthesis modeling. i. the relevance of uncertain aspects of stellar evolution and the initial mass function to the derived physical properties of galaxies

Course Description This is an advanced course in which we explore the field of Statistical Optics. Topics covered include such subjects as the statistical properties of natural (thermal) and laser light, spatial and temporal coherence, effects of partial coherence on optical imaging instruments, effects on imaging due to randomly inhomogeneous media, and a statistical treatment of the detection of light. Development of this more comprehensive model of the behavior of light draws upon the use of tools traditionally available to the electrical engineer, such as linear system theory and the theory of stochastic processes.

http://ieeexplore.ieee.org/iel5/3/23094/01073023.pdf

Statistical optics

We report an imaging method, termed Fourier ptychographic microscopy (FPM), which iteratively stitches together a
number of variably illuminated, low-resolution intensity images in Fourier space to produce a wide-field, high-resolution
complex sample image. By adopting a wavefront correction strategy, the FPM method can also correct for aberrations and
digitally extend a microscope’s depth of focus beyond the physical limitations of its optics. As a demonstration, we built a
microscope prototype with a resolution of 0.78 µm, a field of view of ∼120 mm^2 and a resolution-invariant depth of focus
of 0.3 mm (characterized at 632 nm). Gigapixel colour images of histology slides verify successful FPM operation. The
reported imaging procedure transforms the general challenge of high-throughput, high-resolution microscopy from one
that is coupled to the physical limitations of the system’s optics to one that is solvable through computation.

Wide-field, high-resolution Fourier ptychographic microscopy

The stellar masses, mean ages, metallicities, and star formation histories of galaxies are now commonly estimated via stellar population synthesis (SPS) techniques. SPS relies on stellar evolution calculations from the main sequence to stellar death, stellar spectral libraries, phenomenological dust models, and stellar initial mass functions (IMFs). The present work is the first in a series that explores the impact of uncertainties in key phases of stellar evolution and the IMF on the derived physical properties of galaxies and the expected luminosity evolution for a passively evolving set of stars. A Monte-Carlo Markov-Chain approach is taken to fit near-UV through near-IR photometry of a representative sample of low- and high-redshift galaxies with this new SPS model. Significant results include the following: 1) including uncertainties in stellar evolution, stellar masses at z~0 carry errors of ~0.3 dex at 95% CL with little dependence on luminosity or color, while at z~2, the masses of bright red galaxies are uncertain at the ~0.6 dex level; 2) either current stellar evolution models, current observational stellar libraries, or both, do not adequately characterize the metallicity-dependence of the thermally-pulsating asymptotic giant branch phase; 3) conservative estimates on the uncertainty of the slope of the IMF in the solar neighborhood imply that luminosity evolution per unit redshift is uncertain at the ~0.4 mag level in the K-band, which is a substantial source of uncertainty for interpreting the evolution of galaxy populations across time; 4) The more plausible assumption of a distribution of stellar metallicities, rather than a fixed value as is usually assumed, can have significant effects on the interpretation of colors blueward of the V-band. (ABRIDGED)

The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies

En tomographie, les methodes de reconstruction iteratives necessitent une modelisation discrete du processus d'obtention des mesures. La representation de l'objet d'interet est le point de depart a l'elaboration d'un modele de projection tomographique sur le detecteur, precis et rapide. Les modeles conventionnels ray driven et distance driven, construits a partir d'indicatrices de voxels, ont l'inconvenient d'etre fortement anisotropes. Nous proposons un modele utilisant des fonctions de bases B-splines de degre adequat, nous fournissant un projecteur quasiment isotrope. Une approximation de la projection par une B-spline separable sur le detecteur conduit a un modele efficace en geometrie parallele et conique, de qualite superieure aux modeles conventionnels. Nous montrons notamment que l'erreur de modelisation est amelioree d'un facteur 10 par rapport au modele distance driven. Nous illustrons l'amelioration de la qualite de reconstruction apportee par notre modele sur des simulations du fantome de Shepp-Logan, en utilisant une methode iterative de reconstruction regularisee.

Modèle direct pour la tomographie 3D : apport d'une approximation par B-splines séparables

We investigate the problem of probing the local spatial structure of the magnetic field of the interstellar medium using multi-frequency polarized maps of the synchrotron emission at radio wavelengths. We focus in this paper on the three-dimensional reconstruction of the largest scales of the magnetic field, relying on the internal depolarization (due to differential Faraday rotation) of the emitting medium as a function of electromagnetic frequency. We argue that multi-band spectroscopy in the radio wavelengths, developed in the context of high-redshift extragalactic HI lines, can be a very useful probe of the 3D magnetic field structure of our Galaxy when combined with a Maximum A Posteriori reconstruction technique. When starting from a fair approximation of the magnetic field, we are able to recover the true one by using a linearized version of the corresponding inverse problem. The spectral analysis of this problem allows us to specify the best sampling strategy in electromagnetic frequency and predicts a spatially anisotropic distribution of posterior errors. The reconstruction method is illustrated for reference fields extracted from realistic magneto-hydrodynamical simulations.

Probing magnetic fields with multi-frequency polarized synchrotron emission

We report the observation of T Tau by visible speckle interferometry at the CFHT. The observations were carried out at two wavelengths simultaneously. Each image was split into two, allowing improved data processing. The 4-channel photon-counting detector CP40 was used. We have discovered that 30% of the total energy at Ha is emitted by an extended elliptical component, with a FWHM of 70 maresec. In the neighbouring continuum, T Tau is not resolved by the 3.60 m CFHT. We briefly discuss the astrophysical consequences of this new result.

The Extended Hα Component of T Tau

A technique for reconstructing diffraction-limited image of an object from speckle images without reference star is applied to both simulated and real data. The object spectrum is estimated by blind deconvolution using the power spectrum of the speckle images and the phase is restored from the bispectrum.

Speckle Imaging With the Bispectrum and Without Reference Star

SPID aims at offering a high spectral resolution in both short-exposure (speckle imaging) and long-exposure (adaptive optics with partial compensation) modes. It offers an adjustable spectral resolution (from 60 up to 3000) in the range 400 - 750 nm. For differential observation of astronomical objects, SPID gives images in two spectral bandwidths at the same time. The width and the central wavelength of each bandwidth can be chosen independently. A high image quality is achieved thanks to a new design derived from a Courtes' monochromator. SPID also includes a wavefront sensor for post-compensation processing. A short-exposure mode allows us to achieve diffraction limited images but with a low signal-to-noise ratio. Depending on the object brightness and on the seeing quality, adaptive optics will allow us to improve significantly the signal-to-noise ratio and sometimes to observe a diffraction limited core in long-exposure mode. Depending on the scientific goal, the availability of the two modes will drive the best choice. The current status of SPID is presented together with first results obtained at CFHT in the short-exposure mode.

Éric Thiébaut

Papers

Modèle direct pour la tomographie 3D : apport d'une approximation par B-splines séparables

Probing magnetic fields with multi-frequency polarized synchrotron emission

The Extended Hα Component of T Tau

Speckle Imaging With the Bispectrum and Without Reference Star

SPID: a high spectral resolution diffraction-limited camera