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Éric Thiébaut

Bio: Éric Thiébaut is an academic researcher from University of Lyon. The author has contributed to research in topics: Adaptive optics & Iterative reconstruction. The author has an hindex of 37, co-authored 228 publications receiving 4805 citations. Previous affiliations of Éric Thiébaut include École normale supérieure de Lyon & Lyon College.


Papers
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Journal ArticleDOI
TL;DR: In this paper, a flexible non-parametric inversion method for the interpretation of the integrated light spectra of galaxies, based on synthetic spectra (SSPs) of single stellar populations, is described.
Abstract: In this paper we describe STECMAP (STEllar Content via Maximum A Posteriori), a flexible, non-parametric inversion method for the interpretation of the integrated light spectra of galaxies, based on synthetic spectra of single stellar populations (SSPs). We focus on the recovery of a galaxy's star formation history and stellar age–metallicity relation. We use the high-resolution SSPs produced by pegase-hr to quantify the informational content of the wavelength range λλ= 4000–6800. Regularization of the inversion is achieved by requiring that the solutions are relatively smooth functions of age. The smoothness parameter is set automatically via generalized cross validation. A detailed investigation of the properties of the corresponding simplified linear problem is performed using singular value decomposition. It turns out to be a powerful tool for explaining and predicting the behaviour of the inversion, and may help designing SSP models in the future. We provide means of quantifying the fundamental limitations of the problem considering the intrinsic properties of the SSPs in the spectral range of interest, as well as the noise in these models and in the data. We demonstrate that the information relative to the stellar content is relatively evenly distributed within the optical spectrum. We show that one should not attempt to recover more than about eight characteristic episodes in the star formation history from the wavelength domain we consider. STECMAP preserves optimal (in the cross validation sense) freedom in the characterization of these episodes for each spectrum. We performed a systematic simulation campaign and found that, when the time elapsed between two bursts of star formation is larger than 0.8 dex, the properties of each episode can be constrained with a precision of 0.02 dex in age and 0.04 dex in metallicity from high-quality data [R= 10 000, signal-to-noise ratio (SNR) = 100 per pixel], not taking model errors into account. We also found that the spectral resolution has little effect on population separation provided low- and high-resolution experiments are performed with the same SNR per A. However, higher spectral resolution does improve the accuracy of metallicity and age estimates in double-burst separation experiments. When the fluxes of the data are properly calibrated, extinction can be estimated; otherwise the continuum can be discarded or used to estimate flux correction factors. The described methods and error estimates will be useful in the design and in the analysis of extragalactic spectroscopic surveys.

254 citations

Journal ArticleDOI
TL;DR: In this article, a method for recovering the kinematic properties of a galaxy simultaneously with its stellar content from integrated light spectra is presented. But the method is limited to the case where the velocity distribution of the underlying stars is unknown, and the reconstruction of the stellar age distribution, the age-metallicity relation and the line-of-sight velocity distribution are all non-parametric.
Abstract: We introduce STECKMAP (STEllar Content and Kinematics via Maximum A Posteriori likelihood), a method for recovering the kinematic properties of a galaxy simultaneously with its stellar content from integrated light spectra. It is an extension of STECMAP (presented recently by Ocvirk et al.) to the general case where the velocity distribution of the underlying stars is also unknown. The reconstructions of the stellar age distribution, the age‐metallicity relation and the line-of-sight velocity distribution (LOSVD) are all non-parametric, i.e. no specific shape is assumed. The only a priori conditions that we use are positivity and the requirement that the solution is smooth enough. The smoothness parameter can be set by generalized cross-validation according to the level of noise in the data in order to avoid overinterpretation. We use single stellar populations (SSPs) from P´-HR (R = 10 000, λ = 4 000‐6 800 A, from Le Borgne et al.) to test the method through realistic simulations. Non-Gaussianities in LOSVDs are reliably recovered with signal-to-noise ratio (SNR) as low as 20 per 0.2 A pixel. It turns out that the recovery of the stellar content is not degraded by the simultaneous recovery of the kinematic distribution, so that the resolution in age and error estimates given in Ocvirk et al. remain appropriate when used with STECKMAP. We also explore the case of age-dependent kinematics (i.e. when each stellar component has its own LOSVD). We separate the bulge and disc components of an idealized simplified spiral galaxy in integrated light from high-quality pseudo-data (SNR = 100 per pixel, R = 10 000), and constrain the kinematics (mean projected velocity, projected velocity dispersion) and age of both components. Ke yw ords: methods: data analysis ‐ methods: statistical ‐ techniques: spectroscopic ‐ galaxies: abundances ‐ galaxies: kinematics and dynamics ‐ galaxies: stellar content.

212 citations

Journal ArticleDOI
TL;DR: In this paper, a flexible, nonparametric inversion method for the interpretation of the integrated light spectra of galaxies, based on synthetic Spectra of single stellar populations (SSPs), is presented.
Abstract: This paper describes STECMAP (STEllar Content via Maximum A Posteriori), a flexible, non-parametric inversion method for the interpretation of the integrated light spectra of galaxies, based on synthetic spectra of single stellar populations (SSPs). We focus on the recovery of a galaxy's star formation history and stellar age-metallicity relation. We use the high resolution SSPs produced by PEGASE-HR to quantify the informational content of the wavelength range 4000 - 6800 Angstroms. A detailed investigation of the properties of the corresponding simplified linear problem is performed using singular value decomposition. It turns out to be a powerful tool for explaining and predicting the behaviour of the inversion. We provide means of quantifying the fundamental limitations of the problem considering the intrinsic properties of the SSPs in the spectral range of interest, as well as the noise in these models and in the data. We performed a systematic simulation campaign and found that, when the time elapsed between two bursts of star formation is larger than 0.8 dex, the properties of each episode can be constrained with a precision of 0.04 dex in age and 0.02 dex in metallicity from high quality data (R=10 000, signal-to-noise ratio SNR=100 per pixel), not taking model errors into account. The described methods and error estimates will be useful in the design and in the analysis of extragalactic spectroscopic surveys.

202 citations

Journal ArticleDOI
TL;DR: The spectro-polarimeter VEGA has been installed and successfully tested on the CHARA Array as mentioned in this paper, and the results depend on the observing conditions (seeing, spectral resolution, etc.).
Abstract: Context. Among optical stellar interferometers, the CHARA Array located at Mt Wilson in California offers the potential of very long baselines (up to 330 m) and the prospect of coupling multiple beam combiners. This paper presents the principle and the measured performance of VEGA, Visible spEctroGraph and polArimeter installed in September 2007 at the coherent focus of the array.Aims. With 0.3 ms of arc of spatial resolution and up to of spectral resolution, VEGA intends to measure fundamental parameters of stars, to study stellar activities and to image and analyze circumstellar environments. We describe the observing modes that have been implemented for this spectro-polarimeter and show actual performances measured on the sky during the first observing runs.Methods. The astrophysical programs are described in relation to the observing modes of the instrument, the presentation of the spectrograph and of the interface table is shown and finally the data is presented. We discuss the perspectives of further development in the framework of the CHARA Array.Results. We show that VEGA/CHARA is fully operational. The current limiting magnitude is nearly 7 but the results depend on the observing conditions (seeing, spectral resolution, etc.). We have validated the stability of the instrumental visibility at the level of 1 to 2% over half an hour and of the instrumental polarization for various declinations. Some examples of squared visibility and differential visibility are presented.Conclusions. The spectro-polarimeter VEGA has been installed and successfully tested on CHARA. It will permit stellar physics studies at unprecedented spectral and spatial resolutions.

186 citations

Journal ArticleDOI
TL;DR: Stellar Content and Kinematics via Maximum A Posteriori (STECKMAP) as mentioned in this paper is an extension of STECMAP to the general case where the velocity distribution of the underlying stars is also unknown.
Abstract: We introduce STECKMAP (STEllar Content and Kinematics via Maximum A Posteriori), a method to recover the kinematical properties of a galaxy simultaneously with its stellar content from integrated light spectra. It is an extension of STECMAP (astro-ph/0505209) to the general case where the velocity distribution of the underlying stars is also unknown. %and can be used as is for the analysis of large sets of data. The reconstructions of the stellar age distribution, the age-metallicity relation, and the Line-Of-Sight Velocity Distribution (LOSVD) are all non-parametric, i.e. no specific shape is assumed. The only a propri we use are positivity and the requirement that the solution is smooth enough. The smoothness parameter can be set by GCV according to the level of noise in the data in order to avoid overinterpretation. We use single stellar populations (SSP) from PEGASE-HR (R=10000, lambda lambda = 4000-6800 Angstrom, Le Borgne et al. 2004) to test the method through realistic simulations. Non-Gaussianities in LOSVDs are reliably recovered with SNR as low as 20 per 0.2 Angstrom pixel. It turns out that the recovery of the stellar content is not degraded by the simultaneous recovery of the kinematic distribution, so that the resolution in age and error estimates given in Ocvirk et al. 2005 remain appropriate when used with STECKMAP. We also explore the case of age-dependent kinematics (i.e. when each stellar component has its own LOSVD). We separate the bulge and disk components of an idealized simplified spiral galaxy in integrated light from high quality pseudo data (SNR=100 per pixel, R=10000), and constrain the kinematics (mean projected velocity, projected velocity dispersion) and age of both components.

177 citations


Cited by
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Journal ArticleDOI
TL;DR: In this article, 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 a new SPS model.
Abstract: 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.

1,503 citations

Journal Article
J. Walkup1
TL;DR: 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.
Abstract: 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.

1,364 citations

Journal ArticleDOI
TL;DR: 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, which can also correct for aberrations and digitally extend a microscope's depth-of-focus beyond the physical limitations of its optics.
Abstract: 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.

1,363 citations

Journal ArticleDOI
TL;DR: In this paper, the authors explored 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.
Abstract: 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)

1,303 citations