A new method to improve photometric redshift reconstruction. Applications to the Large Synoptic Survey Telescope

In the next decade, the LSST will become a major facility for the astronomical community. However accurately determining the redshifts of the observed galaxies without using spectroscopy is a major challenge. Reconstruction of the redshifts with high resolution and well-understood uncertainties is mandatory for many science goals, including the study of baryonic acoustic oscillations. We investigate different approaches to establish the accuracy that can be reached by the LSST six-band photometry. We construct a realistic mock galaxy catalog, based on the GOODS survey luminosity function, by simulating the expected apparent magnitude distribution for the LSST. To reconstruct the photometric redshifts (photo-z's), we consider a template-fitting method and a neural network method. The photo-z reconstruction from both of these techniques is tested on real CFHTLS data and also on simulated catalogs. We describe a new method to improve photo-z reconstruction that efficiently removes catastrophic outliers via a likelihood ratio statistical test. This test uses the posterior probability functions of the fit parameters and the colors. We show that the photometric redshift accuracy will meet the stringent LSST requirements up to redshift $\sim2.5$ after a selection that is based on the likelihood ratio test or on the apparent magnitude for galaxies with $S/N>5$ in at least 5 bands. The former selection has the advantage of retaining roughly 35% more galaxies for a similar photo-z performance compared to the latter. Photo-z reconstruction using a neural network algorithm is also described. In addition, we utilize the CFHTLS spectro-photometric catalog to outline the possibility of combining the neural network and template-fitting methods. We conclude that the photo-z's will be accurately estimated with the LSST if a Bayesian prior probability and a calibration sample are used.Comment: 19 pages, 25 figures, accepted for publication in Astronomy and Astrophysics Astronomy and Astrophysics, 201

LSST: from Science Drivers to Reference Design and Anticipated Data Products

(Abridged) We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). A vast array of science will be enabled by a single wide-deep-fast sky survey, and LSST will have unique survey capability in the faint time domain. The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and mapping the Milky Way. LSST will be a wide-field ground-based system sited at Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg$^2$ field of view, and a 3.2 Gigapixel camera. The standard observing sequence will consist of pairs of 15-second exposures in a given field, with two such visits in each pointing in a given night. With these repeats, the LSST system is capable of imaging about 10,000 square degrees of sky in a single filter in three nights. The typical 5$\sigma$ point-source depth in a single visit in $r$ will be $\sim 24.5$ (AB). The project is in the construction phase and will begin regular survey operations by 2022. The survey area will be contained within 30,000 deg$^2$ with $\delta<+34.5^\circ$, and will be imaged multiple times in six bands, $ugrizy$, covering the wavelength range 320--1050 nm. About 90\% of the observing time will be devoted to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg$^2$ region about 800 times (summed over all six bands) during the anticipated 10 years of operations, and yield a coadded map to $r\sim27.5$. The remaining 10\% of the observing time will be allocated to projects such as a Very Deep and Fast time domain survey. The goal is to make LSST data products, including a relational database of about 32 trillion observations of 40 billion objects, available to the public and scientists around the world.Comment: 57 pages, 32 color figures, version with high-resolution figures available from https://www.lsst.org/overvie

Analyse des spectres VLT pour l'expérience SNLS Qualification de transients cosmologiques

The SuperNovae Legacy Survey (SNLS) experiment is an important part of the CFHT Legacy Survey, aiming at improved constraints on the Dark Energy equation of state through reducing the Univers expansion history uncertainty by type Ia supernovae (SN-Ia)observations up to a redshift of 1. The necessary standardisation in order to use SN-Ia's as a distance indicator relies on the precise measurment of their light curves in 4 filters (namely g', r', i' and z' for MegaCam at CFHT). The measurement of the recession velocity, via the redshift, and the validation of the transient as a SN-Ia proceed of the spectroscopic follow-up, achieved on four 8-m class telescops. FORS1 instrument at ESO's Very Large Telescope provides most of this follow-up. The impressive data set, the usual weakness of the signal and the will of testing alternative image calibration and spectrum extraction algorithmes, lead me to develop an independent and automated reduction pipeline. The present work details a comprehensive technique, statistically based, for a homogeneous extraction of the transient spectra and of the host galaxy spectra, when those componants can actually be separated on the combined spectrogramm. This by software development in C++ and Python, interacting with the photometric survey products : the deep field images and the transient's light curves. Then, the classification of the transients remains subjective but is enlightened by the simultaneous fit of the light curves and of the spectrum using SALT2 (an empirical model of SN-Ia, J.Guy 2007). 117 SN-Ia secure or probable (55% and 73% of the observations of the first and second year respectively) are identified this way.Premier volet du CFHT Legacy Survey, le projet SuperNovae Legacy Survey (SNLS) propose de contraindre l'équation d'état de l'énergie noire en réduisant l'incertitude sur l'histoire de l'expansion de l'univers par l'observation de supernovae de type Ia (SN-Ia), jusqu'à un décalage vers le rouge de 1. La standardisation nécessaire à l'utilisation des SN-Ia comme indicateurs de distance repose sur la mesure précise des courbes de lumière dans 4 filtres (g', r', i' et z' pour MégaCam au CFHT). La mesure de la vitesse de récession, via le décalage vers le rouge, ainsi que la validation du transient comme étant bien une SN-Ia procèdent du suivi spectroscopique. L'instrument FORS1 du Very Large Telescope de l'ESO assure l'essentiel de ce suivi. L'imposant lot de données, la fréquente faiblesse du signal et la volonté de tester des algorithmes alternatifs de calibration des images et d'extraction des spectres m'ont amené à développer une chaîne de réduction indépendante et automatisée. Cet ouvrage présente une méthode compréhensive, d'approche statistique, pour une extraction systématisée du spectre du transient et de celui de sa galaxie hôte lorsque ces sources sont effectivement résolues. Ceci par le developpement logiciel en C++ et en Python, en interaction avec les produits du suivi photométrique : les images profondes des champs et les courbes de lumière des transients. La classification des transients reste subjective, mais est éclairée par l'ajustement simultané des courbes de lumières et du spectre par SALT2, un modèle empirique de SN-Ia (J.Guy 2007). 117 SN-Ia certaines ou probables (55% puis 73% des observations des deux premières années) sont ainsi identifiées

Analyse des spectres VLT pour l'expérience SNLS (Qualification de transients cosmologiques)

PARIS7-Bibliothèque centrale (751132105) / SudocMEUDON-Observatoire (920482302) / SudocPARIS-Observatoire (751142302) / SudocAIX-MARSEILLE1.OAMP.Le Verrier (130552205) / SudocSudocFranceF

How to classify permanent grasslands? Conclusions from a French national network

International audienceSince agronomists and ecologists began to study permanent grasslands, they have set various classifications to describe the variety of grasslands or to suggest clear interpretations of their functioning. First classifications due to phytosociologists described the structures of plant communities, and their relations with environment. Recent advances in plant ecology introduced typologies based on life plant traits. It is now clearly established that ecosystemic services provided by grasslands (ie grass production or red list species protection) cannot be predicted by the same characteristics of vegetation. We aimed to compare these contrasted ways of classifying grasslands, estimating the redundancy of vegetation characteristics. Therefore, a national network of 178 grasslands was studied in 2009 with a common experimental protocol to determine the botanical composition. The main result is the low redundancy between taxonomical and functional (i.e. life traits) characteristics of vegetation, despite a dependence between these variables. Single criterion based classifications must thus be avoided for the full prediction of grassland ecosystem services

To what extent do French mountain grasslands provide simultaneously biodiversity and forage services?

International audienceThe key role of permanent grasslands for the conservation of biodiversity, and more generally for the preservation of environment quality, is clearly established. However, the total area covered by grassland is reg- ularly declining in most EU countries. A way to stop the decline of their surfaces is to underline the economic potential of grasslands for breeders and their value. In this perspective, we studied the links between forage services and biodiversity through the survey of a network of 47 permanent grasslands from Pyrenees, Massif Central, Jura and Vosges. Grasslands were followed during 2 years (2009-10) considering the seasonal dynam- ic of forage characteristics and botanical and functional composition. Forage services of grasslands were esti- mated measuring production and quality (digestibility, crude protein content), and biodiversity by indicators of botanical diversity (species richness, percentage of entomophilous species, number of oligotrophic species, botanical families). We showed a strong link between quality of permanent grasslands and biodiversity. The pro- ductive grasslands presented a lesser interest for biodiversity. We also showed that permanent grasslands could provide simultaneously biodiversity and forage services or only forage services or biodiversity

Les typologies des prairies: de quoi s'agit-il ? Comment les utiliser ?

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Assessing population maturity of three perennial grass species: Influence of phenology and tiller demography among mountain and plain pastures

Assessing population maturity of three perennial grass species: Influence of phenology and tiller demography among mountain and plain pastures. 17th Meeting of the FAO-CIHEAM Mountain Pastures Network, Pastoralism and ecosystem conservatio

Sols artificialisés et processus d’artificialisation des sols, Déterminants, impacts et leviers d’action

Dans le pays à économie de marché qu’est la France, c’est principalement sur les marchés que s’expriment les dynamiques foncières et immobilières déterminant la construction. La question comporte trois aspects. Les agents privés, ménages et entrepreneurs, ont des comportements et prennent des décisions qui amènent à construire des terres. Les marchés, foncier et immobilier, agrègent les comportements de ces agents, en conduisant à des équilibres dans lesquels des constructions nouvelles sont réparties dans l’espace. Les pouvoirs publics agissent en utilisateurs directs de terres lorsqu’ils construisent des infrastructures de communication et des bureaux (administrations, services publics) ou en régulateurs des marchés fonciers et immobiliers lorsqu’ils corrigent leur fonctionnement. Les déterminants de la construction sur des terres nouvelles sont divisés en trois domaines. Les deux premiers étudient la construction à l’échelle d’une ville ou d’une aire urbaine, près du centre ou en périphérie, selon des mécanismes qui relèvent de l’économie ou de la géographie urbaine. Il s’agit d’analyser les différentiels (de prix, de coût de construction, de temps de transport, de politiques publiques, de répartition des aménités/nuisances) qui conduisent à construire au centre (en hauteur, en recyclant des terrains, en récupérant des friches ou des réserves foncières) ou en périphérie (par extension horizontale en tâche d’huile ou par sauts, par saupoudrage ou groupage des constructions). L’étude porte sur une aire urbaine, examinée tantôt du point de vue de l’unité urbaine centrale (la ville dense) et tantôt du point de vue de la couronne périurbaine (peu dense). Le troisième domaine porte sur les interactions d’un système de villes (au sens de l’économie géographique) qui constitue l’armature urbaine d’un pays, ce qui conduit à différencier le rythme et la localisation de la construction selon la place d’une ville dans cette armature : capitale, métropole régionale, petite ou moyenne ville, en étudiant les liens et les interactions entre ces centres urbains (concurrence, complémentarité, spécialisation). En bref, il s’agit de distinguer les dynamiques foncières et immobilières vues depuis la ville (point 1), celles vues depuis la couronne périurbaine (point 2) et les dynamiques de l’armature urbaine du pays (point 3), comme les trois dynamiques essentielles déterminant la construction