The DESI Experiment Part I: Science,Targeting, and Survey Design


Authors: DESI collaboration
Journal: ArXiv
Year: 2016
Download: ADS | arXiv



DESI (Dark Energy Spectroscopic Instrument) is a Stage IV ground-based dark energy experiment that will study baryon acoustic oscillations (BAO) and the growth of structure through redshift-space distortions with a wide-area galaxy and quasar redshift survey. To trace the underlying dark matter distribution, spectroscopic targets will be selected in four classes from imaging data. We will measure luminous red galaxies up to $z=1.0$. To probe the Universe out to even higher redshift, DESI will target bright [O II] emission line galaxies up to $z=1.7$. Quasars will be targeted both as direct tracers of the underlying dark matter distribution and, at higher redshifts ($ 2.1 < z < 3.5$), for the Ly-$\alpha$ forest absorption features in their spectra, which will be used to trace the distribution of neutral hydrogen. When moonlight prevents efficient observations of the faint targets of the baseline survey, DESI will conduct a magnitude-limited Bright Galaxy Survey comprising approximately 10 million galaxies with a median $z\approx 0.2$. In total, more than 30 million galaxy and quasar redshifts will be obtained to measure the BAO feature and determine the matter power spectrum, including redshift space distortions.



The DESI Experiment Part II: Instrument Design


Authors: DESI collaboration
Journal: ArXiv
Year: 2016
Download: ADS | arXiv



DESI (Dark Energy Spectropic Instrument) is a Stage IV ground-based dark energy experiment that will study baryon acoustic oscillations and the growth of structure through redshift-space distortions with a wide-area galaxy and quasar redshift survey. The DESI instrument is a robotically-actuated, fiber-fed spectrograph capable of taking up to 5,000 simultaneous spectra over a wavelength range from 360 nm to 980 nm. The fibers feed ten three-arm spectrographs with resolution

R=λ/ΔλR= λ/Δλ

between 2000 and 5500, depending on wavelength. The DESI instrument will be used to conduct a five-year survey designed to cover 14,000 deg


. This powerful instrument will be installed at prime focus on the 4-m Mayall telescope in Kitt Peak, Arizona, along with a new optical corrector, which will provide a three-degree diameter field of view. The DESI collaboration will also deliver a spectroscopic pipeline and data management system to reduce and archive all data for eventual public use.



The XXL survey: First results and future

Authors: M. Pierre et al.
Journal: MNRAS
Year: 2017
Download: ADS | arXiv



The XXL survey currently covers two 25 sq. deg. patches with XMM observations of ~10ks. We summarise the scientific results associated with the first release of the XXL data set, that occurred mid 2016. We review several arguments for increasing the survey depth to 40 ks during the next decade of XMM operations. X-ray (z<2) cluster, (z<4) AGN and cosmic background survey science will then benefit from an extraordinary data reservoir. This, combined with deep multi-


observations, will lead to solid standalone cosmological constraints and provide a wealth of information on the formation and evolution of AGN, clusters and the X-ray background. In particular, it will offer a unique opportunity to pinpoint the z>1 cluster density. It will eventually constitute a reference study and an ideal calibration field for the upcoming eROSITA and Euclid missions.


A new model to predict weak-lensing peak counts III. Filtering technique comparisons

Authors: C. Lin, M. Kilbinger, S. Pires
Journal: A&A
Year: 2016
Download: ADS | arXiv


This is the third in a series of papers that develop a new and flexible model to predict weak-lensing (WL) peak counts, which have been shown to be a very valuable non-Gaussian probe of cosmology. In this paper, we compare the cosmological information extracted from WL peak counts using different filtering techniques of the galaxy shear data, including linear filtering with a Gaussian and two compensated filters (the starlet wavelet and the aperture mass), and the nonlinear filtering method MRLens. We present improvements to our model that account for realistic survey conditions, which are masks, shear-to-convergence transformations, and non-constant noise. We create simulated peak counts from our stochastic model, from which we obtain constraints on the matter density Ωm, the power spectrum normalisation σ8, and the dark-energy parameter w0. We use two methods for parameter inference, a copula likelihood, and approximate Bayesian computation (ABC). We measure the contour width in the Ωm-σ8 degeneracy direction and the figure of merit to compare parameter constraints from different filtering techniques. We find that starlet filtering outperforms the Gaussian kernel, and that including peak counts from different smoothing scales helps to lift parameter degeneracies. Peak counts from different smoothing scales with a compensated filter show very little cross-correlation, and adding information from different scales can therefore strongly enhance the available information. Measuring peak counts separately from different scales yields tighter constraints than using a combined peak histogram from a single map that includes multiscale information. Our results suggest that a compensated filter function with counts included separately from different smoothing scales yields the tightest constraints on cosmological parameters from WL peaks.

CFIS proposal accepted

On the day of the Brexit outcome, so disastrous for Europe and the UK, there is at least good news for the cosmological community: CFIS, the Canada-France Imaging Survey, has been accepted! This survey consists of two parts. The WIQD​(Wide ­ Image Quality ­ Deep) part will cover 5,000 deg2 of the Northern sky, observed in the r-band with the CFHT (Canada-France Hawai’i telescope). The u-band will cover 10,000 deg2 to a lower depth, and is part of LUAU (Legacy for the U­-band All­-sky Universe). 271 nights are granted, observations will start in 8 months from now.CFIS Logo

CFIS will allow us to study properties of dark-matter structures, including filaments between galaxy clusters and groups, stripping of dark-matter halos of satellite galaxies in clusters, and the shapes of dark-matter halos. In addition, the laws of gravity on large scales will be tested, and modifications to Einstein’s theory of general relativity will be looked for. CFIS will observe a very large number of distant, high-redshift galaxies, and will use techniques of galaxy clustering and weak gravitational lensing to achieve its goals.

In addition, CFIS will create synergie with other ongoing and planned surveys: CFIS will provide ground-based optical data for Euclid photometric-redshifts. It will produce a very useful imaging data set for target selection for spectroscopic surveys such as DESI, WEAVE, and MSE. It will further provide optical data of galaxy clusters that will enhance the science outcome of the X-ray mission eRosita.

PIs: Jean-Charles Cuillandre (CEA Saclay/France) & Alan McConnachie (Victoria/Canada).
CosmoStat participants: Martin Kilbinger, Jean-Luc Starck. Sandrine Pires.
Irfu participants: Monique Arnaud, Hervé Aussel, Olivier Boulade, Pierre-Alain Duc, David Elbaz, Christophe Magneville, Yannick Mellier, Marguerite Pierre, Anand Raichoor, Jim Rich, Vanina Ruhlmann-Kleider, Marc Sauvage, Christophe Yèche.

Clustering-based redshift estimation: application to VIPERS/CFHTLS

Authors: V. Scottez, Y. Mellier, B. Granett, T. Moutard, M. Kilbinger et al.
Journal: MNRAS
Year: 2016
Download: ADS | arXiv



We explore the accuracy of the clustering-based redshift estimation proposed by Ménard et al. when applied to VIMOS Public Extragalactic Redshift Survey (VIPERS) and Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) real data. This method enables us to reconstruct redshift distributions from measurement of the angular clustering of objects using a set of secure spectroscopic redshifts. We use state-of-the-art spectroscopic measurements with iAB < 22.5 from the VIPERS as reference population to infer the redshift distribution of galaxies from the CFHTLS T0007 release. VIPERS provides a nearly representative sample to a flux limit of iAB < 22.5 at a redshift of >0.5 which allows us to test the accuracy of the clustering-based redshift distributions. We show that this method enables us to reproduce the true mean colour-redshift relation when both populations have the same magnitude limit. We also show that this technique allows the inference of redshift distributions for a population fainter than the reference and we give an estimate of the colour-redshift mapping in this case. This last point is of great interest for future large-redshift surveys which require a complete faint spectroscopic sample.

École d’été 2016 en cosmologie

C’est l’annonce d’une école d’été en cosmologie qui va avoir lieu cette août, avec participation des membres de CosmoStat.

Dates: 22 au 27 août 2016
Lieu: Narbonne, France
Site web:

Date limite d’inscription: 3 juillet 2016
Comité d’organisation: Alain Blanchard, pour la communité Euclid-France
L’objectif prioritaire est d’offrir à la communauté une initiation et un approfondissement
aux problématiques des grands relevés cosmologiques appropriés aux scientifiques qui
s’impliquent fortement dans le projet Euclid comme dans les autres grands projets dédiés
à l’étude de l’énergie noire.
Programme et intervenants:
* Modèle cosmologique standard, énergie noire (Martin Kunz)
* Clustering des galaxies (Francis Bernardeau, Olivier LeFèvre, Stéphanie Escoffier)
* Lentillage gravitationnel (Martin Kilbinger)
* Amas de galaxies (Alain Blanchard)
* Paramètres de nuisance (Sandrine Codis)

* Outils: Python (Yannick Copin)
* Outils: CLASS (Julien Lesgourgues)

Les cours sont complémentés par des TD.

Slides for weak-lensing (no figure to reduce file size):
Day 1