Session J15: Large Scale Structure - Dark Energy Survey

The Dark Energy Spectroscopic Instrument Survey

The Dark Energy Spectroscopic Instrument (DESI) will measure the history of cosmic acceleration from the present to beyond redshift three with an unprecedented new spectroscopic survey. Over the course of five years starting in 2019, DESI plans to measure spectroscopic redshifts for over 35 million galaxies and quasars across 14000 square degrees. These data will be used by the collaboration to measure the rate of cosmic expansion and the growth of structure with the Baryon Acoustic Oscillation technique and Redshift Space Distortions. DESI will also use the these data to test modified gravity models, inflation, and the measure the sum of neutrino masses. DESI will accomplish these ambitious goals with substantial new instrumentation for the 4-m Mayall telescope at the Kitt Peak National Observatory. I will present an overview of the DESI survey design and forecasts for cosmological and other physical constraints.   

The Dark Energy Spectroscopic Instrument Project

The Dark Energy Spectroscopic Instrument (DESI) will measure spectra of over 35 million galaxies and quasars across 14000 square degrees and use them to take the next step in constraining the nature of Dark Energy. DESI will accomplish these ambitious goals with a major refurbishment of the 4-m Mayall telescope at the Kitt Peak National Observatory and a new prime focus instrument that includes an eight square degree prime focus corrector, 5000 robotic fiber optic positioners, and ten broad-band spectrographs. The project is well underway and expects to begin commissioning in 2019. I will present an overview and status of the DESI project.   

The Dark Energy Survey: Progress Through First 5 Survey Seasons

The Dark Energy Survey (DES) is a currently running optical imaging survey aimed at understanding the accelerating expansion of the universe using four complementary methods: weak gravitational lensing, galaxy cluster counts, baryon acoustic oscillations, and Type Ia supernovae. To perform the 5000 sq-degree wide field and 30 sq-degree supernova surveys, the DES Collaboration built the Dark Energy Camera (DECam), a 3 square degree, 570 Megapixel CCD camera that is installed at the prime focus of the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory. In Feb. 2018 DES completed its fifth 105-night season. We'll start our final (52 night) observing season in August. This presentation will describe DES, the survey strategy and goals, the efficiency of survey operations, and the quality and calibration of the data recorded through Year 5.   

Cosmological Parameter Constraints from the Dark Energy Survey Supernova Program Three Year Spectroscopic Sample

We present cosmological parameter constraints from 251 spectroscopically confirmed Type Ia Supernovae (0.02 $<$ z $<$ 0.85) discovered during the first 3 years of the Dark Energy Survey Supernova Program. Type Ia supernovae, used as standardizable candles, probe the acceleration of the universe and are sensitive to the equation of state parameter for dark energy. The photometric calibration, photometric pipeline, additional low-z supernovae samples (z$<$.1), as well as the final cosmological results and systematics analysis are discussed.   

Photometric Calibration for the Dark Energy Survey

The Dark Energy Survey (DES) science results depend upon photometric calibration that map measured fluxes within an optical filter bandpass to objects identified through processing of the imaging data. The steps in the calibration process are many. An overview of these steps will be given with particular emphasis on the determination of the bandpass of the various filters used by DES and the establishment of an absolute flux scale. The importance of the calibration will then be described in terms of its impact on selected DES science results.   

The Halo Population Statistics of DES redMaPPer Selected Clusters

We present results of ongoing efforts to determine the population statistics of halos at fixed optical richness for the Dark Energy Survey (DES) clusters identified with the redMaPPer algorithm. First, we discuss the scaling properties of X-ray observables of DES Year-1 redMaPPer with z>0.2, obtained from archival Chandra and XMM observations. Then, we constrain the scatter in the log of halo mass at fixed optical-richness via the halo population model of Evrard et al. (2014). Finally, the cosmological constraints from the DES-Year 1 Data will be presented, in which these above results are the key input of the cosmological pipeline. We finally discuss the state of systematic uncertainties and provide the roadmap of the cluster cosmology.   

Galaxy Cluster Cosmology with DES Y1 Data

Constraining LambdaCDM cosmology with galaxy cluster abundance is one of the fundamental goals of the Dark Energy Survey (DES). Many thousands of clusters out to redshift 0.65 have been identified in DES data. We carry out weak-lensing and multi-wavelength studies to constrain the masses of clusters and provide input for cosmology analysis. A cosmology pipeline that considers various systematic effects such as cluster projections and mis-centering is used to derive constraints on LambdaCDM cosmology parameters. This presentation will discuss DES galaxy cluster cosmology analyses based on data collected in the first year.   

Producing an SDSS-BOSS CMASS sample with imaging from the Dark Energy Survey to test gravity

We propose a test of gravity on cosmological scales using the newly-defined DES CMASS analogue (DMASS) sample. The CMASS sample is originally designed from the Sloan Digital Sky Survey and provides the most powerful redshift-space galaxy clustering measurements to date. A joint analysis of redshift-space distortions (such as those probed by CMASS) and weak gravitational lensing (currently best measured by the Dark Energy Survey) can provide a powerful cosmological-scale test of General Relativity. Unfortunately, the DES and SDSS-BOSS footprints suffer minimal overlap, primarily on the celestial equator near the SDSS Stripe 82 region.~ We have built a robust Bayesian model to select DMASS galaxies in the DES footprint specifically to address this lack of overlap. We show that the DMASS sample selected by this model has a fairly good match with the CMASS sample through various validations. To test models of modified gravity, we construct a data vector consisting of Year 1 DES galaxy-galaxy lensing measurements around the galaxies in the DMASS sample and the existing SDSS redshift-space distortion measurements from BOSS.   

Density split statistics: Cosmological constraints from counts and lensing in cells in DES Y1

We derive cosmological constraints from the probability distribution function (PDF) of evolved large-scale matter density fluctuations. We do this by splitting lines of sight by density based on their count of tracer galaxies, and by measuring both gravitational shear around and counts-in-cells in overdense and underdense lines of sight, in Dark Energy Survey (DES) First Year data. Our analysis uses a perturbation theory model and is validated using N-body simulation realizations and log-normal mocks. It allows us to constrain cosmology, bias and stochasticity of galaxies w.r.t. matter density and, in addition, the skewness of the matter density field. Our constraints on $\Omega_m$ and $\sigma_8$ are consistent with the DES 3x2pt results under the assumption that there is only mild stochasticity in galaxy count. As an additional test of gravity, we compare the skewness $S_3$ of the matter density PDF to its $\Lambda$CDM prediction. We find no evidence of excess skewness, with better than 25 per cent relative precision. Notably, on the scales we use, the measurement of the skewness of the PDF improves the uncertainty on cosmological parameters by factors of $\approx$2 relative to a measurement of variance alone.