The DES project

Ramon Miquel

Since 2005, a group at IFAE, together with a group at ICE (Institut de Ciències de l’Espai), and another at CIEMAT (Centro de Investigaciones Energéticas, Medio Ambientales y Tecnológicas) and Universidad Autónoma de Madrid (UAM), collaborates in the DES (Dark Energy Survey) international project, led by Fermilab (USA). In late 2016 the group joined the Large Survey Synoptic Telescope (LSST), led by SLAC (USA).

Introduction

The Dark Energy Survey started in 2013 and is the largest photometric galaxy survey ever, having, in six years, imaged 5000 sq. deg. (an octant of the sky) in five optical and near-infrared bands (grizY) to unprecedented depth (iAB ~ 24), and measuring the position on the sky, distance and shape of almost 300 million galaxies up to redshift z ~ 1.4. IFAE is a founding member of the collaboration, and designed and produced a large fraction of the readout electronics of the DES camera, DECam. The survey has as its main goal to unveil the nature of the mysterious dark-energy component of the universe that powers its current accelerated expansion.

2021 Activities

In 2021, DES presented the cosmological results from the data taken in the first three years of observations (DES-Y3), including more than 100 million distant galaxies, whose location on the sky, redshift, and shape and orientation have been carefully measured and calibrated.
The bulk of the DES papers containing the main cosmological results from the first year of observations (Y1) were published in 2017. The next major step has been the publication of the results of the cosmological analyses of the first three years of observations (DES-Y3), which took place in May 2021. The group at IFAE led the DES paper presenting the galaxy-shape (“shear”) catalog corresponding to DES-Y3, with over 100 million galaxies, as well as the paper presenting the calibration of the redshift distribution of the same galaxy sample, arguably the two most critical DES measurements. Both papers were discussed in the 2020 memoire.

Figure 1: Constraints on the cosmological parameters Ωm (the total matter density now in units of the critical density) and S8=σ8(Ωm/0.3)1/2, where σ8 is the amplitude of the power spectrum of matter density fluctuations in 8 Mpc/h scales, obtained from a combination of galaxy clustering, cosmic shear and galaxy-galaxy lensing measurements in the DES-Year3 data set (gray filled contours). The constraints are compared to the latest results from the CMB obtained by the Planck satellite (green filled contours). The agreement between both measurement represents a test of the ΛCDM cosmological model. Also, results from other galaxy surveys are shown for comparison. Taken from DES Collaboration 2022, “Dark Energy Survey Year 3 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing,” PRD 105, 023520 [arXiv:2105.13549].

In 2021, DES presented the cosmological results from the data taken in the first three years of observations (DES-Y3), including more than 100 million distant galaxies, whose location on the sky, redshift, and shape and orientation have been carefully measured and calibrated.

The most stringent cosmological constraints come from the combination of measurements of the galaxy clustering in a flux-limited sample of nearby galaxies, the shear-shear correlations in the weak-lensing sample of 100 million distant galaxies, and the tangential shear of the galaxies in the weak-lensing sample around the galaxies in the nearby sample (galaxy-galaxy lensing). This combination maximizes the constraining power of the data, in particular by breaking the degeneracy between the galaxy bias of the nearby sample and the amplitude of the fluctuations in the underlying matter distribution. The result of the overall combination of the three two-point correlation functions is presented in Fig. 1, which shows the 68% and 95% allowed contours in the S8 – Ωm plane. Here S8 measures the inhomogeneity of the matter distribution now: it is related to the standard deviation of the matter density fluctuations in spheres of 8 Mpc/h radius, while Ωm is the fraction of matter in the total matter-energy of the Universe now. In Fig. 1, we can see that the DES results are in fair agreement with those from the Planck satellite.

Planck measures the tiny inhomogeneity of the early Universe at redshift ~1100, and extrapolates to the current epoch assuming the ΛCDM cosmology. The agreement between DES and Planck is, then, a test of the validity of ΛCDM.

Furthermore, an IFAE PhD student co-led the paper that presented the mass map derived from the DES-Y3 weak-lensing galaxy sample: N. Jeffrey, M. Gatti et al. (DES Collaboration) 2021, “Dark Energy Survey Year 3 results: curved-sky weak lensing mass map reconstruction,” MNRAS 505, 4626 [arXiv:2105.13539]. Due to the weak gravitational lensing effect, the observed size, shape and orientation of distant galaxies are slightly distorted by the gravitational pull of the masses between them and us. Then, the statistical properties of a large set of images of distant galaxies can be studied to determine the location of the intervening (mostly dark) matter to produce mass maps, as the one shown in Fig. 2, which is by far the largest mass map produced to date.

Figure 2: Mass map from the Dark Energy Survey (DES), corresponding to over 4100 square degrees in the sky. Brighter areas contain more (mostly dark) matter, while darker areas contain less. The green circles correspond to visible galaxy clusters, also observed by DES. The inset shows a zoom into the randomly chosen small cyan area in the main map. One can clearly see that areas with more dark matter are rich in galaxy clusters, while areas with less dark matter are also relatively empty of ordinary matter. This is the largest dark-matter map produced to date. From N. Jeffrey, M. Gatti et al. (DES Collaboration) 2021, “Dark Energy Survey Year 3 results: curved-sky weak lensing mass map reconstruction,” MNRAS 505, 4626 [arXiv:2105.13539].

Summary

Due to the weak gravitational lensing effect, the observed size, shape and orientation of distant galaxies are slightly distorted by the gravitational pull of the masses between them and us. Then, the statistical properties of a large set of images of distant galaxies can be studied to determine the location of the intervening (mostly dark) matter to produce mass maps as the one shown in the figure, produced by the Dark Energy Survey analyzing over 100 million distant galaxies corresponding to the data from their first three years of observations.
Brighter areas in the map contain more (mostly dark) matter, while darker areas contain less. The green circles correspond to visible galaxy clusters, also observed by DES. The inset shows a zoom into the randomly chosen small cyan area in the main map. One can clearly see that areas with more dark matter are rich in galaxy clusters, while areas with less dark matter are also relatively empty of ordinary matter. This is by far the largest dark-matter map produced to date. From N. Jeffrey, M. Gatti et al. (DES Collaboration) 2021, “Dark Energy Survey Year 3 results: curved-sky weak lensing mass map reconstruction,” MNRAS 505, 4626. At the time, Marco Gatti was a PhD student at IFAE.