The DES and DESI projects

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) and, since 2015, in the development of DESI (Dark Energy Spectroscopic Instrument), led by LBNL (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 measured the position on the sky, distance and shape of almost 300 million galaxies up to redshift z ~ 1.4. Starting in spring 2021, the Dark Energy Spectroscopic Instrument (DESI) will collect spectra for over 30 million galaxies and quasars covering the whole redshift range 0.2 < z < 3.5, and will become the largest spectroscopic galaxy survey ever. IFAE is a founding member of both collaborations. In DES, IFAE designed and produced a large fraction of the readout electronics of the DES camera, while in DESI, IFAE designed and produced the 10 GFA cameras necessary for the guiding, focusing and alignment of the 5000 fibers in the DESI focal plane. Both surveys have as their main goal to unveil the nature of the mysterious dark-energy component of the universe that powers its current accelerated expansion.

Dark Energy Spectroscopic Instrument (DESI)

During early 2019, the IFAE group finished the delivery of all the GFA cameras for DESI, complete with mechanical enclosures, filters, CCDs, readout electronics, thermal control, etc., which were then mounted in the focal plane of the DESI instrument. The commissioning of the instrument started in late summer 2019 and finished in early March 2020. The pandemic then forced a six-month shutdown. Re-commissioning took place in fall 2020, and in December 2020 the Survey Validation phase of observations started. Along the way, the GFAs have performed very well, needing only limited upgrades to their firmware.

Sloan Digital Sky Survey (SDSS)

After 20 years of operations, in July 2020 the SDSS collaboration presented its final cosmological results, including the most precise measurement of the history of the expansion of the Universe.

DESI can be considered the successor of the Sloan Digital Sky Survey (SDSS), which in 1998 pioneered the field of large galaxy surveys. IFAE researchers were involved in the second phase of SDSS, between 2006 and 2011, and then in the fourth phase (2014-2019) through the eBOSS project. In July 2020, the eBOSS Collaboration presented the final cosmological results after two decades of spectroscopic observations at Apache Point. The results represent the most precise measurement of the history of the expansion rate of the Universe from redshift 2.5 (when the age of the Universe was only a fifth of its current age) until now, setting tight constraints on possible deviations from the standard ΛCDM cosmological model. Combining these data with cosmic microwave background and supernova measurements, a stringent upper limit on the sum of neutrino masses of about 0.1 eV is found, at the 95% confidence level. Figure 1 shows the measurements of the expansion rate (top panel) and the growth rate (bottom panel) as a function of redshift from different samples of SDDS galaxies and quasars.

Figure 1: A summary of the SDSS measurements of the expansion (top) and growth (bottom) rates from different galaxy and quasar samples. Red points correspond to distance measurements across the line of sight, while green points correspond to radial distances. The lines represent the Planck best fit predictions in a flat ΛCDM model. From Alam et al. (eBOSS Collaboration) 2020, arXiv:2007.08991.

Dark Energy Survey (DES)

In 2020, the group at IFAE led the DES paper presenting the galaxy-shape catalog corresponding to the first three years of observations, as well as the paper presenting the calibration of the redshift distribution of the same galaxy sample, two of the most critical DES measurements

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 will be the publication of the results of the cosmological analyses of the first three years of observations (Y3), now expected in spring 2021. In 2020, the first papers with the ingredients necessary for the Y3 cosmological analyses were published. Two of them were led by IFAE researchers.

An IFAE PhD student led the paper that presented the catalog of galaxy shapes used in the Y3 weak lensing analysis, the cornerstone of DES cosmology: M. Gatti et al. (DES Collaboration) 2020, “Dark Energy Survey Year 3 Results: Weak Lensing Shape Catalogue”, arXiv:2011.03408. In the paper, the “shear” (shape and orientation) measurements of over 100 million distant galaxies are presented, together with the battery of tests performed to identify possible systematic errors and the calibration procedure relating the measured galaxy ellipticities with the physical shear. Figure 2 shows the measured mean shear response of the ellipticity measurements across the 4143 sq. deg. DES-Y3 footprint.

Figure 2: Map of the mean shear response (the linear coefficient relating the measured galaxy ellipticity with the weak lensing shear) in the 4143 sq. deg. DES Y3 footprint. The response is extracted directly from the data, using a self-calibration technique. From Gatti et al. (DES Collaboration) 2020, arXiv:2011.03408.

Two IFAE PhD students led the paper presenting the calibration of the redshift distribution of the distant galaxies in the weak lensing sample described above. The paper (M. Gatti, G. Giannini et al. (DES Collaboration) 2020, “Dark Energy Survey Year 3 Results: Clustering Redshifts – Calibration of the Weak Lensing Source Redshift Distributions with redMaGiC and BOSS/eBOSS”, arXiv:2012.08569) uses the cross-correlation technique, pioneered at IFAE for DES-Y1, to calibrate the redshift estimates provided by a self-organizing map. The technique measures the 3D spatial correlation between the weak lensing galaxy sample and galaxy samples with well-known redshifts, either photometric (redMaGiC) or spectroscopic (BOSS/eBOSS), to infer the unknown redshift distribution.