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 foremost photometric galaxy survey in this decade, having just finished imaging 5000 sq. deg. (an octant of the sky) in five optical and near-infrared bands (grizY) to unprecedented depths (iAB ~ 24), and measuring the position on the sky, distance and shape of almost 300 million galaxies up to a redshift z ~ 1.4. Starting in early 2020, 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 foremost spectroscopic galaxy survey in the next decade. 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 has designed and produced the 10 (+2 spares) 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.

GFA cameras for the DESI project

In 2018, the group at IFAE completed the production of the GFA cameras for the DESI project

During 2018, the IFAE group finished the production of the GFA cameras, complete with mechanical enclosures, filters, CCDs, readout electronics, thermal control, etc. Figure 1 shows one of the finished cameras, while Fig. 2 shows it at Lawrence Berkeley National Lab, once installed in one of the 10 petals that constitute the DESI focal plane.

Figure 1: A picture of the first GFA production camera completed at IFAE, without its filter cover.

Figure 2: A picture of the first complete petal of the 10 in the DESI focal plane, with its 500 fiber positioners and its GFA camera on the top-right corner.

Cross-correlations between DES and CMB measurements

In 2018, the group at IFAE led two papers involving cross-correlations between DES and CMB measurements

After the onslaught of DES papers published in 2017 and containing the main cosmological results from the first year of observations (Y1), 2018 was a relatively quiet year. In 2019, DES expects to publish the results of the cosmological analysis of the first three years of observations (Y3). All the same, two DES analyses published in 2018 were led by IFAE researchers.

A PhD student at IFAE led the analysis of the ratio between the gravitational lensing of background DES galaxies and that of CMB photons around the same sample of foreground DES galaxies, using DES-Y1, SPT and Planck data. This ratio turns out to be a purely kinematical probe of dark energy, sensitive to the history of the expansion rate of the universe. The results show good agreement with the canonical ΛCDM model and were published in J. Prat et al. (DES Collaboration) 2018, “Cosmological lensing ratios with DES Y1, SPT and Planck”, arXiv:1810.02212 [astro-ph.CO].

Figure 3: Stacked CMB excess temperature maps in and around supervoids in the Jubilee simulation (left) and in the Planck vs. DES-Y3 sample (right). Dashed circles mark the (scaled) void radius. Taken from A. Kovács et al. (DES Collaboration) 2018, “More out of less: an excess integrated Sachs-Wolfe signal from supervoids mapped out by the Dark Energy Survey”, arXiv:1811.07812 [astro-ph.CO].

Then, a post-doctoral researcher at IFAE led a continuation of his earlier analysis of the correlation between the positions of DES supervoids and the temperature of the CMB in the Planck maps. CMB photons are expected to cool down as they cross large matter underdensities in an accelerating expanding universe (the so-called late-time ISW effect). In his Y1 analysis, an ISW signal well in excess of that predicted by ΛCDM was found. In his follow-up paper, using both DES Y3 data and public BOSS data, he recovers an intriguing excess with respect to the ΛCDM prediction at almost three standard deviations. The results are reported in A. Kovács et al. (DES Collaboration) 2018, “More out of less: an excess integrated Sachs-Wolfe signal from supervoids mapped out by the Dark Energy Survey”, arXiv:1811.07812 [astro-ph.CO]. Figure 3 shows the stacked ISW signal maps in and around the DES supervoids (right panel) compared to the ΛCDM expectations (left panel).