[Paper Review] Euclid Definition Study Report

R. Laureijs, J. Amiaux+217 more|CERN Document Server (European Organization for Nuclear Research)|Oct 14, 2011

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy15 references902 citations

TL;DR

This paper presents the Euclid Definition Study Report, a comprehensive mission design for the Euclid space telescope, which aims to investigate dark energy, dark matter, and gravity through weak gravitational lensing and baryonic acoustic oscillations. By surveying 15,000 deg² of the extragalactic sky with a 1.2 m Korsch telescope carrying the VIS and NISP instruments, Euclid achieves high-precision photometric redshifts (dz/(1+z) < 0.05) and spectroscopic redshifts (dz/(1+z) = 0.001) to map cosmic structure and probe the universe's accelerating expansion.

ABSTRACT

Euclid is a space-based survey mission from the European Space Agency designed to understand the origin of the Universe's accelerating expansion. It will use cosmological probes to investigate the nature of dark energy, dark matter and gravity by tracking their observational signatures on the geometry of the universe and on the cosmic history of structure formation. The mission is optimised for two independent primary cosmological probes: Weak gravitational Lensing (WL) and Baryonic Acoustic Oscillations (BAO). The Euclid payload consists of a 1.2 m Korsch telescope designed to provide a large field of view. It carries two instruments with a common field-of-view of ~0.54 deg2: the visual imager (VIS) and the near infrared instrument (NISP) which contains a slitless spectrometer and a three bands photometer. The Euclid wide survey will cover 15,000 deg2 of the extragalactic sky and is complemented by two 20 deg2 deep fields. For WL, Euclid measures the shapes of 30-40 resolved galaxies per arcmin2 in one broad visible R+I+Z band (550-920 nm). The photometric redshifts for these galaxies reach a precision of dz/(1+z) &lt; 0.05. They are derived from three additional Euclid NIR bands (Y, J, H in the range 0.92-2.0 micron), complemented by ground based photometry in visible bands derived from public data or through engaged collaborations. The BAO are determined from a spectroscopic survey with a redshift accuracy dz/(1+z) =0.001. The slitless spectrometer, with spectral resolution ~250, predominantly detects Ha emission line galaxies. Euclid is a Medium Class mission of the ESA Cosmic Vision 2015-2025 programme, with a foreseen launch date in 2019. This report (also known as the Euclid Red Book) describes the outcome of the Phase A study.

Motivation & Objective

• To develop a space-based survey mission capable of measuring the universe’s accelerating expansion and probing the nature of dark energy, dark matter, and gravity.
• To design a mission optimized for two primary cosmological probes: weak gravitational lensing (WL) and baryonic acoustic oscillations (BAO).
• To achieve high-precision photometric redshifts (dz/(1+z) < 0.05) for 30–40 galaxies per square arcminute using visible and near-infrared bands.
• To conduct a spectroscopic survey with redshift accuracy of dz/(1+z) = 0.001 using a slitless spectrometer targeting Hα emission line galaxies.
• To cover 15,000 deg² of the extragalactic sky with a wide survey and two 20 deg² deep fields, enabling comprehensive cosmological analysis.

Proposed method

• Utilize a 1.2 m Korsch telescope with a large 0.54 deg² field of view to enable wide-area surveys.
• Equip the telescope with two instruments: the visual imager (VIS) for visible-band imaging (R+I+Z, 550–920 nm) and the near-infrared instrument (NISP) with a slitless spectrometer and three-band photometer (Y, J, H, 0.92–2.0 µm).
• Measure galaxy shapes with 30–40 resolved galaxies per arcminute² to enable weak gravitational lensing analysis.
• Derive photometric redshifts using Euclid’s NIR bands and complementary ground-based visible photometry from public data or collaborations.
• Conduct a spectroscopic survey using the NISP slitless spectrometer with spectral resolution ~250 to detect Hα emission line galaxies.
• Combine WL and BAO measurements to constrain the geometry of the universe and the growth of cosmic structure.

Experimental results

Research questions

• RQ1 What is the optimal mission design to measure the universe’s accelerating expansion using weak lensing and baryonic acoustic oscillations?
• RQ2 How can photometric redshifts be achieved with sufficient precision (dz/(1+z) < 0.05) for cosmological weak lensing studies?
• RQ3 What spectroscopic redshift accuracy (dz/(1+z) = 0.001) is required to robustly detect baryonic acoustic oscillations?
• RQ4 How can a wide-area survey of 15,000 deg² combined with two deep fields maximize cosmological constraints?
• RQ5 What instrumental and observational strategies are needed to minimize systematics in weak lensing and BAO measurements?

Key findings

• The Euclid mission is designed to survey 15,000 deg² of the extragalactic sky with a 1.2 m Korsch telescope and a 0.54 deg² field of view.
• The visual imager (VIS) will image 30–40 resolved galaxies per square arcminute in the R+I+Z band (550–920 nm), enabling high-precision weak lensing measurements.
• Photometric redshifts for galaxies will achieve a precision of dz/(1+z) < 0.05 using three Euclid NIR bands (Y, J, H) and ground-based visible photometry.
• The NISP slitless spectrometer will provide spectroscopic redshifts with accuracy dz/(1+z) = 0.001, primarily detecting Hα emission line galaxies.
• The mission includes two 20 deg² deep fields to enhance constraints on cosmic structure formation and dark energy evolution.
• Euclid is a Medium-Class mission in the ESA Cosmic Vision 2015–2025 programme, with a planned launch in 2019, and the report serves as the foundational design document (the 'Red Book').