Euclid Definition Study Report

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) < 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.

Figures (137)

• Figure 1: Definition Study Report
• Figure 2: Left panel: The Universe evolves from a homogeneous state after the big bang through cooling and expansion. The small initial inhomogeneities grow through gravity to produce the large-scale structures that are observed today. Right panel: The mass-energy budget at our cosmological epoch. The Universe appears to be dominated by ingredients, dark energy and dark matter, whose nature poses some of the most pressing questions in fundamental physics.
• Figure 3: (Middle panel) Raw gravitational lensing data obtained from HST observations (Massey et al., 2007). Each line/stick shows the average ellipticity of about 200 galaxies. Lines of sight without coherent lensing produce circular average galaxies, with zero ellipticity, represented by a dot; the line length indicates the magnitude of the lensing effect, and the orientation shows the major axis. (Right panel) Foreground dark matter lenses are mapped by filtering the observed shear field for the circular patterns. (Left panel) The clumpiness of dark matter on different physical scales can be quantified statistically via the correlation between shear along different lines of sight (ground-based results from Fu et al., 2008). An independent estimate of systematics is indicated by the black open symbols in the left panel; it should be consistent with zero but is not quite because of residual systematics, inherent to ground-based weak lensing studies
• Figure 4: (Left panel) The galaxy distribution in the largest surveys of the local Universe, compared to simulated distributions from the Millennium Run (Springel et al., 2005); (Right panel) The two-point correlation function of SDSS "luminous red galaxies", in which the BAO peak at$\sim 105 h^{-1}$ Mpc has been clearly detected (Eisenstein et al., 2005).
• Figure 5: The expected constraints from Euclid in the dynamical dark energy parameter space. We show lensing only (green), galaxy clustering only (blue), all the Euclid probes (lensing+galaxy clustering+clusters+ISW; orange) and all Euclid with Planck CMB constraints (red). The cross shows a cosmological constant model. Left panel: the expected$68 \%$ confidence contours in the ( $w_{p}, w_{a}$ ). Right panel: the $1 \sigma$ constraints on the function $w(z)$ parameterised by ( $w_{p}, w_{a}$ ) as a function of redshift (green-lensing alone, blue-galaxy clustering alone, orange-all of the Euclid probes, red-Euclid combined with Planck).