Euclid

Abstract:

Abstract:

<jats:p>We measured the cross-correlation between galaxy weak lensing data from the Kilo Degree Survey (KiDS-1000, DR4) and cosmic microwave background (CMB) lensing data from the Atacama Cosmology Telescope (ACT, DR4) and the <jats:italic>Planck</jats:italic> Legacy survey. We used two samples of source galaxies, selected with photometric redshifts, (0.1 &lt; <jats:italic>z</jats:italic><jats:sub>B</jats:sub> &lt; 1.2) and (1.2 &lt; <jats:italic>z</jats:italic><jats:sub>B</jats:sub> &lt; 2), which produce a combined detection significance of the CMB lensing and weak galaxy lensing cross-spectrum of 7.7<jats:italic>σ</jats:italic>. With the lower redshift galaxy sample, for which the cross-correlation was detected at a significance of 5.3<jats:italic>σ</jats:italic>, we present joint cosmological constraints on the matter density parameter, Ω<jats:sub>m</jats:sub>, and the matter fluctuation amplitude parameter, <jats:italic>σ</jats:italic><jats:sub>8</jats:sub>, marginalising over three nuisance parameters that model our uncertainty in the redshift and shear calibration as well as the intrinsic alignment of galaxies. We find our measurement to be consistent with the best-fitting flat ΛCDM cosmological models from both <jats:italic>Planck</jats:italic> and KiDS-1000. We demonstrate the capacity of CMB weak lensing cross-correlations to set constraints on either the redshift or shear calibration by analysing a previously unused high-redshift KiDS galaxy sample (1.2 &lt; <jats:italic>z</jats:italic><jats:sub>B</jats:sub> &lt; 2), with the cross-correlation detected at a significance of 7<jats:italic>σ</jats:italic>. This analysis provides an independent assessment for the accuracy of redshift measurements in a regime that is challenging to calibrate directly owing to known incompleteness in spectroscopic surveys.</jats:p>

Abstract:

The analysis of weak gravitational lensing in wide-field imaging surveys is considered to be a major cosmological probe of dark energy. Our capacity to constrain the dark energy equation of state relies on an accurate knowledge of the galaxy mean redshift ⟨z⟩. We investigate the possibility of measuring ⟨z⟩ with an accuracy better than 0.002 (1 + z) in ten tomographic bins spanning the redshift interval 0.2 < z < 2.2, the requirements for the cosmic shear analysis of Euclid. We implement a sufficiently realistic simulation in order to understand the advantages and complementarity, as well as the shortcomings, of two standard approaches: the direct calibration of ⟨z⟩ with a dedicated spectroscopic sample and the combination of the photometric redshift probability distribution functions (zPDFs) of individual galaxies. We base our study on the Horizon-AGN hydrodynamical simulation, which we analyse with a standard galaxy spectral energy distribution template-fitting code. Such a procedure produces photometric redshifts with realistic biases, precisions, and failure rates. We find that the current Euclid design for direct calibration is sufficiently robust to reach the requirement on the mean redshift, provided that the purity level of the spectroscopic sample is maintained at an extremely high level of > 99.8%. The zPDF approach can also be successful if the zPDF is de-biased using a spectroscopic training sample. This approach requires deep imaging data but is weakly sensitive to spectroscopic redshift failures in the training sample. We improve the de-biasing method and confirm our finding by applying it to real-world weak-lensing datasets (COSMOS and KiDS+VIKING-450).