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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:

Covariance matrices are among the most difficult pieces of end-to-end cosmological analyses. In principle, for two-point functions, each component involves a four-point function, and the resulting covariance often has hundreds of thousands of elements. We investigate various compression mechanisms capable of vastly reducing the size of the covariance matrix in the context of cosmic shear statistics. This helps identify which of its parts are most crucial to parameter estimation. We start with simple compression methods, by isolating and “removing” 200 modes associated with the lowest eigenvalues, then those with the lowest signal-to-noise ratio, before moving on to more sophisticated schemes like compression at the tomographic level and, finally, with the massively optimized parameter estimation and data compression (MOPED). We find that, while most of these approaches prove useful for a few parameters of interest, like Ωm, the simplest yield a loss of constraining power on the intrinsic alignment (IA) parameters as well as S8. For the case considered—cosmic shear from the first year of data from the Dark Energy Survey—only MOPED was able to replicate the original constraints in the 16-parameter space. Finally, we apply a tolerance test to the elements of the compressed covariance matrix obtained with MOPED and confirm that the IA parameter AIA is the most susceptible to inaccuracies in the covariance matrix.

Abstract:

This paper presents the ESA Voyage 2050 White Paper for a concept of TeraHertz Exploration and Zooming-in for Astrophysics (THEZA). It addresses the science case and some implementation issues of a space-borne radio interferometric system for ultra-sharp imaging of celestial radio sources at the level of angular resolution down to (sub-) microarcseconds. THEZA focuses at millimetre and sub-millimetre wavelengths (frequencies above $\sim$300~GHz), but allows for science operations at longer wavelengths too. The THEZA concept science rationale is focused on the physics of spacetime in the vicinity of supermassive black holes as the leading science driver. The main aim of the concept is to facilitate a major leap by providing researchers with orders of magnitude improvements in the resolution and dynamic range in direct imaging studies of the most exotic objects in the Universe, black holes. The concept will open up a sizeable range of hitherto unreachable parameters of observational astrophysics. It unifies two major lines of development of space-borne radio astronomy of the past decades: Space VLBI (Very Long Baseline Interferometry) and mm- and sub-mm astrophysical studies with "single dish" instruments. It also builds upon the recent success of the Earth-based Event Horizon Telescope (EHT) -- the first-ever direct image of a shadow of the super-massive black hole in the centre of the galaxy M87. As an amalgam of these three major areas of modern observational astrophysics, THEZA aims at facilitating a breakthrough in high-resolution high image quality studies in the millimetre and sub-millimetre domain of the electromagnetic spectrum.Comment: White Paper submitted in response to the ESA Call Voyage 205