Cosmology

Abstract:

We present cosmic shear constraints from the completed Kilo-Degree Survey (KiDS), where the cosmological parameter S 8 ≡ σ 8 √Ω m /0.3 = 0.81 +0.016 −0.021 is found to be in agreement (0.73 σ ) with results from the Planck Legacy cosmic microwave background experiment. The final KiDS footprint spans 1347 square degrees of deep nine-band imaging across the optical and near-infrared (NIR), along with an extra 23-square degrees of KiDS-like calibration observations of deep spectroscopic surveys. Improvements in our redshift distribution estimation methodology, combined with our enhanced calibration data and multi-band image simulations, allowed us to extend our lensed sample out to a photometric redshift of z B ≤ 2.0. Compared to previous KiDS analyses, the increased survey area and redshift depth results in a ∼32% improvement in constraining power in terms of Σ 8 ≡ σ 8 (Ω m /0.3) α = 0.821 +0.014 −0.016 , where α = 0.58 has been optimised to match the revised degeneracy direction of σ 8 and Ω m for our current survey at higher redshift. We adopted a new physically motivated intrinsic alignment (IA) model that jointly depends on the galaxy sample’s halo mass and spectral type distributions, and which is informed by previous direct alignment measurements. We also marginalised over our uncertainty on the impact of baryon feedback on the non-linear matter power spectrum. Compared to previous KiDS analyses, we conclude that the increase seen in S 8 primarily results from our improved redshift distribution estimation and calibration, as well as a new survey area and improved image reduction. Our companion paper presents a full suite of internal and external consistency tests (including joint constraints with other datasets), finding the KiDS-Legacy dataset to be the most internally robust sample produced by KiDS to date.

Abstract:

The James Webb Space Telescope has ushered in an era of abundant high-redshift observations of young stellar populations characterized by strong emission lines, motivating us to integrate nebular emission into the new Maraston stellar population model which incorporates the latest Geneva stellar evolutionary tracks for massive stars with rotation. We use the photoionization code Cloudy to obtain the emergent nebular continuum and line emission for a range of modelling parameters, then compare our results to observations on various emission line diagnostic diagrams. We carry out a detailed comparison with several other models in the literature assuming different input physics, including modified prescriptions for stellar evolution and the inclusion of binary stars, and find close agreement in the H , H , [N ii], and [S ii] luminosities between the models. However, we find significant differences in lines with high ionization energies, such as He ii1640 and [O iii], due to large variations in the hard ionizing photon production rates. The models differ by a maximum of , where these differences are mostly caused by the assumed stellar rotation and effective temperatures for the Wolf Rayet phase. Interestingly, rotation and uncorrected effective temperatures in our single star population models alone generate [O iii] ionizing photon production rates higher than models including binary stars with ages between 1 to 6 Myr. These differences highlight the dependence of derived properties from SED fitting on the assumed model, as well as the sensitivity of predictions from cosmological simulations.

Abstract:

We combine deep photometric data in the COSMOS and XMM-LSS fields with high-resolution cosmological hydrodynamical simulations to explore two key questions: (1) how does the galaxy stellar mass function, particularly in the dwarf ( 10 M) regime, vary with environment, defined as the distance from large-scale structure (LSS) traced by nodes and filaments in the cosmic web? (2) is there a generic ‘missing dwarfs’ problem in Lambda cold dark matter (CDM) predictions when all environments – and not just satellites around Milky Way like galaxies – are considered? The depth of the observational data used here enables us to construct complete, unbiased samples of galaxies, down to 10 M and out to . Strong environmental differences are found for the galaxy stellar mass function when considering distance from LSS. As we move closer to LSS, the dwarf mass function becomes progressively flatter and the knee of the mass function shifts to larger stellar masses, both of which result in a higher ratio of massive to dwarf galaxies. While the stellar mass functions from the three simulations (NewHorizon, TNG50, and FIREbox) considered here do not completely agree across the dwarf regime, there is no evidence of a generic missing dwarfs problem in the context of CDM, akin to the results of recent work that demonstrates that there is no missing satellites problem around Galactic analogues.

Abstract:

We present a second-order calculation of relativistic large-scale-structure observables in cosmological perturbation theory, specifically the “cosmic rulers and clock”, which are the building-blocks of any other large-scale-structure observable, including galaxy number counts, on large scales. We calculate the scalar rulers (longitudinal perturbation and magnification) and the cosmic clock to second order, using a fully non-linear covariant definition of the observables. We validate our formulæ on three non-trivial space-time metrics: two of them are null tests on metrics which are obtained by applying a gauge transformation to the background space-time, while the third is the “separate universe” curved background, for which we can also compute the observables exactly. We then illustrate the results by evaluating the second-order observables in a simplified symmetric setup. On large scales, they are suppressed over the linear contributions by ∼10-4, while they become comparable to the linear contributions on mildly non-linear scales. The results of this paper form a significant (and the most complicated) part of the relativistic galaxy number density at second order.