Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

We introduce a fast method to measure the conversion gain in complementary metal-oxide-semiconductor active pixel sensors, which accounts for nonlinearity and interpixel capacitance (IPC). The standard mean-variance method is biased because it assumes that pixel values depend linearly on the signal, and existing methods to correct for nonlinearity still introduce significant biases. While current IPC correction methods are prohibitively slow for a per-pixel application, our new method uses separate measurements of the IPC kernel to calculate the gain almost instantaneously. Using test data from a flight detector of the ESA Euclid mission, the IPC correction recovers the results of slower methods with 0.1% accuracy. The nonlinearity correction ensures that the estimated gain is independent of signal, correcting a bias of more than 2.5%.

Abstract:

We present the results of a search for galaxy clusters in the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) microwave sky maps covering 16293 square degrees in three frequency bands, using data obtained over the lifetime of the project (2008–2022). We report redshifts and mass estimates for 10040 clusters detected via their Sunyaev-Zel’dovich (SZ) effect with signal-to-noise greater than 4 at a 2.4 arcminute filter scale. The catalog includes 1180 clusters at redshifts greater than 1, and 124 clusters at redshifts greater than 1.5. Using a relation between cluster SZ signal and mass that is consistent with recent weak-lensing measurements, we estimate that clusters detected with signalto-noise greater than 5 form a sample which is 90% complete for clusters with masses greater than 5 × 10¹⁴M⊙ (measured within a spherical volume with mean density 500 times the critical density). El Gordo, a cluster found in an initial ACT survey of 755 square degrees, remains the most extreme cluster in mass and redshift; we find no cluster with a mass and redshift combination high enough to falsify the standard ΛCDM cosmology with Gaussian initial perturbations. We make public a variety of data products, including the full cluster candidate list, noise maps, and sky masks, along with our software for cluster detection and instructions for reproducing our cluster catalogs from the public ACT maps.

Abstract:

The late 20th and early 21st century became an extraordinary era for the Savilian chair and the astrophysics subdepartment at Oxford. It coincided with the golden age of physical cosmology, in which the study of the origin and evolution of the Universe was transformed from a speculative and data-starved backwater, looked upon with some suspicion by astronomers and physicists alike, to a precise, data-driven, and well-resourced field, a jewel in the crown of modern physics. By chance, the Savilian professors in post during that period, George Efstathiou (from 1988 to 1998) and Joseph (‘Joe’) Silk (from 1999 to 2010), were two of the leading figures in the genesis of the modern age of cosmology, instrumental in laying down the groundwork for the revolution that was to come.

Abstract:

The thermal Sunyaev–Zel’dovich (tSZ) effect provides a powerful probe of the thermal pressure of ionised gas in galaxy clusters and the cosmic web; constrained simulations reconstruct the mass and velocity fields of the local Universe. We explore how these two may be mutually informative: the tSZ signal provides a benchmark for assessing the fidelity of constrained simulations, and constrained simulations contribute information on the positions, total masses and density profiles of cosmic web structures for use in tSZ studies. We focus on cluster predictions in the Bayesian Origin Reconstruction from Galaxies (BORG) paradigm, introducing CSiBORG-Manticore, a new state-of-the-art suite of digital twins—data-constrained posterior simulations whose initial conditions are inferred via Bayesian forward modelling. We develop a framework for scoring constrained simulations on their ability to match measured Planck Compton-y maps around clusters, and use it to demonstrate improvement from previous BORG reconstructions. We further validate halo masses against weak-lensing-calibrated X-ray masses from eROSITA. We also show how high-fidelity digital twins offer a practical route to extracting additional information from tSZ data through a novel calibration of the mass–observable relation, and provide a complementary framework to purely statistical analyses of Compton-y maps. This paves the way for integrating the large-scale structure information inherent in constrained simulations into the study of CMB secondary anisotropies.