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Supermassive Black Hole Growth in Hierarchically Merging Nuclear Star Clusters

The Astrophysical Journal American Astronomical Society 991:1 (2025) 58

Authors:

Konstantinos Kritos, Ricarda S Beckmann, Joseph Silk, Emanuele Berti, Sophia Yi, Marta Volonteri, Yohan Dubois, Julien Devriendt

Abstract:

Supermassive black holes are prevalent at the centers of massive galaxies, and their masses scale with galaxy properties, increasing evidence suggesting that these trends continue to low stellar masses. Seeds are needed for supermassive black holes, especially at the highest redshifts explored by the James Webb Space Telescope. We study the hierarchical merging of galaxies via cosmological merger trees and argue that the seeds of supermassive black holes formed in nuclear star clusters via stellar black hole mergers at early epochs. Observable tracers include intermediate-mass black holes, nuclear star clusters, and early gas accretion in host dwarf galaxies, along with a potentially detectable stochastic gravitational-wave background, ejection of intermediate and supermassive black holes, and consequences of a significant population of early tidal disruption events and extreme mass ratio inspirals.

Gravitational turbulence: The small-scale limit of the cold-dark-matter power spectrum

Physical Review D American Physical Society (APS) 112:6 (2025) 063501

Authors:

Yonadav Barry Ginat, Michael L Nastac, Robert J Ewart, Sara Konrad, Matthias Bartelmann, Alexander A Schekochihin

Abstract:

The matter power spectrum, P ( k ) , is one of the fundamental quantities in the study of large-scale structure in cosmology. Here, we study its small-scale asymptotic limit, and show that for cold dark matter in d spatial dimensions, P ( k ) has a universal k − d asymptotic scaling with the wave number k , for k ≫ k nl , where k nl − 1 denotes the length scale at which nonlinearities in gravitational interactions become important. We propose a theoretical explanation for this scaling, based on a nonperturbative analysis of the system’s phase-space structure. Gravitational collapse is shown to drive a turbulent phase-space flow of the quadratic Casimir invariant, where the linear and nonlinear time scales are balanced, and this balance dictates the k dependence of the power spectrum. A parallel is drawn to Batchelor turbulence in hydrodynamics, where large scales mix smaller ones via tidal interactions. The k − d scaling is also derived by expressing P ( k ) as a phase-space integral in the framework of kinetic field theory, which is analyzed by the saddle-point method; the dominant critical points of this integral are precisely those where the time scales are balanced. The coldness of the dark-matter distribution function—its nonvanishing only on a d -dimensional submanifold of phase space—underpins both approaches. The theory is accompanied by 1D Vlasov-Poisson simulations, which confirm it.

Calibrating baryonic effects in cosmic shear with external data in the LSST era

Monthly Notices of the Royal Astronomical Society (2025)

Authors:

Amy Wayland, David Alonso, and Matteo Zennaro

Abstract:

Cosmological constraints derived from weak lensing (WL) surveys are limited by baryonic effects, which suppress the non-linear matter power spectrum on small scales. By combining WL measurements with data from external tracers of the gas around massive structures, it is possible to calibrate baryonic effects and, therefore, obtain more precise cosmological constraints. In this study, we generate mock data for a Stage-IV weak lensing survey such as the Legacy Survey of Space and Time (LSST), X-ray gas fractions, and stacked kinetic Sunyaev-Zel'dovich (kSZ) measurements, to jointly constrain cosmological and astrophysical parameters describing baryonic effects (using the Baryon Correction Model - BCM). First, using WL data alone, we quantify the level to which the BCM parameters will need to be constrained to recover the cosmological constraints obtained under the assumption of perfect knowledge of baryonic feedback. We identify the most relevant baryonic parameters and determine that they must be calibrated to a precision of ∼10-20% to avoid significant degradation of the fiducial WL constraints. We forecast that long-term X-ray data from ∼5000 clusters should be able to reach this threshold for the parameters that characterise the abundance of hot virialised gas. Constraining the distribution of ejected gas presents a greater challenge, however, but we forecast that long-term kSZ data from a CMB-S4-like experiment should achieve the level of precision required for full self-calibration.

Cosmological simulations of the same spiral galaxy: satellite properties, the role of baryonic physics and star formation history in shaping dark matter cores/cusps

(2025)

Authors:

A Nunez-Castineyra, E Nezri, P Mollitor, L Michel-Dansac, Julien Devriendt, R Teyssier

Evidence for inverse Compton scattering in high-redshift Lyman-break galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press 543:1 (2025) 507-517

Authors:

IH Whittam, MJ Jarvis, Eric J Murphy, NJ Adams, RAA Bowler, A Matthews, RG Varadaraj, CL Hale, I Heywood, K Knowles, L Marchetti, N Seymour, F Tabatabaei, AR Taylor, M Vaccari, A Verma

Abstract:

Radio continuum emission provides a unique opportunity to study star formation unbiased by dust obscuration. However, if radio observations are to be used to accurately trace star formation to high redshifts, it is crucial that the physical processes that affect the radio emission from star-forming galaxies are well understood. While inverse Compton (IC) losses from the cosmic microwave background (CMB) are negligible in the local universe, the rapid increase in the strength of the CMB energy density with redshift [] means that this effect becomes increasingly important at . Using a sample of high-redshift () Lyman-break galaxies selected in the rest-frame ultraviolet (UV), we have stacked radio observations from the MIGHTEE survey to estimate their 1.4-GHz flux densities. We find that for a given rest-frame UV magnitude, the 1.4-GHz flux density and luminosity decrease with redshift. We compare these results to the theoretical predicted effect of energy losses due to IC scattering off the CMB, and find that the observed decrease is consistent with this explanation. We discuss other possible causes for the observed decrease in radio flux density with redshift at a given UV magnitude, such as a top-heavy initial mass function at high redshift or an evolution of the dust properties, but suggest that IC scattering is the most compelling explanation.