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1. Introducing the BRAHMA simulation suite: signatures of low-mass black hole seeding models in cosmological simulations

   https://doi.org/10.1093/mnras/stae1386  

   Bhowmick, Aklant_K; Blecha, Laura; Torrey, Paul; Kelley, Luke_Zoltan; Weinberger, Rainer; Vogelsberger, Mark; Hernquist, Lars; Somerville, Rachel_S; Evans, Analis_Eolyn (June 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT While the first “seeds” of supermassive black holes (BH) can range from $$\sim 10^2-10^6 \rm ~{\rm M}_{\odot }$$, the lowest mass seeds ($$\lesssim 10^3~\rm {\rm M}_{\odot }$$) are inaccessible to most cosmological simulations due to resolution limitations. We present our new BRAHMA simulations that use a novel flexible seeding approach to predict the $$z\ge 7$$ BH populations for low-mass seeds. We ran two types of boxes that model $$\sim 10^3~\rm {\rm M}_{\odot }$$ seeds using two distinct but mutually consistent seeding prescriptions at different simulation resolutions. First, we have the highest resolution $$[9~\mathrm{Mpc}]^3$$ (BRAHMA-9-D3) boxes that directly resolve $$\sim 10^3~\rm {\rm M}_{\odot }$$ seeds and place them within haloes with dense, metal-poor gas. Second, we have lower resolution, larger volume $$[18~\mathrm{Mpc}]^3$$ (BRAHMA-18-E4), and $$\sim [36~\mathrm{Mpc}]^3$$ (BRAHMA-36-E5) boxes that seed their smallest resolvable $$\sim 10^4~\&~10^5~\mathrm{{\rm M}_{\odot }}$$ BH descendants using new stochastic seeding prescriptions calibrated using BRAHMA-9-D3. The three boxes together probe key BH observables between $$\sim 10^3\,\mathrm{ and}\,10^7~\rm {\rm M}_{\odot }$$. The active galactic nuclei (AGN) luminosity function variations are small (factors of $$\sim 2-3$$) at the anticipated detection limits of potential future X-ray facilities ($$\sim 10^{43}~ \mathrm{ergs~s^{-1}}$$ at $$z\sim 7$$). Our simulations predict BHs $$\sim 10-100$$ times heavier than the local $$M_*$$ versus $$M_{\mathrm{ bh}}$$ relations, consistent with several JWST-detected AGN. For different seed models, our simulations merge binaries at $$\sim 1-15~\mathrm{kpc}$$, with rates of $$\sim 200-2000$$ yr−1 for $$\gtrsim 10^3~\rm {\rm M}_{\odot }$$ BHs, $$\sim 6-60$$ yr−1 for $$\gtrsim 10^4~\rm {\rm M}_{\odot }$$ BHs, and up to $$\sim 10$$ yr−1 amongst $$\gtrsim 10^5~\rm {\rm M}_{\odot }$$ BHs. These results suggest that Laser Interferometer Space Antenna mission has promising prospects for constraining seed models. 

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2. C ¹ actions on manifolds by lattices in Lie groups

   https://doi.org/10.1112/S0010437X22007278  

   Brown, Aaron; Damjanović, Danijela; Zhang, Zhiyuan (March 2022, Compositio Mathematica)

   In this paper we study Zimmer's conjecture for $$C^{1}$$ actions of lattice subgroup of a higher-rank simple Lie group with finite center on compact manifolds. We show that when the rank of an uniform lattice is larger than the dimension of the manifold, then the action factors through a finite group. For lattices in $${\rm SL}(n, {{\mathbb {R}}})$$ , the dimensional bound is sharp. 

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3. Regularized Stein Variational Gradient Flow

   https://doi.org/10.1007/s10208-024-09663-w  

   He, Ye; Balasubramanian, Krishnakumar; Sriperumbudur, Bharath K; Lu, Jianfeng (July 2024, Foundations of Computational Mathematics)

   Abstract The stein variational gradient descent (SVGD) algorithm is a deterministic particle method for sampling. However, a mean-field analysis reveals that the gradient flow corresponding to the SVGD algorithm (i.e., the Stein Variational Gradient Flow) only provides a constant-order approximation to the Wasserstein gradient flow corresponding to the KL-divergence minimization. In this work, we propose the Regularized Stein Variational Gradient Flow, which interpolates between the Stein Variational Gradient Flow and the Wasserstein gradient flow. We establish various theoretical properties of the Regularized Stein Variational Gradient Flow (and its time-discretization) including convergence to equilibrium, existence and uniqueness of weak solutions, and stability of the solutions. We provide preliminary numerical evidence of the improved performance offered by the regularization. 

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4. The fate of rotating massive stars across cosmic times

   https://doi.org/10.1093/mnras/staf1470  

   Hirschi, R.; Goodman, K.; Meynet, G.; Maeder, A.; Ekström, S.; Eggenberger, P.; Georgy, C.; Sibony, Y.; Yusof, N.; Martinet, S.; et al (September 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The initial mass and metallicity of stars both have a strong impact on their fate. Stellar axial rotation also has a strong impact on the structure and evolution of massive stars. In this study, we exploit the large grid of GENEC models, covering initial masses from 9 to 500 $${\rm M}_{\odot }$$ and metallicities ranging from $$Z=10^{-5}$$ (nearly zero) to 0.02 (supersolar), to determine the impact of rotation on their fate across cosmic times. Using the carbon–oxygen core mass and envelope composition as indicators of their fate, we predict stellar remnants, supernova engines, and spectroscopic supernova types for both rotating and non-rotating stars. We derive rates of the different supernova and remnant types considering two initial mass functions to help solve puzzles such as the absence of observed pair-instability supernovae. We find that rotation significantly alters the remnant type and supernova engine, with rotating stars favouring black hole formation at lower initial masses than their non-rotating counterparts. Additionally, we confirm the expected strong metallicity dependence of the fates with a maximum black hole mass predicted to be below 50 $${\rm M}_{\odot }$$ at SMC or higher metallicities. A pair-instability mass gap is predicted between about 90 and 150 $${\rm M}_{\odot }$$, with the most massive black holes below the gap found at the lowest metallicities. Considering the fate of massive single stars has far-reaching consequences across many different fields within astrophysics, and understanding the impact of rotation and metallicity will improve our understanding of how massive stars end their lives, and their impact on the Universe. 

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5. Flows, growth rates, and the veering polynomial

   https://doi.org/10.1017/etds.2022.63  

   LANDRY, MICHAEL P.; MINSKY, YAIR N.; TAYLOR, SAMUEL J. (January 2022, Ergodic Theory and Dynamical Systems)

   Abstract For a pseudo-Anosov flow $$\varphi $$ without perfect fits on a closed $$3$$ -manifold, Agol–Guéritaud produce a veering triangulation $$\tau $$ on the manifold M obtained by deleting the singular orbits of $$\varphi $$ . We show that $$\tau $$ can be realized in M so that its 2-skeleton is positively transverse to $$\varphi $$ , and that the combinatorially defined flow graph $$\Phi $$ embedded in M uniformly codes the orbits of $$\varphi $$ in a precise sense. Together with these facts, we use a modified version of the veering polynomial, previously introduced by the authors, to compute the growth rates of the closed orbits of $$\varphi $$ after cutting M along certain transverse surfaces, thereby generalizing the work of McMullen in the fibered setting. These results are new even in the case where the transverse surface represents a class in the boundary of a fibered cone of M . Our work can be used to study the flow $$\varphi $$ on the original closed manifold. Applications include counting growth rates of closed orbits after cutting along closed transverse surfaces, defining a continuous, convex entropy function on the ‘positive’ cone in $H^1$ of the cut-open manifold, and answering a question of Leininger about the closure of the set of all stretch factors arising as monodromies within a single fibered cone of a $$3$$ -manifold. This last application connects to the study of endperiodic automorphisms of infinite-type surfaces and the growth rates of their periodic points. 

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