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1. Constraints on Nonthermal Pressure at Galaxy Cluster Outskirts from a Joint SPT and XMM-Newton Analysis

   https://doi.org/10.3847/2041-8213/adc676  

   Sarkar, Arnab; McDonald, Michael; Bleem, Lindsey; Bautz, Mark; Benson, Bradford A; Chakraborty, Priyanka; Grant, Catherine E; Jones, Christine; Kéruzoré, Florian; Miller, Eric D; et al (May 2025, The Astrophysical Journal Letters)

   Abstract We present joint South Pole Telescope and XMM-Newton observations of eight massive galaxy clusters (0.8–2 × 10¹⁵M_⊙) spanning a redshift range of 0.16–0.35. Employing a novel Sunyaev–Zel’dovich + X-ray fitting technique, we effectively constrain the thermodynamic properties of these clusters out to the virial radius. The resulting best-fit electron density, deprojected temperature, and deprojected pressure profiles are in good agreement with previous observations of massive clusters. For the majority of the cluster sample (five out of eight clusters), the entropy profiles exhibit a self-similar behavior near the virial radius. We further derive hydrostatic mass, gas mass, and gas fraction profiles for all clusters up to the virial radius. Comparing the enclosed gas fraction profiles with the universal gas fraction profile, we obtain nonthermal pressure fraction profiles for our cluster sample at  >0.5R₅₀₀, demonstrating a steeper increase betweenR₅₀₀andR₂₀₀that is consistent with the hydrodynamical simulations. Our analysis yields nonthermal pressure fraction ranges of 8%–28% (median: 15% ± 11%) atR₅₀₀and 21%–35% (median: 27% ± 12%) atR₂₀₀. Notably, weak-lensing mass measurements are available for only four clusters in our sample, and our recovered total cluster masses, after accounting for nonthermal pressure, are consistent with these measurements. 

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2. Mass proxy quality of massive halo properties in the illustrisTNG and flamingo simulations: I. Hot gas

   https://doi.org/10.1093/mnras/staf1665  

   Aljamal, Eddie; Evrard, August E; Farahi, Arya; Pillepich, Annalisa; Nelson, Dylan; Schaye, Joop; Schaller, Matthieu; Braspenning, Joey (October 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We examine scale and redshift dependence of mass–property relations (MPRs) for five hot gas properties of two large group- bservationsand cluster-scale halo samples realized by the Illustris tng, TNG-Cluster, and FLAMINGO cosmological hydrodynamical simulations. For intrinsic properties of (i) hot gas mass ($$M_{\rm gas}$$), (ii) spectroscopic-like temperature ($$T_{\rm sl}$$), (iii) soft-band X-ray luminosity ($$L_{\rm X}$$), and (iv) X-ray ($$Y_{\rm X}$$), and (v) Sunyaev–Zel’dovich ($$Y_{\rm SZ}$$) thermal energies, we use MPR parameters to infer mass proxy quality (MPQ) – the implied scatter in total halo mass conditioned on a property – for haloes with $$M_{\rm 500c}\ge 10^{13}\, {\rm M}_\odot$$ at redshifts, $$z \in \lbrace 0, 0.5, 1, 2\rbrace$$. We find: (1) in general, scaling relation slopes and covariance display moderate to strong dependence on halo mass, with redshift dependence secondary; (2) for haloes with $$M_{\rm 500c}> 10^{14}\, {\rm M}_\odot$$, scalings of $$M_{\rm gas}$$ and $$Y_{\rm SZ}$$ simplify toward self-similar slope and constant intrinsic scatter (5 and 10 per cent, respectively) nearly independent of scale, making both measures ideal for cluster finding and characterization to $z=2$; (3) halo mass-conditioned likelihoods of hot gas mass and thermal energy closely follow a lognormal form; and (4) despite normalization differences up to 0.4 dex between the two simulations, higher order scaling features such as slopes and property covariance show much better agreement. Slopes show appreciable redshift dependence at the group scale, while redshift dependence of the scatter is exhibited by low-mass flamingo haloes only; (5) property correlations are largely consistent between the simulations, with values that mainly agree with existing empirical measurements. We close with a literature survey placing our MPR slopes and intrinsic scatter estimates into community context. 

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3. Augmenting astrophysical scaling relations with machine learning: Application to reducing the Sunyaev–Zeldovich flux–mass scatter

   https://doi.org/10.1073/pnas.2202074120  

   Wadekar, Digvijay; Thiele, Leander; Villaescusa-Navarro, Francisco; Hill, J. Colin; Cranmer, Miles; Spergel, David N.; Battaglia, Nicholas; Anglés-Alcázar, Daniel; Hernquist, Lars; Ho, Shirley (March 2023, Proceedings of the National Academy of Sciences)

   Complex astrophysical systems often exhibit low-scatter relations between observable properties (e.g., luminosity, velocity dispersion, oscillation period). These scaling relations illuminate the underlying physics, and can provide observational tools for estimating masses and distances. Machine learning can provide a fast and systematic way to search for new scaling relations (or for simple extensions to existing relations) in abstract high-dimensional parameter spaces. We use a machine learning tool called symbolic regression (SR), which models patterns in a dataset in the form of analytic equations. We focus on the Sunyaev-Zeldovich flux−cluster mass relation ( Y SZ − M ), the scatter in which affects inference of cosmological parameters from cluster abundance data. Using SR on the data from the IllustrisTNG hydrodynamical simulation, we find a new proxy for cluster mass which combines Y SZ and concentration of ionized gas ( c gas ): M ∝ Y conc 3/5 ≡ Y SZ 3/5 (1 − A c gas ). Y conc reduces the scatter in the predicted M by ∼20 − 30% for large clusters ( M ≳ 10 14 h −1 M ⊙ ), as compared to using just Y SZ . We show that the dependence on c gas is linked to cores of clusters exhibiting larger scatter than their outskirts. Finally, we test Y conc on clusters from CAMELS simulations and show that Y conc is robust against variations in cosmology, subgrid physics, and cosmic variance. Our results and methodology can be useful for accurate multiwavelength cluster mass estimation from upcoming CMB and X-ray surveys like ACT, SO, eROSITA and CMB-S4. 

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4. Studying galaxy cluster morphological metrics with mock-X

   https://doi.org/10.1093/mnras/stab605  

   Cao, Kaili; Barnes, David J; Vogelsberger, Mark (March 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Dynamically relaxed galaxy clusters have long played an important role in galaxy cluster studies because it is thought their properties can be reconstructed more precisely and with less systematics. As relaxed clusters are desirable, there exist a plethora of criteria for classifying a galaxy cluster as relaxed. In this work, we examine 9 commonly used observational and theoretical morphological metrics extracted from $$54\, 000$$mock-X synthetic X-ray images of galaxy clusters taken from the IllustrisTNG, BAHAMAS, and MACSIS simulation suites. We find that the simulated criteria distributions are in reasonable agreement with the observed distributions. Many criteria distributions evolve as a function of redshift, cluster mass, numerical resolution, and subgrid physics, limiting the effectiveness of a single relaxation threshold value. All criteria are positively correlated with each other, however, the strength of the correlation is sensitive to redshift, mass, and numerical choices. Driven by the intrinsic scatter inherent to all morphological metrics and the arbitrary nature of relaxation threshold values, we find the consistency of relaxed subsets defined by the different metrics to be relatively poor. Therefore, the use of relaxed cluster subsets introduces significant selection effects that are non-trivial to resolve. 

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5. Deep Synoptic Array Science: Two Fast Radio Burst Sources in Massive Galaxy Clusters

   https://doi.org/10.3847/2041-8213/acd3ea  

   Connor, Liam; Ravi, Vikram; Catha, Morgan; Chen, Ge; Faber, Jakob T.; Lamb, James W.; Hallinan, Gregg; Harnach, Charlie; Hellbourg, Greg; Hobbs, Rick; et al (May 2023, The Astrophysical Journal Letters)

   Abstract The hot gas that constitutes the intracluster medium (ICM) has been studied at X-ray and millimeter/submillimeter wavelengths (Sunyaev–Zel’dovich effect) for decades. Fast radio bursts (FRBs) offer an additional method of directly measuring the ICM and gas surrounding clusters via observables such as dispersion measure (DM) and Faraday rotation measure. We report the discovery of two FRB sources detected with the Deep Synoptic Array whose host galaxies belong to massive galaxy clusters. In both cases, the FRBs exhibit excess extragalactic DM, some of which likely originate in the ICM of their respective clusters. FRB 20220914A resides in the galaxy cluster A2310 at z = 0.1125 with a projected offset from the cluster center of 520 ± 50 kpc. The host of a second source, FRB 20220509G, is an elliptical galaxy at z = 0.0894 that belongs to the galaxy cluster A2311 at the projected offset of 870 ± 50 kpc. These sources represent the first time an FRB has been localized to a galaxy cluster. We combine our FRB data with archival X-ray, Sunyaev–Zel'dovich (SZ), and optical observations of these clusters in order to infer properties of the ICM, including a measurement of gas temperature from DM and y SZ of 0.8–3.9 keV. We then compare our results to massive cluster halos from the IllustrisTNG simulation. Finally, we describe how large samples of localized FRBs from future surveys will constrain the ICM, particularly beyond the virial radius of clusters. 

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