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Abstract The microphysics of the intracluster medium (ICM) in galaxy clusters is still poorly understood. Observational evidence suggests that the effective viscosity is suppressed by plasma instabilities that reduce the mean free path of particles. Measuring the effective viscosity of the ICM is crucial to understanding the processes that govern its physics on small scales. The trails of ionized interstellar medium left behind by the so-called jellyfish galaxies can trace the turbulent motions of the surrounding ICM and constrain its local viscosity. We present the results of a systematic analysis of the velocity structure function (VSF) of the Hαline for ten galaxies from the GASP sample. The VSFs show a sublinear power-law scaling below 10 kpc that may result from turbulent cascading and extends to 1 kpc, which is below the supposed ICM dissipation scales of tens of kpc expected in a fluid described by Coulomb collisions. Our result constrains the local ICM viscosity to be 0.3%–25% of the expected Spitzer value. Our findings demonstrate that either the ICM particles have a smaller mean free path than expected in a regime defined by Coulomb collisions or that we are probing effects due to collisionless physics in the ICM turbulence. 

Aims.We present a method for refining photometric redshift galaxy catalogs based on a comparison of their color-space matching with overlapping spectroscopic calibration data. We focus on cases where photometric redshifts (photo-z) are estimated empirically. Identifying galaxies that are poorly represented in spectroscopic data is crucial, as their photo-zmay be unreliable due to extrapolation beyond the training sample. Methods.Our approach uses a self-organizing map (SOM) to project a multidimensional parameter space of magnitudes and colors onto a 2D manifold, allowing us to analyze the resulting patterns as a function of various galaxy properties. Using SOM, we compared the Kilo-Degree Survey’s bright galaxy sample (KiDS-Bright), limited tor < 20 mag, with various spectroscopic samples, including the Galaxy And Mass Assembly (GAMA). Results.Our analysis reveals that GAMA tends to underrepresent KiDS-Bright at its faintest (r ≳ 19.5) and highest-redshift (z ≳ 0.4) ranges; however, no strong trends are seen in terms of color or stellar mass. By incorporating additional spectroscopic data from the SDSS, 2dF, and early DESI, we identified SOM cells where the photo-zvalues are estimated suboptimally. We derived a set of SOM-based criteria to refine the photometric sample and improve photo-zstatistics. For the KiDS-Bright sample, this improvement is modest, namely, it excludes the least represented 20% of the sample reduces photo-zscatter by less than 10%. Conclusions.We conclude that GAMA, used for KiDS-Bright photo-ztraining, is sufficiently representative for reliable redshift estimation across most of the color space. Future spectroscopic data from surveys such as DESI should be better suited for exploiting the full improvement potential of our method. 

We present Weak Gravitational Lensing measurements of a sample of 157 clusters within the Kilo Degree Survey (KiDS), detected with a > 5σthermal Sunyaev-Zel’dovich (SZ) signal by the Atacama Cosmology Telescope (ACT). Using a halo-model approach, we constrained the average total cluster mass,M_WL, accounting for the ACT cluster selection function of the full sample. We find that the SZ cluster mass estimateM_SZ, which was calibrated using X-ray observations, is biased withM_SZ/M_WL = (1 − b_SZ) = 0.65 ± 0.05. Separating the sample into six mass bins, we find no evidence of a strong mass dependency for the mass bias, (1 − b_SZ). Adopting this ACT-KiDS SZ mass calibration would bring thePlanckSZ cluster count into agreement with the counts expected from thePlanckcosmic microwave background ΛCDM cosmological model, although it should be noted that the cluster sample considered in this work has a lower average massM_SZ, uncor = 3.64 × 10¹⁴ M_⊙compared to thePlanckcluster sample which has an average mass in the rangeM_SZ, uncor = (5.5 − 8.5)×10¹⁴ M_⊙, depending on the sub-sample used.