Why is there a bimodal distribution of central cooling times in galaxy clusters? X-ray observations show approximately half of clusters have short central cooling times (cool-core) while the rest have longer cooling times (non-cool-core). Simulations struggle to reproduce this dichotomy for massive clusters. Is this a failure of AGN jet feedback modeling, overly massive black holes, or a signature of cosmic rays?
Does heating by AGN jets solve the cooling flow problem? Observed star formation rates in cluster cores are suppressed well below those expected from unimpeded radiative cooling (up to several hundred M☉ yr−1). Which of many possible heating mechanisms — AGN lobes, sound waves, turbulence, cosmic rays — is responsible?
Why is the turbulent energy density in the bulk ICM only a few percent of thermal energy? Previous simulations predicted 20–40%, but recent XRISM micro-calorimeter observations show turbulent pressure support of only 2–4%. Is this an attribution error of bulk flows to turbulence, or observational bias?
The ICM is weakly collisional, so heat and momentum should be transported along magnetic field lines (Braginskii MHD). Are there observable consequences? Anisotropic heat conduction implies sharper contact discontinuities, anisotropic viscosity suppresses Kelvin-Helmholtz instabilities, and whistler-electron interactions suppress thermal conduction.
How can magnetic fields grow on scales below the particle mean free path of today's cluster? The plasma dynamo scenario requires kinetic plasma physics at early times followed by a small-scale dynamo — where turbulence drives a fluctuating dynamo that amplifies seed fields. PICO-Clusters instead shows that galactic feedback during proto-cluster formation grows magnetic coherence lengths to exceed the mean free path early on, supporting MHD descriptions of the small-scale cluster dynamo.
What powers extended radio halos in merging galaxy clusters? Radio halos are found predominantly in merging clusters (70% at high masses, 35% at lower masses). Their origin likely involves turbulent re-acceleration of electrons and hadronic cosmic-ray interactions, but the relative importance of these mechanisms and the connection to cluster mass remain unclear.
Do radio relics trace merger-driven shocks, and how are electrons accelerated at low-Mach shocks? Tensions exist between Mach numbers inferred from radio vs. X-ray data, between magnetic field estimates from cooling lengths vs. surrounding ICM, and in spectral index variations. Recent PICO-Cluster work (Whittingham et al. 2026) shows that the interaction of merger shocks, accretion shocks, and density fluctuations can explain these discrepancies.
Why have galaxy clusters not been detected at gamma-ray energies despite predictions from cosmic-ray models? Reducing acceleration efficiencies and including cosmic-ray streaming and diffusion can suppress gamma-ray emission below current limits, but whether these solutions hold in cosmological MHD simulations with modern CR transport treatments remains to be shown.
How do jellyfish galaxies form and how does star formation proceed in their violent tails? Ram-pressure stripping of the ISM, magnetic field draping over galaxies, and tidal interactions fundamentally transform infalling galaxies. PICO-Clusters provides the cosmological context for population-level studies of these phenomena.
What fraction of stars resides in cluster galaxies versus the diffuse intracluster light (ICL)? Simulations tend to predict brighter ICL than observed, though the tension may partly reflect observational challenges. The ICL fraction is rather insensitive to resolution once the dominant stellar-mass galaxies are well resolved, but tidal stripping of low-mass satellites is resolution-sensitive and may affect detailed ICL properties. Some intracluster stars may also form in situ in cold gas clouds of jellyfish tails, which may only be partly captured in current simulations.
Are galaxy groups and clusters baryonically closed, with baryon-to-dark-matter ratios matching the cosmic mean? A persistent tension exists between CMB-inferred baryon fractions and late-time X-ray and SZ constraints at group scales (M200 ∼ 1013–1014 M☉). Recent eROSITA stacking and kinetic SZ analyses find lower gas fractions in groups than earlier studies, while simulations with different AGN feedback prescriptions — EAGLE, IllustrisTNG, SIMBA — give discrepant results, suggesting that improved baryonic physics models may be key to resolving this long-standing challenge.