🔭 astrophysics

Dianoga simulations of galaxy clusters and groups: Properties of the baryonic components

The Dianoga simulations utilize the OpenGadget3 code to investigate how specific implementations of AGN feedback and star formation affect galaxy clusters and groups, revealing that while the reference model broadly matches observations, the precise details of how AGN energy interacts with the sub-resolution interstellar medium are critical for accurately reproducing observed baryonic properties and cluster thermodynamics.

Original authors: Stefano Borgani, Alice Damiano, Elena Rasia, Tiago Castro, Michela Esposito, Ilaria Marini, Giuseppe Murante, Milena Valentini, Veronica Biffi, Klaus Dolag, Gian Luigi Granato, Antonio Ragagnin, Cinth

Published 2026-08-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Stefano Borgani, Alice Damiano, Elena Rasia, Tiago Castro, Michela Esposito, Ilaria Marini, Giuseppe Murante, Milena Valentini, Veronica Biffi, Klaus Dolag, Gian Luigi Granato, Antonio Ragagnin, Cinthia Ragone-Figueroa, Alex Saro, Luca Tornatore

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep in the vast cosmic web, where gravity has pulled matter together over billions of years, lie the largest structures in the universe: galaxy clusters. These are not just random collections of stars; they are massive, dynamic ecosystems containing hundreds or thousands of galaxies, swirling clouds of superheated gas, and the invisible scaffolding of dark matter that holds it all together. At the heart of the most massive galaxies within these clusters sit supermassive black holes, objects so dense that their gravity traps everything nearby, including light. For decades, astronomers have struggled to understand how these three components—the galaxies, the hot gas, and the black holes—grow and change together. If the gas in a cluster were left to its own devices, it would cool down rapidly, collapse, and form an explosion of new stars, creating a universe that looks nothing like the one we see today. Something must be heating the gas and stopping this runaway star formation, and the leading suspect is the central black hole, which can release tremendous amounts of energy. However, reproducing this delicate balance in computer models has proven to be one of the most difficult challenges in modern astrophysics.

A team of researchers led by Stefano Borgani has tackled this problem using a new set of high-resolution computer simulations called the Dianoga project. Instead of trying to simulate the entire universe, they focused their computational power on twenty-eight specific regions, each centered on a massive galaxy cluster, along with a smaller box of space to serve as a control group. These simulations track the behavior of nearly three hundred distinct groups and clusters of galaxies, following the movement of billions of particles representing dark matter, gas, and stars. The goal was to see if they could recreate the observed properties of these cosmic structures, particularly the relationship between the mass of a central black hole and the mass of the stars in its host galaxy. To do this, they started with a minimal set of rules for how black holes feed and release energy, calibrating them only to match the local relationship between black hole mass and galaxy size, and then let the simulations run to see what happened to the rest of the cluster.

The results revealed both successes and persistent puzzles. The simulations successfully reproduced the general distribution of galaxy sizes in the universe, matching observations for smaller galaxies and groups quite well. However, when it came to the most massive clusters, the models produced a problem: the central galaxies, known as Brightest Cluster Galaxies, grew too large. In the real universe, these galaxies are surprisingly modest in size, but in the simulations, they accumulated far too many stars, becoming significantly heavier than their real-world counterparts. This suggests that the mechanism intended to stop star formation in the center of these massive clusters was not strong enough. The black holes in the simulations did release energy, but it was not enough to prevent the surrounding gas from cooling and collapsing into new stars.

The researchers also examined the hot gas that fills the space between the galaxies, known as the intra-cluster medium. In the outer regions of the clusters, far from the center, the simulations matched the real universe perfectly, showing that the gas behaves exactly as gravity and basic physics predict. The trouble was confined to the very center. In the real universe, the gas in the cores of clusters often forms "cool cores," where the gas is dense and relatively cool, yet it does not collapse into stars because the black hole gently heats it. In the Dianoga simulations, the central gas failed to develop these low-entropy cool cores; instead, the entropy and temperature profiles were less "cool-cored" than observed. The models failed to create the observed low-entropy cores, indicating that the feedback loop between the black hole and the gas was not self-regulating correctly. The black hole was either not heating the gas efficiently enough, or the way the energy was transferred to the gas was flawed.

To solve this, the team tested several variations of their model, tweaking how the black hole energy interacts with the gas. They discovered that the key was not just how much energy the black hole released, but how that energy was delivered to the smallest, coldest clumps of gas within the star-forming regions. In their standard model, the energy was deposited in a way that allowed the gas to quickly cool back down and form stars. However, when they modified the model to allow the black hole's energy to evaporate these cold clumps of gas before they could turn into stars, the results improved dramatically. This change prevented the central galaxies from growing too large and helped the simulated gas cores become cooler and more similar to what astronomers observe in the real universe.

This finding highlights a crucial insight for the future of cosmological simulations: the details of how different physical processes connect are just as important as the total amount of energy involved. It is not enough to simply have a powerful black hole; the way that power is transferred to the surrounding environment must be precise. The researchers found that by adjusting this specific interaction, they could bring their simulated clusters much closer to reality, solving the mystery of why the central galaxies were too big and why the gas cores were not forming the observed cool structures. While the simulations still face challenges, particularly in perfectly matching the most massive systems, this work demonstrates that the co-evolution of galaxies and black holes is a tightly regulated dance, where the smallest details of energy transfer determine the fate of the largest structures in the cosmos. The study confirms that current models are on the right track but require a more nuanced understanding of how black holes influence the birth of stars in the densest environments of the universe.

Technical Summary: Dianoga Simulations of Galaxy Clusters and Groups

Problem and Context
Cosmological hydrodynamical simulations face a persistent challenge in reproducing the observed co-evolution of cluster galaxies, the thermodynamical properties of the intra-cluster and intra-group media (ICM/IGM), and central supermassive black holes (SMBHs). While simulations have successfully modeled ICM properties outside cluster cores, they struggle in the core regions where "overcooling" leads to excessive star formation and overly massive Brightest Cluster Galaxies (BCGs). Observational evidence indicates a self-regulated cooling-feedback loop driven by Active Galactic Nuclei (AGN), yet current implementations often fail to simultaneously reproduce the galaxy stellar mass function (GSMF), BCG masses, and the "cool-core" entropy profiles observed in relaxed clusters. A specific tension exists between high-resolution simulations, which often overpredict BCG masses, and lower-resolution runs, which sometimes reproduce cool cores but lack statistical power.

Methodology
The authors present the "Dianoga" set of cosmological hydrodynamical simulations, utilizing the OpenGadget3 code (a TreePM-SPH evolution of GADGET3). The dataset comprises:

28 Lagrangian zoom-in regions centered on massive halos, containing a total of 293 halos with $M_{200} \ge 1.5 \times 10^{13} M_\odot$ (including 74 clusters with $M_{200} \ge 10^{14} M_\odot$ and 23 with $M_{200} \ge 10^{15} M_\odot$ ).
One cosmological box ( $\approx 50 h^{-1}$ Mpc) simulated at the same resolution to serve as a field reference.
Resolution: Dark matter particle mass $m_{DM} \approx 3.38 \times 10^7 h^{-1} M_\odot$ ; initial gas particle mass $m_{gas} \approx 6.24 \times 10^6 h^{-1} M_\odot$ . Stellar and BH particles have a gravitational softening of 250 $h^{-1}$ cpc.
Physics Models: The simulations include metallicity-dependent radiative cooling, a redshift-dependent UV background, a multi-phase interstellar medium (ISM) model for star formation (Springel & Hernquist 2003), Type-II supernova feedback, and chemical enrichment.
AGN Feedback: The core of the study involves a reference model (M1) and six alternative configurations (M2–M7). The reference model uses a purely thermal feedback mechanism where a fraction of the radiated energy heats surrounding gas. The SMBH model includes seeding in halos with sufficient stellar mass, Bondi-like gas accretion (with cold/hot boost factors $\alpha_c, \alpha_h$ ), Eddington-limited accretion, and BH mergers. Crucially, the reference model is minimally calibrated: parameters are tuned only to match the local $M_{BH} - M_*$ relation for field galaxies, leaving all cluster-scale properties as predictions.
Variations: The alternative models test:
- Reduced feedback efficiency ( $\epsilon_f = 0.05$ vs $0.1$).
- Lower Bondi boost factors.
- Removal of the Eddington limit.
- Evaporation of the cold ISM component: A sub-resolution mechanism where AGN feedback energy evaporates the cold phase of multi-phase gas particles, preventing immediate energy loss.

Key Results

Galaxy Stellar Mass Function (GSMF): The reference model (M1) reproduces the shape of the observed GSMF at low and intermediate masses ( $\log(M_*/M_\odot) \lesssim 11.25$ ) in both the field and cluster environments. However, it overestimates the number density of high-mass galaxies. Notably, the model successfully reproduces the steepening of the GSMF at the low-mass end ( $\log(M_*/M_\odot) \lesssim 10.5$ ) observed in recent data, indicating that stellar feedback (SN-driven winds) is efficient enough for low-mass systems.
BCG and Stellar Mass Fractions: In massive clusters ( $M_{500} \sim 10^{15} M_\odot$ ), the reference model systematically overpredicts BCG stellar masses and the total stellar mass fraction (by a factor of $\sim 3$ compared to observations). This discrepancy is less severe in lower-mass groups. The overmassive BCGs are linked to episodes of runaway accretion of high-density cold gas that the feedback model fails to suppress.
ICM Scaling Relations: The simulations predict gas mass fractions ( $f_g$ ) and mass-temperature ( $M_g-T$ ) relations that generally agree with observations from groups to clusters. The $Y_X-M_{500}$ relation (where $Y_X$ is the X-ray analogue of the Compton- $y$ parameter) shows excellent agreement with data, confirming its robustness as a mass proxy. The simulated clusters appear slightly cooler than observed at fixed mass, a tension partially alleviated by accounting for a $\sim 20\%$ hydrostatic mass bias in X-ray observations.
Thermodynamical Profiles: Outside the core ( $R \gtrsim 0.15 R_{500}$ ), the simulated profiles of electron density, temperature, entropy, and pressure match observations and self-similar expectations. However, in the core regions, the simulated clusters are less "cool-cored" than observed; they exhibit flatter entropy profiles and steeper temperature gradients. The average entropy level in the cores is higher than observed, suggesting the feedback does not provide a self-regulated compensation for radiative losses effectively enough to maintain low-entropy cool cores.
Impact of Alternative Models: The study finds that simply tuning the total feedback efficiency ( $\epsilon_f$ ) or accretion boost factors does not resolve the overmassive BCG issue. However, models that include the evaporation of the cold ISM component (M4–M6) successfully reduce BCG stellar masses and stellar mass fractions to levels closer to observations. These models also increase the "cool-coreness" of the entropy profiles in test cases, demonstrating that the interface between the AGN feedback and the sub-resolution star formation model is critical.

Significance and Conclusions
The paper demonstrates that the details of the interface between AGN energy injection and the sub-resolution interstellar medium model are at least as critical as the total feedback efficiency itself. While the reference model successfully reproduces many global properties with minimal tuning, it highlights a specific failure mode in high-resolution simulations: the inability to regulate star formation in the most massive halos without overproducing stars or failing to create realistic cool cores.

The authors conclude that current implementations of AGN feedback in cosmological simulations can be tuned to produce realistic cool cores at lower resolutions, but as resolution increases, achieving self-regulation becomes more difficult. The results suggest that future improvements must move beyond simple thermal feedback tuning to include explicit treatments of the kinetic mode of AGN feedback (jets), more sophisticated SMBH accretion models linked to halo cooling, and star formation models that explicitly resolve mass and energy flows between ISM phases. The Dianoga simulations provide a framework for investigating these complex baryonic cycles and the co-evolution of galaxies and SMBHs.

More like this