arXiv subjects

R. Valdarnini

Publications and source records attributed to R. Valdarnini.

At least 19 recordsLinked to original sources

arXiv2026

Hydrodynamical simulations of the merging cluster El Gordo in a two-state self-interacting dark matter scenario

A large suite of N-body/hydrodynamical simulations is used to demonstrate that the physical properties of the merging cluster El Gordo are well reproduced within a two-state self-interacting dark matter scenario. Our findings show that, in addition to elastic scattering, the presence of an inelastic, endothermic, up-scattering channel is essential to satisfy observational constraints derived from the measured weak lensing profiles of the two colliding clusters. We find that the observational properties of El Gordo are adequately matched by our self-interacting dark matter model within the following interval of parameters: an elastic cross-section in the range of ~ 4 -5 cm^2/gr, and an up-scattering channel bounded by an inelastic cross-section of approximately ~ 2 -4 cm^2/gr with a threshold velocity of ~1200 - 1600 kms, respectively. We argue that our conclusions depend critically on current uncertainties in the primary mass and the projected separation d_{DM}. In fact, our findings suggest that if error estimates in weak lensing measurements were significantly reduced - with future observations showing evidence of a low primary mass (<~ 10^{15} M_sun ) and a larger projected separation d_{DM} >~ 740 kpc - it would then be difficult for the proposed self-interacting scenario to be reconciled with observational constraints. We further suggest that future lensing surveys of massive, high-velocity merging clusters could be decisive in testing the proposed dark matter scenario.

astro-ph.CO↗

arXiv2025

An SIDM simulation of the merging cluster El Gordo and its tension between the post collision DM density profiles and weak lensing constraints

We review recent findings from a detailed simulation study of the merging cluster El Gordo and present new results inferred from weak lensing data. We found that the observed spatial offsets between the different mass components are well reproduced in merging simulations that include self-interacting dark matter (DM), with an elastic cross-section per unit mass of approximately \sigma_DM/m_X ~ 4 -5 cm^2/gr. Moreover, a relative line-of-sight peculiar velocity on the order of several hundred km/s is found between the two stellar components of the colliding subclusters. These findings strongly suggest the possibility that, in a very energetic cluster collision, DM could possess collisional properties. However, the self-interacting DM merger model presented here is not without difficulties. The values found for \sigma_DM/m_X being in conflict with the current upper bounds on cluster scales. As a solution to this tension we argue that in major cluster mergers the physical modeling of DM interactions, based on the scattering of DM particles, should be considered too simplistic. Additionally, the DM halos of the post-collision clusters have cored density profiles with core radii r_c ~ 300 kpc. Consequently, the associated reduced tangential shear lensing profiles consistently tend to zero at angles \theta <~ 40^{''}. This result is inconsistent with what is deduced from the measured profiles. These profiles exhibit a diverging behavior when \theta --> 0, as predicted by an NFW mass model. We argue that such contradictions cannot be easily reconciled within the DM models presented so far as an alternative to the collisionless paradigm. However, we suggest that this tension can be used as a unique test bed to probe new DM physics.

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arXiv2024

An N-body/hydrodynamical simulation study of the merging cluster El Gordo: A compelling case for self-interacting dark matter?

We use a large set N-body/hydrodynamical simulations to study the physical properties of the merging cluster El Gordo. We find that the observed X-ray structures, along with other data, can be matched fairly well by simulations with collision velocities 2,000 kms <= V <= 2,500 kms and impact parameters 600 kpc <= P <= 800 kpc. The mass of the primary is constrained to be between 10^{15} M_sun and ~ 1.6 10^{15} M_sun, in accordance with recent lensing-based mass measurements. Moreover, a returning, post-apocenter, scenario is not supported by our head-on simulations. We considered merger models that incorporate dark matter self-interactions. The simulation results show that the observed spatial offsets between the different mass components are well reproduced in self-interacting dark matter models with an elastic cross-section in the range σ_DM/m_X ~ 4 -5 cm^2/gr. In addition, the mean relative line-of-sight radial velocity between the two brightest cluster galaxies is found to be on the order of several hundred km/s. We argue that these findings provide an unambiguous signature of a dark matter behavior that exhibits collisional properties in a very energetic high-redshift cluster collision. The range of allowed values we find for sigma_DM/m_X is, however, inconsistent with present upper limits. To resolve this tension we suggest the possibility that the self-interacting dark matter model used here be considered as only a low order approximation, and that the underlying physical processes that describe the interaction of dark matter in major cluster mergers are more complex than can be adequately represented by the commonly assumed approach based on the scattering of dark matter particles.

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arXiv2021

A study of cool core resiliency and entropy mixing in simulations of galaxy cluster mergers

We present results from a suite of binary merging cluster simulations. The hydrodynamical cluster simulations are performed employing a smoothed particle hydrodynamics (SPH) formulation in which gradient errors are strongly reduced by means of an integral approach. We consider adiabatic as well as radiative simulations, in which we include gas cooling, star formation and energy feedback from supernovae. We explore the effects of merging on the thermodynamic structure of the intracluster gas of the final merger remnant. In particular, we study how core entropy is generated during the merging and the stability properties of the initial cool-core profile against disruption. To this end, we consider a range of initial mass ratio and impact parameters. Final entropy profiles of our adiabatic merging simulations are in good accord with previous findings (ZuHone 2011), with cool-cores being disrupted for all of the initial merging setups. For equal-mass off-axis mergers, we find that a significant contribution to the final primary core entropy is due to hydrodynamic instabilities generated by rotational motions, which are induced by tidal torques during the first pericenter passage. In radiative simulations, cool-cores are more resilient against heating processes; nonetheless, they are able to maintain their integrity only in the case of off-axis mergers with very unequal masses. We suggest that these results are robust against changes in the gas physical modeling, in particular to the inclusion of AGN thermal feedback. Our findings support the view that the observed core cluster morphology emerges naturally in a merging cluster context, and conclude that the merging angular momentum is a key parameter in shaping the thermodynamical properties of the final merger remnant.

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arXiv2019

A multifiltering study of turbulence in a large sample of simulated galaxy clusters

We present results from a large set of N-body/SPH hydrodynamical cluster simulations aimed at studying the statistical properties of turbulence in the ICM. The numerical hydrodynamical scheme employs a SPH formulation in which gradient errors are strongly reduced by using an integral approach. We consider both adiabatic and radiative simulations. We construct clusters subsamples according to the cluster dynamical status or gas physical modeling, from which we extract small-scale turbulent velocities obtained by applying to cluster velocities different multiscale filtering methods. The velocity power spectra of non-radiative relaxed clusters are mostly solenoidal and steeper than Kolgomorov. Cooling runs are distinguished by much shallower spectra, a feature which we interpret as the injection of turbulence at small scales due to the interaction of compact cool gas cores with the ICM. Turbulence in galaxy clusters is then characterized by multiple injection scales, with the small scale driving source acting in addition to the large scale injection mechanisms. Cooling runs of relaxed clusters exhibit enstrophy profiles with a power-law behavior over more than two decades in radius, and a turbulent-to-thermal energy ratio ~1 %. In accord with Hitomi observations, in the core of a highly relaxed cluster we find low level of gas motions. In addition, the estimated cluster radial profile of the sloshing oscillation period exhibits an associated Froude number satisfying Fr ~ 0.1 within r / r_{200} <~ 0.1. Our findings suggest that in cluster cores ICM turbulence approaches a stratified anisotropic regime, with weak stirring motions dominated by gravity buoyancy forces and strongly suppressed along the radial direction. We conclude that turbulent heating cannot be considered the main heating source in cluster cores.

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arXiv2016

Improved Performances in Subsonic Flows of an SPH Scheme with Gradients Estimated using an Integral Approach

In this paper we present results from a series of hydrodynamical tests aimed at validating the performance of a smoothed particle hydrodynamics (SPH) formulation in which gradients are derived from an integral approach. We specifically investigate the code behavior with subsonic flows, where it is well known that zeroth-order inconsistencies present in standard SPH make it particularly problematic to correctly model the fluid dynamics. In particular we consider the Gresho-Chan vortex problem, the growth of Kelvin-Helmholtz instabilities, the statistics of driven subsonic turbulence and the cold Keplerian disc problem. We compare simulation results for the different tests with those obtained, for the same initial conditions, using standard SPH. We also compare the results with the corresponding ones obtained previously with other numerical methods, such as codes based on a moving-mesh scheme or Godunov-type Lagrangian meshless methods. We quantify code performances by introducing error norms and spectral properties of the particle distribution, in a way similar to what was done in other works. We find that the new SPH formulation exhibits strongly reduced gradient errors and outperforms standard SPH in all of the tests considered. In fact, in terms of accuracy we find good agreement between the simulation results of the new scheme and those produced using other recently proposed numerical schemes. These findings suggest that the proposed method can be successfully applied for many astrophysical problems in which the presence of subsonic flows previously limited the use of SPH, with the new scheme now being competitive in these regimes with other numerical methods.

astro-ph.IM↗

arXiv2012

Hydrodynamic capabilities of an SPH code incorporating an artificial conductivity term with a gravity-based signal velocity

This paper investigates the hydrodynamic performances of an SPH code incorporating an artificial heat conductivity term in which the adopted signal velocity is applicable when gravity is present. In accordance with previous findings it is shown that the performances of SPH to describe the development of Kelvin-Helmholtz instabilities depend strongly on the consistency of the initial condition set-up and on the leading error in the momentum equation due to incomplete kernel sampling. An error and stability analysis shows that the quartic B-spline kernel (M_5) possesses very good stability properties and we propose its use with a large neighbor number, between ~50 (2D) to ~ 100 (3D), to improve convergence in simulation results without being affected by the so-called clumping instability. SPH simulations of the blob test show that in the regime of strong supersonic flows an appropriate limiting condition, which depends on the Prandtl number, must be imposed on the artificial conductivity SPH coefficients in order to avoid an unphysical amount of heat diffusion. Results from hydrodynamic simulations that include self-gravity show profiles of hydrodynamic variables that are in much better agreement with those produced using mesh-based codes. In particular, the final levels of core entropies in cosmological simulations of galaxy clusters are consistent with those found using AMR codes. Finally, results of the Rayleigh-Taylor instability test demonstrate that in the regime of very subsonic flows the code has still several difficulties in the treatment of hydrodynamic instabilities. These problems being intrinsically due to the way in which in standard SPH gradients are calculated and not to the implementation of the artificial conductivity term.

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arXiv2010

The impact of numerical viscosity in SPH simulations of galaxy clusters

A SPH code employing a time-dependent artificial viscosity scheme is used to construct a large set of N-body/SPH cluster simulations for studying the impact of artificial viscosity on the thermodynamics of the ICM and its velocity field statistical properties. Spectral properties of the gas velocity field are investigated by measuring for the simulated clusters the velocity power spectrum E(k). The longitudinal component E_c(k) exhibits over a limited range a Kolgomorov-like scaling k^{-5/3}, whilst the solenoidal power spectrum component E_s(k) is strongly influenced by numerical resolution effects. The dependence of the spectra E(k) on dissipative effects is found to be significant at length scales 100-300Kpc, with viscous damping of the velocities being less pronounced in those runs with the lowest artificial viscosity. The turbulent energy density radial profile E_{turb}(r) is strongly affected by the numerical viscosity scheme adopted in the simulations, with the turbulent-to-total energy density ratios being higher in the runs with the lowest artificial viscosity settings and lying in the range between a few percent and ~10%. These values are in accord with the corresponding ratios extracted from previous cluster simulations realized using mesh-based codes. At large cluster radii, the mass correction terms to the hydrostatic equilibrium equation are little affected by the numerical viscosity of the simulations, showing that the X-ray mass bias is already estimated well in standard SPH simulations. Finally, simulations in which the gas can cool radiatively are characterized by the presence in the cluster inner regions of high levels of turbulence, generated by the interaction of the compact cool gas core with the ambient medium.

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arXiv2006

X-ray temperature spectroscopy of simulated cooling clusters

Results from a large sample of hydrodynamical/N-body simulations of galaxy clusters in a LCDM cosmology are used to simulate cluster X-ray observations as expected from Chandra observations. The physical modeling of the gas includes radiative cooling, star formation, energy feedback and metal enrichment. The biasing of spectral temperatures with respect to mass-weighted temperatures is found to be influenced by two independent processes. The first scale dependency is absent in adiabatic runs and is due to cooling, whose efficiency to transform cold gas into stars is higher for cool clusters and this in turn implies a strong dependency of the spectral versus mass-weighted temperature relation on the cluster mass. The second dependency is due to photon emission because of cool gas which is accreted during merging events and biases the spectral fits. These events have been quantified according to the power ratio method and a robust correlation is found to exist between the spectral bias and the amount of cluster substructure. The shape of the simulated temperature profiles is not universal and it is steeper at the cluster center for cool clusters than for the massive ones. The profiles are in good agreement with data in the radial range between $\sim 0.1 r_{vir}$ and $\sim 0.4 r_{vir}$; at small radii ($r< 0.1 r_{vir}$) the cooling runs fail to reproduce the shape of the observed profiles. The fit is improved if one considers a hierarchical merging scenario in which cluster cores can accrete cooler gas through merging with cluster subclumps, though the shape of the temperature profiles is modified in a significant way only in the regime where the mass of the substructure is a large fraction of the cluster mass.

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arXiv2003

X-ray Cluster Properties in SPH Simulations of Galaxy Clusters

Results from a large set of hydrodynamical SPH simulations of galaxy clusters in a flat LCDM cosmology are used to investigate cluster X-ray properties. The physical modeling of the gas includes radiative cooling, star formation, energy feedback and metal enrichment that follows from the explosions of SNe type II and Ia. The metallicity dependence of the cooling function is also taken into account. It is found that the luminosity-temperature relation of simulated clusters is in good agreement with the data, and the X-ray properties of cool clusters are unaffected by the amount of feedback energy that has heated the intracluster medium (ICM). The fraction of hot gas $f_g$ at the virial radius increases with $T_X$ and the distribution obtained from the simulated cluster sample is consistent with the observational ranges.

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arXiv2003

Parallelization of a treecode

I describe here the performance of a parallel treecode with individual particle timesteps. The code is based on the Barnes-Hut algorithm and runs cosmological N-body simulations on parallel machines with a distributed memory architecture using the MPI message-passing library. For a configuration with a constant number of particles per processor the scalability of the code was tested up to P=128 processors on an IBM SP4 machine. In the large $P$ limit the average CPU time per processor necessary for solving the gravitational interactions is $\sim 10 %$ higher than that expected from the ideal scaling relation. The processor domains are determined every large timestep according to a recursive orthogonal bisection, using a weighting scheme which takes into account the total particle computational load within the timestep. The results of the numerical tests show that the load balancing efficiency $L$ of the code is high ($>=90%$) up to P=32, and decreases to $L\sim 80%$ when P=128. In the latter case it is found that some aspects of the code performance are affected by machine hardware, while the proposed weighting scheme can achieve a load balance as high as $L\sim 90%$ even in the large $P$ limit.

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arXiv2002

Performance characteristics of a parallel treecode

I describe here the performances of a parallel treecode with individual particle timesteps. The code is based on the Barnes-Hut algorithm and runs cosmological N-body simulations on parallel machines with a distributed memory architecture using the MPI message passing library. For a configuration with a constant number of particles per processor the scalability of the code has been tested up to P=32 processors. The average CPU time per processor necessary for solving the gravitational interactions is within $\sim 10 %$ of that expected from the ideal scaling relation. The load balancing efficiency is high ($\simgt90%$) if the processor domains are determined every large timestep according to a weighting scheme which takes into account the total particle computational load within the timestep.

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arXiv2002

Iron abundances in hydrodynamical simulations of galaxy clusters

Hydrodynamical SPH simulations of galaxy clusters are used to investigate the metal enrichment of the intracluster medium. The final metallicity abundances of the simulated clusters are determined according to the numerical resolution and a number of model parameters. For a fiducial set of model prescriptions the results of the simulations indicate iron abundances in broad agreement with data. Final X-ray properties are not sensitive to the heating of the ICM. This supports a scenario where the ICM evolution of cool clusters is driven by radiative cooling.

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arXiv2002

Iron abundances and heating of the ICM in hydrodynamical simulations of galaxy clusters

Results from a large set of hydrodynamical SPH simulations of galaxy clusters in a flat LCDM cosmology are used to investigate the metal enrichment and heating of the ICM. The physical modeling of the gas includes radiative cooling, star formation, energy feedback and metal enrichment that follow from the explosions of SNe of type II and Ia. The metallicity dependence of the cooling function is also taken into account. For a fiducial set of model prescriptions the results indicate radial iron profiles in broad agreement with observations; global iron abundances are also consistent with data. It is found that the iron distribution in the ICM is critically dependent on the shape of the metal deposition profile. For low temperatu re clusters simulations yield iron abundances below the allowed observational range, unless it is introduced a minimum diffusion length of metals in the ICM. The simulated emission-weighted radial temperature profiles are in good agreement with data for cooling flow clusters, but at very small distances from the cluster centres ($\sim 2%$ of the virial radii) the temperatures are a factor $\sim$ two higher than the measured spectral values. The luminosity-temperature relation is in excellent agreement with the data, cool clusters ($T_X\sim 1keV$) have a core excess entropy of $\sim 200 keVcm^2$ and their X-ray properties are unaffected by the amount of feedback energy that has heated the ICM. The fraction of hot gas $f_g$ at the virial radius increases with $T_X$ and the distribution obtained from the simulated cluster sample is consistent with the observational ranges.

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arXiv2001

Radiative cooling in SPH hydrodynamical simulations of X-ray clusters

The results from hydrodynamical TREESPH simulations of galaxy clusters are used to investigate the dependence of the final cluster X-ray properties upon the numerical resolution and the assumed star formation models for the cooled gas. When cold gas particles are allowed to convert into stars the final gas profiles show a well defined core radius and the temperature profiles are nearly flat. Final X-ray luminosities are found to be numerically stable, with uncertainties of a factor 2.

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arXiv2001

Detection of non-random patterns in large-scale structure

A new method for analyzing the morphological features of point patterns is presented. The method is taken from the study of molecular liquids, where it has been introduced for making a statistical description of anisotropic distributions. The statistical approach is based on the spherical harmonic expansion of angular correlations.

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arXiv2001

Numerical Convergence of Hydrodynamical SPH Simulations of Cooling Clusters

The results from hydrodynamical TREESPH simulations of galaxy clusters are used to investigate the dependence of the final cluster X-ray properties upon the numerical resolution and the assumed star formation models for the cooled gas. A comparison between runs with different star formation methods shows that the results of simulations, based on star formation methods in which gas conversion into stars is controlled by an efficiency parameter c_{star}, are sensitive to the simulation numerical resolution. In this respect star formation methods based instead on a local density threshold, are shown to give more stable results. Final X-ray luminosities are found to be numerically stable, with uncertainties of a factor 2.

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arXiv2001

Numerical convergence of physical variables in hydrodynamical simulations of cooling clusters

Results from hydrodynamical SPH simulations of galaxy clusters are used to investigate the dependence of the final cluster X-ray properties on the numerical resolution and the assumed models for the physical gas processes. Two different spatially flat cosmological models have been considered: a low-density cold dark matter universe with a vacuum energy density Omega_L=0.7 (LCDM) and a cold+hot dark matter model (CHDM). These simulations first include radiative cooling and then also conversion of cold gas particles into stars. When cold gas particles are allowed to convert into stars the final gas profiles show a well defined core radius and the temperature profiles are nearly flat. For the most massive test cluster in the LCDM model, these simulations show a prominent cooling flow in the cluster core. This cluster was analyzed in detail, running simulations with different star formation methods and increasing numerical resolution. A comparison between different runs shows that the results of simulations, based on star formation methods in which gas conversion into stars is controlled by an efficiency parameter c_star, are sensitive to the numerical resolution of the simulation. In this respect star formation methods based instead on a local density threshold, as in Navarro and White (1993), are shown to give more stable results. Final X-ray luminosities are found to be numerically stable, with uncertainties of a factor two.

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