Vincenzo Antonuccio-Delogu

Simulating galaxy clusters – I. Thermal and chemical properties of the intracluster medium

We have performed a series of N-body/hydrodynamical (TREESPH) simulations of clusters and groups of galaxies, selected from a cosmological volume within a Lambda cold dark matter (� CDM) framework: these objects have been resimulated at higher resolution to z = 0, in order to follow also the dynamical, thermal and chemical input on to the intracluster medium (ICM) from stellar populations within galaxies. The simulations include metallicitydependent radiative cooling, star formation according to different initial mass functions (IMFs), energy feedback as strong starburst-driven galactic superwinds, chemical evolution with noninstantaneous recycling of gas and heavy elements, effects of a metagalactic ultraviolet (UV) field and thermal conduction in the ICM. In this paper, the first in a series of three, we derive results, mainly at z = 0, on the temperature and entropy profiles of the ICM, its X-ray luminosity, the cluster cold components [cold fraction as well as mass-to-light ratio (MLR)] and the metal distribution between ICM and stars. In general, models with efficient superwinds (produced by the action of supernovae and, in some simulations, of active galactic nuclei (AGNs), along with a top-heavy stellar IMF, are able to reproduce fairly well the observed LX‐T relation, the entropy profiles and the cold fraction: both features are found to be needed in order to remove high-density and low-entropy cold gas at core scales, although additional alternative feedback mechanisms would still be required to prevent late-time central cooling flows, and subsequent overproduction of stars and heavy elements at the centre. Observed radial ICM temperature profiles can be matched, except for the gradual decline in temperature inside r ∼ 0.1Rvir. Metal enrichment of the ICM gives rise to somewhat steep inner iron gradients; yet, the global level of enrichment compares well to observational estimates when a top-heavy IMF is adopted, and after correcting for the stars formed at late times at the base of the cooling flows, the metal partition between stars and ICM gets into good agreement with observations. The overall abundance and profile of iron in the ICM is found essentially unchanged from z = 1 to present time. Finally, the α/Fe of the gas is found to increase steadily with radius, decreasing over time.

Triggered star formation in the inner filament of Centaurus A

We present recent Hubble Space Telescopeobservations of the inner filament of Centaurus A, using the new Wide Field Camera 3 (WFC3) F225W, F657N and F814W filters. We find a young stellar population near the south-west tip of the filament. Combining the WFC3 dataset with archival Advanced Camera for Surveys (ACS) F606W observations, we are able to constrain the ages of these stars to ∼10 Myrs, with best-fit ages of 1-4 Myrs. No further recent star-formation is found along the filament. Based on the location and age of this stellar population, and the fact that there is no radio lobe or jet activity near the star-formation, we propose an u pdated explanation for the origin of the inner filament. Sutherland et al. suggested that radio jet-induced shocks can drive the observed optical line emission. We argue that such shocks can naturally arise due to a weak cocoon-driven bow shock (rather than from the radio jet directly), propagating through the diffuse interstellar medium from a location near the inner northern radio lobe. The shock can overrun a molecular cloud, triggering star-formation in th e dense molecular cores. Ablation and shock heating of the diffuse gas then gives rise to the observed optical line and X-ray emission. Deeper X-ray observations should show more diffuse emission along the filament.

Jet interactions with a giant molecular cloud in the Galactic centre and ejection of hypervelocity stars

Reproduced with permission from Astronomy & Astrophysics,

Stellar population gradients from cosmological simulations: dependence on mass and environment in local galaxies

The age and metallicity gradients for a sample of group and cluster galaxies from N-body+hydrodynamical simulation are analysed in terms of galaxy stellar mass. Dwarf galaxies show null age gradient with a tail of high and positive values for systems in groups and cluster outskirts. Massive systems have generally zero-age gradients which turn to positive for the most massive ones. Metallicity gradients are distributed around zero in dwarf galaxies and become more negative with mass; massive galaxies have steeper negative metallicity gradients, but the trend flattens with mass. In particular, fossil groups are characterized by a tighter distribution of both age and metallicity gradients. We find a good agreement with both local observations and independent simulations. Interestingly, our results suggest that environment differently affects the gradients at low and high masses. The results are also discussed in terms of the central age and metallicity, as well as the total colour, specific star formation and velocity dispersion.

Weak Lensing Mass Reconstruction of the Galaxy Cluster Abell 209

Context. Weak lensing applied to deep optical images of clusters of galaxies provides a powerful tool to reconstruct the distribution of the gravitating mass associated to these structures. Aims. We use the shear signal extracted by an analysis of deep exposures of a region centered around the galaxy cluster ABCG209, at redshift z � 0.2, to derive both a map of the projected mass distribution and an estimate of the total mass within a characteristic radius. Methods. We use a series of deep archival R-band images from CFHT-12k, covering an area of � 0.3deg 2 . We determine the shear of background galaxy images using a new implementation of the modified Kaiser-Squires-Broadhurst KSB+ pipeline for shear determination, which we has been tested against the “Shear TEsting Program 1 and 2” simulations. We use mass aperture statistics to produce maps of the 2 dimensional density distribution, and parametric fits using both Navarro-Frenk-White and singular-isothermal-sphere profiles to constrain the total mass. Results. The projected mass distribution shows a pronounced asymmetry, with an elongated structure extending from the SE to the NW. This is in general agreement with the optical distribution previously found by other authors. A similar elongation was previously detected in the X-ray emission map, and in the distribution of galaxy colours. The circular NFW mass profile fit gives a total mass of M200 = 7.7 +4.3 −2.7 10 14 M⊙ inside the virial radius r200 = 1.8 ± 0.3Mpc. Conclusions. The weak lensing profile reinforces the evidence for an elongated structure of ABCG209, as previously suggested by studies of the galaxy distribution and velocities.

Comptonization of the cosmic microwave background by high energy particles residing in AGN cocoons

Context. X-ray cavities and extended radio sources (“cocoons”) surrounding active galactic nuclei (AGN) have been detected by the Chandra X-ray mission and radio interferometers. A joint analysis of X-ray and radio maps suggests that pressure values of non-thermal radio-emitting particles derived from the radio maps are not sufficient to inflate the X-ray cavities. We propose using the Sunyaev-Zel’dovich (SZ) effect, whose intensity strongly depends on the pressure, to find the hitherto undetected, dynamicallydominant component in the radio cocoons. Aims. Numerical simulations show that plasma with a high temperature (10 9 −10 10 K) is a good candidate for inflating the AGN cocoons. To constrain the population of high energy electrons inside AGN cocoons that is predicted by numerical simulations, we study different methods for maximizing the contribution of such energetic electrons to the SZ effect. Methods. Our calculations of intensity maps of the SZ effect include relativistic corrections and utilize both analytic models and numerical 2D simulations. Results. We demonstrate that the spectral function at a frequency of 217 GHz has an absolute maximum at a temperature higher than 10 9 K, therefore the measurement of the SZ effect at this frequency is a powerful tool for potentially revealing the dynamicallydominant component inside AGN jet-driven radio cocoons. A new method is proposed for excluding the contribution from the low energy, non-relativistic electrons to the SZ effect by means of observations at two frequencies. We show how one may correct for a possible contribution from the kinematic SZ effect. The intensity maps of the SZ effect are calculated for the self-similar Sedov solution, and application of a predicted ring-like structure on the SZ map at a frequency of 217 GHz is proposed to determine the energy released during the active jet stage. The SZ intensity map for an AGN cocoon in a distant elliptical is calculated using a 2D numerical simulation and including relativistic corrections to the SZ effect. We show the intensity spectrum of the SZ effect is flat at high frequencies if gas temperature is as high as kbTe = 500 keV.

FLY: MPI-2 high resolution code for LSS cosmological simulations

Cosmological simulations of structures and galaxies formations have played a fundamental role in the study of the origin, formation and evolution of the Universe. These studies improved enormously with the use of supercomputers and parallel systems and, recently, grid based systems and Linux clusters. Now we present the new version of the tree N-body parallel code FLY that runs on a PC Linux Cluster using the one side communication paradigm MPI-2 and we show the performances obtained. FLY is included in the Computer Physics Communication Program Library. This new version was developed using the Linux Cluster of CINECA, an IBM Cluster with 1024 Intel Xeon Pentium IV 3.0 GHz. The results show that it is possible to run a 64 million particle simulation in less than 15 minutes for each time-step, and the code scalability with the number of processors is achieved. This leads us to propose FLY as a code to run very large N-body simulations with more than 109 particles with the higher resolution of a pure tree code. The FLY new version is available at the CPC Program Library, http://cpc.cs.qub.ac.uk/summaries/ADSC_v2_0.html [U. Becciani, M. Comparato, V. Antonuccio-Delogu, Comput Phys. Comm. 174 (2006) 605].

A parallel tree code for large N-body simulation: dynamic load balance and data distribution on a CRAY T3D system

Abstract N-body algorithms for long-range unscreened interactions like gravity belong to a class of highly irregular problems whose optimal solution is a challenging task for present-day massively parallel computers. In this paper we describe a strategy for optimal memory and work distribution which we have applied to our parallel implementation of the Barnes & Hut (1986) recursive tree scheme on a Cray T3D using the CRAFT programming environment. We have performed a series of tests to find an optimal data distribution in the T3D memory, and to identify a strategy for the Dynamic Load Balance in order to obtain good performances when running large simulations (more than 10 million particles). The results of tests show that the step duration depends on two main factors: the data locality and the T3D network contention. Increasing data locality we are able to minimize the step duration if the closest bodies (direct interaction) tend to be located in the same PE local memory (contiguous block subdivision, high granularity), whereas the tree properties have a fine grain distributuion. In a very large simulation, due to network contention, an unbalanced load arises. To remedy this we have devised an automatic work redistribution mechanism which provided a good Dynamic Load Balance at the price of an insignificant overhead.

Backflows by active galactic nuclei jets: global properties and influence on supermassive black hole accretion

Jets from Active Galactic Nuclei (AGN) inflate large cavities in the hot gas environment around galaxies and galaxy clusters. The large-scale gas circulation promoted within such cavities by the jet itself gives rise to backflows that propagate back to the centre of the jet-cocoon system, spanning all the physical scales relevant for the AGN. Using an Adaptive Mesh Refinement code, we study these backflows through a series of numerical experiments, aiming at understanding how their global properties depend on jet parameters. We are able to characterize their mass flux down to a scale of a few kiloparsecs to about 0.5 M yr −1 for as long as 15 or 20 Myr, depending on jet power. We find that backflows are both spatially coherent and temporally intermittent, independently of jet power in the range 1043−45 erg/s. Using the mass flux thus measured, we model analytically the effect of backflows on the central accretion region, where a Magnetically Arrested Disc lies at the centre of a thin circumnuclear disc. Backflow accretion onto the disc modifies its density profile, producing a flat core and tail. We use this analytic model to predict how accretion beyond the BH magnetopause is modified, and thus how the jet power is temporally modulated. Under the assumption that the magnetic flux stays frozen in the accreting matter, and that the jets are always launched via the Blandford-Znajek (1977) mechanism, we find that backflows are capable of boosting the jet power up to tenfold during relatively short time episodes (a few Myr).

Cosmological Simulations and Data Exploration: a Testcase on the Usage of Grid Infrastructure

While new infrastructures for large computational challenges begin to be widely accessible to researchers, computational codes need to be re-designed to exploit new facilities. The Grid and the cloud computing concepts are changing the computational resource distribution and availability, and much effort start to be made to develop new codes for a better exploitation of new resources. This paper presents an example of the use of Grid resources, based on gLite middleware, to run cosmological simulations, that, up to now, are normally executed on Supercomputers. We have also used the Grid to explore and visualize the dataset. We discuss non particular the performance of FLY a parallel code implementing the octal-tree algorithm introduced by J. Barnes and P. Hut to compute the gravitational field efficiently. It simulates the evolution of the collisionless component of the material content of our Universe. FLY was originally developed to run on mainframe systems using the one-side communication paradigm, but we are now presenting a modified version of the computational algorithm to exploit the Grid environment. We also integrated the data exploration and visualization process on the Grid, to obtain preliminary results using the distributed facilities.