arXiv subjects

Veronica Biffi

Publications and source records attributed to Veronica Biffi.

At least 37 records · Page 2Linked to original sources

arXiv2022

The Hateful Eight: Connecting Massive Substructures in Galaxy Clusters like Abell 2744 to their Dynamical Assembly State using the Magneticum Simulations

Substructures are known to be good tracers for the dynamical states and recent accretion histories of the most massive collapsed structures in the Universe, galaxy clusters. Observations find extremely massive substructures in some clusters, especially Abell 2744, which are potentially in tension with the $Λ$CDM paradigm since they are not found in simulations directly. However, the methods to measure substructure masses strongly differ between observations and simulations. Using the fully hydrodynamical cosmological simulation suite Magneticum Pathfinder we develop a method to measure substructure masses in projection from simulations, similar to the observational approach. We identify a simulated Abell 2744 counterpart that not only has eight substructures of similar mass fractions but also exhibits similar features in the hot gas component. This cluster formed only recently through a major merger together with at least 6 massive minor merger events since z=1, where prior the most massive component had a mass of less than $1\times10^{14}M_\odot$. We show that the mass fraction of all substructures and of the eighth substructure separately are excellent tracers for the dynamical state and assembly history for all galaxy cluster mass ranges, with high fractions indicating merger events within the last 2Gyr. Finally, we demonstrate that the differences between subhalo masses measured directly from simulations as bound and those measured in projection are due to methodology, with the latter generally 2-3 times larger than the former. We provide a predictor function to estimate projected substructure masses from SubFind masses for future comparison studies between simulations and observations.

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arXiv2022

Chemical abundances in the outskirts of nearby galaxy groups measured with joint Suzaku and Chandra observations

We report results from deep Suzaku and mostly snapshot Chandra observations of four nearby galaxy groups: MKW4, Antlia, RXJ1159+5531, and ESO3060170. Their peak temperatures vary over 2-3 keV, making them the smallest systems with gas properties constrained to their viral radii. The average Fe abundance in the outskirts (R $>$ 0.25R$_{200}$) of their intragroup medium (IGrM) is $Z_{\rm Fe}=0.309\pm0.018$ $Z_\odot$ with $χ^2$ = 14 for 12 degrees of freedom, which is remarkably uniform and strikingly similar to that of massive galaxy clusters, and is fully consistent with the numerical predictions from the IllustrisTNG cosmological simulation. Our results support an early-enrichment scenario among galactic systems over an order of magnitude in mass, even before their formation. When integrated out to R$_{200}$, we start to see a tension between the measured Fe content in ICM and what is expected from supernovae yields. We further constrain their O, Mg, Si, S, and Ni abundances. The abundance ratios of those elements relative to Fe are consistent with the predictions (if available) from IllustrisTNG. Their Type Ia supernovae fraction varies between 14%-21%. A pure core collapsed supernovae enrichment at group outskirts can be ruled out. Their cumulative iron-mass-to-light ratios within R$_{200}$ are half that of the Perseus cluster, which may imply that galaxy groups do not retain all of their enriched gas due to their shallower gravitational potential wells, or that groups and clusters may have different star formation histories.

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arXiv2022

CHEX-MATE: Morphological analysis of the sample

In this work, we performed an analysis of the X-ray morphology of the 118 CHEX-MATE (Cluster HEritage project with XMM-Newton - Mass Assembly and Thermodynamics at the Endpoint of structure formation) galaxy clusters, with the aim to provide a classification of their dynamical state. To investigate the link between the X-ray appearance and the dynamical state, we considered four morphological parameters: the surface brightness concentration, the centroid shift, and the second- and third-order power ratios. These indicators result to be: strongly correlated with each other, powerful in identifying the disturbed and relaxed population, characterised by a unimodal distribution and not strongly influenced by systematic uncertainties. In order to obtain a continuous classification of the CHEX-MATE objects, we combined these four parameters in a single quantity, M, which represents the grade of relaxation of a system. On the basis of the M value, we identified the most extreme systems of the sample, finding 15 very relaxed and 27 very disturbed galaxy clusters. From a comparison with previous analysis on X-ray selected samples, we confirmed that the Sunyaev-Zeldovich (SZ) clusters tend to be more disturbed. Finally, by applying our analysis on a simulated sample, we found a general agreement between the observed and simulated results, with the only exception of the concentration. This latter behaviour, is partially related to the presence of particles with high smoothed-particle hydrodynamics density in the central regions of the simulated clusters due to the action of the idealised isotropic thermal Active Galactic Nuclei (AGN) feedback.

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arXiv2022

Chemical enrichment in groups and clusters

As building blocks of dust, rocky planets, and even complex life, the chemical elements heavier than hydrogen (H) and helium (He) - called "metals" in astronomy - play an essential role in our Universe and its evolution. Up to Fe and Ni, these metals are known to be created by stars and stellar remnants via nuclear fusion and to be ejected into their immediate surroundings to enrich new stellar generations. A spectacular finding, however, is that these processed elements are found even outside stellar systems, in particular in the hot, X-ray atmospheres surrounding early-type galaxies and pervading galaxy clusters and groups. These large-scales structures are thus a remarkable fossil record of the integrated history of the enrichment of our Universe. In this Chapter, we briefly discuss the chemical properties of this intracluster - or intragroup - medium (ICM). After introducing the concept of chemical abundances, and recalling which stellar sources produce which elements, we review the method to derive abundance measurements from observations of the ICM and detail some chemical models implemented in numerical hydrodynamical simulations of cosmic structures. In particular, we explore how synergies between X-ray observations and numerical simulations help us to understand (i) the cosmic epoch at which the bulk of the enrichment occurred, (ii) the physics of these stellar sources responsible for this enrichment, and (iii) the main mechanisms responsible for metal diffusion and mixing outside galaxies.

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arXiv2022

The eROSITA view of the Abell 3391/95 field: The Northern Clump. The largest infalling structure in the longest known gas filament observed with eROSITA, XMM-Newton, and Chandra

SRG/eROSITA PV observations revealed the A3391/95 cluster system and the Northern Clump (MCXC J0621.7-5242 galaxy cluster) are aligning along a cosmic filament in soft X-rays, similarly to what has been seen in simulations before. We aim to understand the dynamical state of the Northern Clump as it enters the atmosphere ($3\times R_{200}$) of A3391. We analyzed joint eROSITA, XMM-Newton, and Chandra observations to probe the morphological, thermal, and chemical properties of the Northern Clump from its center out to a radius of 988 kpc ($R_{200}$). We utilized the ASKAP/EMU radio data, DECam optical image, and Planck y-map to study the influence of the WAT radio source on the Northern Clump central ICM. From the Magneticum simulation, we identified an analog of the A3391/95 system along with an infalling group resembling the Northern Clump. The Northern Clump is a WCC cluster centered on a WAT radio galaxy. The gas temperature over $0.2-0.5R_{500}$ is $k_BT_{500}=1.99\pm0.04$ keV. We employed the $M-T$ scaling relation and obtained a mass estimate of $M_{500}=(7.68\pm0.43)\times10^{13}M_{\odot}$ and $R_{500}=(636\pm12)$ kpc. Its atmosphere has a boxy shape and deviates from spherical symmetry. We identify a southern surface brightness edge, likely caused by subsonic motion relative to the filament gas. At $\sim\! R_{500}$, the southern atmosphere appears to be 42% hotter than its northern atmosphere. We detect a downstream tail pointing toward the north with a projected length of $\sim318$ kpc, plausibly the result of ram pressure stripping. The analog group in the Magneticum simulation is experiencing changes in its gas properties and a shift between the position of the halo center and that of the bound gas while approaching the main cluster pair.

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arXiv2022

The velocity structure of the intracluster medium during a major merger: simulated microcalorimeter observations

Major mergers between galaxy clusters can produce large turbulent and bulk flow velocities in the intra-cluster medium and thus imprint diagnostic features in X-ray spectral emission lines from heavy ions. As demonstrated by Hitomi in observations of the Perseus cluster, measurements of gas velocities in clusters from high-resolution X-ray spectra will be achievable with upcoming X-ray calorimeters like those on board XRISM, Athena, or a Lynx like mission. We investigate this possibility for interesting locations across a major cluster merger from a hydrodynamical simulation, via X-ray synthetic spectra with a few eV energy resolution. We observe the system from directions perpendicular to the plane of the merger and along the merger axis. In these extreme geometrical configurations, we find clear non-Gaussian shapes of the iron He-like K_alpha line at 6.7keV. The velocity dispersion predicted from the simulations can be retrieved for the brightest 100ks pointings with XRISM Resolve, despite some discrepancy related to the complex non-Gaussian line shapes. Measurements in faint regions require however high S/N and the larger collecting area of the Athena X-IFU calorimeter is thus needed. With the latter, we also investigate the gas temperature and velocity gradient across the merger bow shock edge, from 20"-wide annuli extracted from a single 1Ms X-IFU pointing. We find best-fit temperature and velocity dispersion values that are consistent with predictions from the simulations within 1-sigma, but the uncertainties on the inferred velocity dispersion are too large to place any stringent constraints on the shallow gradient downstream of the shock. We also present simulated images of the thermal and kinetic Sunyaev-Zeldovich effects, using the above viewing configurations, and compare the results at angular resolutions appropriate for future observatories such as CMB-S4 and AtLAST.

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arXiv2021

Exploring the role of cosmological shock waves in the Dianoga simulations of galaxy clusters

Cosmological shock waves are ubiquitous to cosmic structure formation and evolution. As a consequence, they play a major role in the energy distribution and thermalization of the intergalactic medium (IGM). We analyze the Mach number distribution in the Dianoga simulations of galaxy clusters performed with the SPH code GADGET-3. The simulations include the effects of radiative cooling, star formation, metal enrichment, supernova and active galactic nuclei feedback. A grid-based shock-finding algorithm is applied in post-processing to the outputs of the simulations. This procedure allows us to explore in detail the distribution of shocked cells and their strengths as a function of cluster mass, redshift and baryonic physics. We also pay special attention to the connection between shock waves and the cool-core/non-cool core (CC/NCC) state and the global dynamical status of the simulated clusters. In terms of general shock statistics, we obtain a broad agreement with previous works, with weak (low-Mach number) shocks filling most of the volume and processing most of the total thermal energy flux. As a function of cluster mass, we find that massive clusters seem more efficient in thermalising the IGM and tend to show larger external accretion shocks than less massive systems. We do not find any relevant difference between CC and NCC clusters. However, we find a mild dependence of the radial distribution of the shock Mach number on the cluster dynamical state, with disturbed systems showing stronger shocks than regular ones throughout the cluster volume.

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arXiv2021

The eROSITA view of the Abell 3391/95 field: a case study from the Magneticum cosmological simulation

We investigated the origin and gas properties of a simulated galaxy cluster pair, to connect simulation predictions to the SRG/eROSITA X-ray observations of the Abell 3391/95 field. The simulated system has been extracted from the (352 cMpc/h)^3 volume of the Magneticum Pathfinder cosmological simulations at z=0.07. We tracked back in time the main progenitors of the pair clusters and surrounding groups to study the assembly history of the system and its evolution. Similarly to the observed A3391/95 system, the simulated pair is embedded in a complex network of gas filaments, with structures aligned over more than 20 projected Mpc and the whole region collapsing towards the central overdense node. The spheres of influence (3*R200) of the two main clusters already overlap at z=0.07, but their virial boundaries are still physically separated. The diffuse gas located in the interconnecting bridge closely reflects the WHIM, with typical temperature of ~1 keV and overdensity $\sim 100$, with respect to the mean baryon density of the Universe, and lower enrichment level compared to the ICM in clusters. We find that most of the bridge gas collapsed from directions roughly orthogonal to the intra-cluster gas accretion directions, and its origin is mostly unrelated to the two cluster progenitors. We find clear signatures in the surrounding groups of infall motion towards the pair, such as significant radial velocities and slowdown of gas compared to dark matter. These findings further support the picture of the Northern Clump (MCXC J0621.7-5242) cluster infalling along a cosmic gas filament towards Abell 3391, possibly merging with it. We conclude that, in such a configuration, the pair clusters of the A3391/95-like system are in a pre-merger phase, and did not interact yet. The diffuse gas in the interconnecting bridge is mostly warm filament gas, rather than tidally-stripped cluster gas.

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arXiv2021

The metal content of the hot atmospheres of galaxy groups

Galaxy groups host the majority of matter and more than half of all the galaxies in the Universe. Their hot ($10^7$ K), X-ray emitting intra-group medium (IGrM) reveals emission lines typical of many elements synthesized by stars and supernovae. Because their gravitational potentials are shallower than those of rich galaxy clusters, groups are ideal targets for studying, through X-ray observations, feedback effects, which leave important marks on their gas and metal contents. Here, we review the history and present status of the chemical abundances in the IGrM probed by X-ray spectroscopy. We discuss the limitations of our current knowledge, in particular due to uncertainties in the modeling of the Fe-L shell by plasma codes, and coverage of the volume beyond the central region. We further summarize the constraints on the abundance pattern at the group mass scale and the insight it provides to the history of chemical enrichment. Parallel to the observational efforts, we review the progress made by both cosmological hydrodynamical simulations and controlled high-resolution 3D simulations to reproduce the radial distribution of metals in the IGrM, the dependence on system mass from group to cluster scales, and the role of AGN and SN feedback in producing the observed phenomenology. Finally, we highlight future prospects in this field, where progress will be driven both by a much richer sample of X-ray emitting groups identified with eROSITA, and by a revolution in the study of X-ray spectra expected from micro-calorimeters onboard XRISM and ATHENA.

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arXiv2021

Exploring the hydrostatic mass bias in MUSIC clusters: application to the NIKA2 mock sample

Clusters of galaxies are useful tools to constrain cosmological parameters, only if their masses can be correctly inferred from observations. In particular, X-ray and Sunyaev-Zeldovich (SZ) effect observations can be used to derive masses within the framework of the hydrostatic equilibrium. Therefore, it is crucial to have a good control of the possible mass biases that can be introduced when this hypothesis is not valid. In this work, we analyzed a set of 260 synthetic clusters from the MUSIC simulation project, at redshifts $0 \leq z \leq 0.82$. We estimate the hydrostatic mass of the MUSIC clusters from X-ray only (temperature and density) and from X-ray and SZ (density and pressure). Then, we compare them with the true 3D dynamical mass. The biases are of the order of 20%. We find that using the temperature instead of the pressure leads to a smaller bias, although the two values are compatible within 1$σ$. Non-thermal contributions to the total pressure support, arising from bulk motion and turbulence of the gas, are also computed and show that they are sufficient to account for this bias. We also present a study of the correlation between the mass bias and the dynamical state of the clusters. A clear correlation is shown between the relaxation state of the clusters and the bias factor. We applied the same analysis on a subsample of 32 objects, already selected for supporting the NIKA2 SZ Large Program.

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arXiv2020

Iron in X-COP: tracing enrichment in cluster outskirts with high accuracy abundance profiles

We present the first metal abundance profiles for a representative sample of massive clusters. Our measures extend to $R_{500}$ and are corrected for a systematic error plaguing previous outskirt estimates. Our profiles flatten out at large radii, admittedly not a new result, however the radial range and representative nature of our sample extends its import well beyond previous findings. We find no evidence of segregation between cool-core and non-cool-core systems beyond $\sim 0.3 R_{500}$, implying that, as was found for thermodynamic properties (Ghirardini et al, 2019), the physical state of the core does not affect global cluster properties. Our mean abundance within $R_{500}$ shows a very modest scatter, $< $15%, suggesting the enrichment process must be quite similar in all these massive systems. This is a new finding and has significant implications on feedback processes. Together with results from thermodynamic properties presented in a previous X-COP paper, it affords a coherent picture where feedback effects do not vary significantly from one system to another. By combing ICM with stellar measurements we have found the amount of Fe diffused in the ICM to be about ten times higher than that locked in stars. Although our estimates suggest, with some strength, that the measured iron mass in clusters is well in excess of the predicted one, systematic errors prevent us from making a definitive statement. Further advancements will only be possible when systematic uncertainties, principally those associated to stellar masses, both within and beyond $R_{500}$, can be reduced.

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arXiv2019

Black hole mass of central galaxies and cluster mass correlation in cosmological hydro-dynamical simulations

Recently, relations connecting the SMBH mass of central galaxies and global properties of the hosting cluster, such as temperature and mass, were observed. We investigate the correlation between SMBH mass and cluster mass and temperature, their establishment and evolution. We compare their scatter to that of the classical $M_{\rm BH}-M_{\rm BCG}$ relation. We study how gas accretion and BH-BH mergers contribute to SMBH growth across cosmic time. We employed 135 groups and clusters with a mass range $1.4\times 10^{13}M_{\odot}-2.5\times 10^{15} M_{\odot}$ extracted from a set of 29 zoom-in cosmological hydro-dynamical simulations where the baryonic physics is treated with various sub-grid models, including feedback by AGN. In our simulations we find that $M_{\rm BH}$ correlates well with $M_{500}$ and $T_{500}$, with the scatter around these relations compatible within $2σ$ with the scatter around $M_{\rm BH}-M_{\rm BCG}$ at $z=0$. The $M_{\rm BH}-M_{500}$ relation evolves with time, becoming shallower at lower redshift as a direct consequence of hierarchical structure formation. On average, in our simulations the contribution of gas accretion to the total SMBH mass dominates for the majority of the cosmic time ($z>0.4$), while in the last 2 Gyr the BH-BH mergers become a larger contributor. During this last process, substructures hosting SMBHs are disrupted in the merger process with the BCG and the unbound stars enrich the diffuse stellar component rather than increase BCG mass. From the results obtained in our simulations with simple sub-grid models we conclude that the scatter around the $M_{\rm BH}-T_{500}$ relation is comparable to the scatter around the $M_{\rm BH}-M_{\rm BCG}$ relation and that, given the observational difficulties related to the estimation of the BCG mass, clusters temperature and mass can be a useful proxy for the SMBHs mass, especially at high redshift.

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arXiv2019

Probing Macro-Scale Gas Motions and Turbulence in Diffuse Cosmic Plasmas

Clusters of galaxies, the largest collapsed structures in the Universe, are located at the intersection of extended filaments of baryons and dark matter. Cosmological accretion onto clusters through large scale filaments adds material at cluster outskirts. Kinetic energy in the form of bulk motions and turbulence due to this accretion provides a form of pressure support against gravity, supplemental to thermal pressure. Significant amount of non-thermal pressure support could bias cluster masses derived assuming hydrostatic equilibrium, the primary proxy for cluster cosmology studies. Sensitive measurements of Doppler broadening and shift of astrophysical lines, and the relative fluctuations in thermodynamical quantities (e.g., density, pressure, and entropy) are primary diagnostic tools. Forthcoming planned and proposed X-ray (with large etendue, throughput, and high spectral resolution) and SZ observatories will provide crucial information on the assembly and virialisation processes of clusters, involving turbulent eddies cascading at various spatial scales and larger gas bulk motions in their external regions to the depth or their potential wells.

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arXiv2019

Mass-Metallicity Relation from Cosmological Hydrodynamical Simulations and X-ray Observations of Galaxy Groups and Clusters

Recent X-ray observations of galaxy clusters show that the distribution of intra-cluster medium (ICM) metallicity is remarkably uniform in space and time. In this paper, we analyse a large sample of simulated objects, from poor groups to rich clusters, to study the dependence of the metallicity and related quantities on the mass of the systems. The simulations are performed with an improved version of the Smoothed-Particle-Hydrodynamics \texttt{GADGET-3} code and consider various astrophysical processes including radiative cooling, metal enrichment and feedback from stars and active galactic nuclei (AGN). The scaling between the metallicity and the temperature obtained in the simulations agrees well in trend and evolution with the observational results obtained from two data samples characterised by a wide range of masses and a large redshift coverage. We find that the iron abundance in the cluster core ($r<0.1R_{500}$) does not correlate with the temperature nor presents a significant evolution. The scale invariance is confirmed when the metallicity is related directly to the total mass. The slope of the best-fitting relations is shallow ($β\sim-0.1$) in the innermost regions ($r<0.5R_{500}$) and consistent with zero outside. We investigate the impact of the AGN feedback and find that it plays a key role in producing a constant value of the outskirts metallicity from groups to clusters. This finding additionally supports the picture of early enrichment.

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arXiv2018

A web portal for hydrodynamical, cosmological simulations

This article describes a data center hosting a web portal for accessing and sharing the output of large, cosmological, hydro-dynamical simulations with a broad scientific community. It also allows users to receive related scientific data products by directly processing the raw simulation data on a remote computing cluster. The data center has a multi-layer structure: a web portal, a job control layer, a computing cluster and a HPC storage system. The outer layer enables users to choose an object from the simulations. Objects can be selected by visually inspecting 2D maps of the simulation data, by performing highly compounded and elaborated queries or graphically by plotting arbitrary combinations of properties. The user can run analysis tools on a chosen object. These services allow users to run analysis tools on the raw simulation data. The job control layer is responsible for handling and performing the analysis jobs, which are executed on a computing cluster. The innermost layer is formed by a HPC storage system which hosts the large, raw simulation data. The following services are available for the users: (I) {\sc ClusterInspect} visualizes properties of member galaxies of a selected galaxy cluster; (II) {\sc SimCut} returns the raw data of a sub-volume around a selected object from a simulation, containing all the original, hydro-dynamical quantities; (III) {\sc Smac} creates idealised 2D maps of various, physical quantities and observables of a selected object; (IV) {\sc Phox} generates virtual X-ray observations with specifications of various current and upcoming instruments.

astro-ph.IM↗

arXiv2018

The origin of ICM enrichment in the outskirts of present-day galaxy clusters from cosmological hydrodynamical simulations

The uniformity of the intra-cluster medium (ICM) enrichment level in the outskirts of nearby galaxy clusters suggests that chemical elements were deposited and widely spread into the intergalactic medium before the cluster formation. This observational evidence is supported by numerical findings from cosmological hydrodynamical simulations, as presented in Biffi et al. (2017), including the effect of thermal feedback from active galactic nuclei. Here, we further investigate this picture, by tracing back in time the spatial origin and metallicity evolution of the gas residing at z=0 in the outskirts of simulated galaxy clusters. In these regions, we find a large distribution of iron abundances, including a component of highly-enriched gas, already present at z=2. At z>1, the gas in the present-day outskirts was distributed over tens of virial radii from the the main cluster and had been already enriched within high-redshift haloes. At z=2, about 40% of the most Fe-rich gas at z=0 was not residing in any halo more massive than 1e11 Msun/h in the region and yet its average iron abundance was already 0.4, w.r.t. the solar value by Anders & Grevesse (1989). This confirms that the in situ enrichment of the ICM in the outskirts of present-day clusters does not play a significant role, and its uniform metal abundance is rather the consequence of the accretion of both low-metallicity and pre-enriched (at z>2) gas, from the diffuse component and through merging substructures. These findings do not depend on the mass of the cluster nor on its core properties.

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arXiv2015

Neutral hydrogen in galaxy clusters: impact of AGN feedback and implications for intensity mapping

By means of zoom-in hydrodynamic simulations we quantify the amount of neutral hydrogen (HI) hosted by groups and clusters of galaxies. Our simulations, which are based on an improved formulation of smoothed particle hydrodynamics (SPH), include radiative cooling, star formation, metal enrichment and supernova feedback, and can be split in two different groups, depending on whether feedback from active galactic nuclei (AGN) is turned on or off. Simulations are analyzed to account for HI self-shielding and the presence of molecular hydrogen. We find that the mass in neutral hydrogen of dark matter halos monotonically increases with the halo mass and can be well described by a power-law of the form $M_{\rm HI}(M,z)\propto M^{3/4}$. Our results point out that AGN feedback reduces both the total halo mass and its HI mass, although it is more efficient in removing HI. We conclude that AGN feedback reduces the neutral hydrogen mass of a given halo by $\sim50\%$, with a weak dependence on halo mass and redshift. The spatial distribution of neutral hydrogen within halos is also affected by AGN feedback, whose effect is to decrease the fraction of HI that resides in the halo inner regions. By extrapolating our results to halos not resolved in our simulations we derive astrophysical implications from the measurements of $Ω_{\rm HI}(z)$: halos with circular velocities larger than $\sim25~{\rm km/s}$ are needed to host HI in order to reproduce observations. We find that only the model with AGN feedback is capable of reproducing the value of $Ω_{\rm HI}b_{\rm HI}$ derived from available 21cm intensity mapping observations.

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arXiv2015

How does our choice of observable influence our estimation of the centre of a galaxy cluster? Insights from cosmological simulations

Galaxy clusters are an established and powerful test-bed for theories of both galaxy evolution and cosmology. Accurate interpretation of cluster observations often requires robust identification of the location of the centre. Using a statistical sample of clusters drawn from a suite of cosmological simulations in which we have explored a range of galaxy formation models, we investigate how the location of this centre is affected by the choice of observable - stars, hot gas, or the full mass distribution as can be probed by the gravitational potential. We explore several measures of cluster centre: the minimum of the gravitational potential, which would expect to define the centre if the cluster is in dynamical equilibrium; the peak of the density; the centre of BCG; and the peak and centroid of X-ray luminosity. We find that the centre of BCG correlates more strongly with the minimum of the gravitational potential than the X-ray defined centres, while AGN feedback acts to significantly enhance the offset between the peak X-ray luminosity and minimum gravitational potential. These results highlight the importance of centre identification when interpreting clusters observations, in particular when comparing theoretical predictions and observational data.

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