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.Comment: 11 pages, 6 figures, MNRAS accepte

On the dynamical state of galaxy clusters: insights from cosmological simulations - II.

Using a suite of cosmology simulations of a sample of >120 galaxy clusters with log (MDM, vir) 64 14.5. We compare clusters that form in purely dark matter (DM) run and their counterparts in hydro-runs and investigate four independent parameters that are normally used to classify dynamical state. We find that the virial ratio \u3b7 in hydro-dynamical runs is \u2dc10 per cent lower than in the DM run, and there is no clear separation between the relaxed and unrelaxed clusters for any parameter. Further, using the velocity dispersion deviation parameter \u3b6, which is defined as the ratio between cluster velocity dispersion \u3c3 and the theoretical prediction \u3c3 _t = 1a{G M_{total}/R}, we find that there is a linear correlation between the virial ratio \u3b7 and this \u3b6 parameter. We propose to use this \u3b6 parameter, which can be easily derived from observed galaxy clusters, as a substitute of the \u3b7 parameter to quantify the cluster dynamical state

Diffuse light and galaxy interactions in the core of nearby clusters

The kinematics of the diffuse light in the densest regions of the nearby clusters can be unmasked using the planetary nebulae (PNs) as probes of the stellar motions. The position-velocity diagrams around the brightest cluster galaxies (BCGs) identify the relative contributions from the outer halos and the intracluster light (ICL), defined as the light radiated by the stars floating in the cluster potential. The kinematics of the ICL can then be used to asses the dynamical status of the nearby cluster cores and to infer their formation histories. The cores of the Virgo and Coma are observed to be far from equilibrium, with mergers currently on-going, while the ICL properties in the Fornax and Hydra clusters show the presence of sub-components being accreted in their cores, but superposed to an otherwise relaxed population of stars. Finally the comparison of the observed ICL properties with those predicted from Lambda-CDM simulations indicates a qualitative agreement and provides insights on the ICL formation. Both observations and simulations indicate that BCG halos and ICL are physically distinct components, with the ``hotter" ICL dominating at large radial distances from the BCGs halos as the latter become progressively fainter.Comment: 14 pages, 5 figures. Invited review to appear in the proceedings of "Galaxies and their masks" eds. Block, D.L., Freeman, K.C. and Puerari, I., 2010, Springer (New York

Intragroup diffuse light in compact groups of galaxies II. HCG 15, 35 and 51

This continuing study of intragroup light in compact groups of galaxies aims to establish new constraints to models of formation and evolution of galaxy groups, specially of compact groups, which are a key part in the evolution of larger structures, such as clusters. In this paper we present three additional groups (HCG 15, 35 and 51) using deep wide field $B$ and $R$ band images observed with the LAICA camera at the 3.5m telescope at the Calar Alto observatory (CAHA). This instrument provides us with very stable flatfielding, a mandatory condition for reliably measuring intragroup diffuse light. The images were analyzed with the OV\_WAV package, a wavelet technique that allows us to uncover the intragroup component in an unprecedented way. We have detected that 19, 15 and 26% of the total light of HCG 15, 35 and 51, respectively, is in the diffuse component, with colours that are compatible with old stellar populations and with mean surface brightness that can be as low as $28.4 {\rm B mag arcsec^{-2}}$. Dynamical masses, crossing times and mass to light ratios were recalculated using the new group parameters. Also tidal features were analyzed using the wavelet technique.Comment: 12 pages, 5 figures. Accepted for publication in MNRAS. See http://www.eso.org/~cdarocha/publications/DaRochaetal2008_IGL_HCG.pdf for full resolution version. Complementary reference adde

Diffuse light and building history of the galaxy cluster Abell 2667

We have searched for diffuse intracluster light in the galaxy cluster Abell 2667 (z=0.233) from HST images in three filters. We have applied to these images an iterative multi-scale wavelet analysis and reconstruction technique, which allows to subtract stars and galaxies from the original images. We detect a zone of diffuse emission south west of the cluster center (DS1), and a second faint object (ComDif), within DS1. Another diffuse source (DS2) may be detected, at lower confidence level, north east of the center. These sources of diffuse light contribute to 10-15% of the total visible light in the cluster. Whether they are independent entities or are part of the very elliptical external envelope of the central galaxy remains unclear. VLT VIMOS integral field spectroscopy reveals a faint continuum at the positions of DS1 and ComDif but do not allow to compute a redshift. A hierarchical substructure detection method reveals the presence of several galaxy pairs and groups defining a similar direction as the one drawn by the DS1-central galaxy-DS2 axis. The analysis of archive XMM-Newton and Chandra observations shows X-ray emission elongated in the same direction. The X-ray temperature map shows the presence of a cool core, a broad cool zone stretching from north to south and hotter regions towards the north east, south west and north west. This possibly suggests shock fronts along these directions produced by infalling material. These various data are consistent with a picture in which diffuse sources are concentrations of tidal debris and harassed matter expelled from infalling galaxies by tidal stripping and undergoing an accretion process onto the central cluster galaxy; as such, they are expected to be found along the main infall directions.Comment: Accepted for publication in Astronomy and Astrophysic

The optical/X-ray connection: ICM iron content and galaxy optical luminosity in 20 galaxy clusters

X-ray observations of galaxy clusters have shown that the intra-cluster gas has iron abundances of about one third of the solar value. These observations also show that part (if not all) of the intra-cluster gas metals were produced within the member galaxies. We present a systematic analysis of 20 galaxy clusters to explore the connection between the iron mass and the total luminosity of early-type and late-type galaxies, and of the brightest cluster galaxies (BCGs). From our results, the intra-cluster medium (ICM) iron mass seems to correlate better with the luminosity of the BCGs than with that of the red and blue galaxy populations. As the BCGs cannot produce alone the observed amount of iron, we suggest that ram-pressure plus tidal stripping act together to enhance, at the same time, the BCG luminosities and the iron mass in the ICM. Through the analysis of the iron yield, we have also estimated that SN Ia are responsible for more than 50% of the total iron in the ICM. This result corroborates the fact that ram-pressure contributes to the gas removal from galaxies to the inta-cluster medium, being very efficient for clusters in the temperature range 2 < kT (keV)< 10Comment: Accepted for publication in MNRAS (11 pg, 9 figures and 3 tables

Mass assembly of galaxies: Smooth accretion versus mergers

Galaxies accrete their mass by means of both smooth accretion from the cosmic web, and the mergers of smaller entities. We wish to quantify the respective role of these two modes of accretion, which could determine the morphological types of galaxies observed today. Multi-zoom cosmological simulations are used to estimate as a function of time the evolution of mass in bound systems, for dark matter as well as baryons. The baryonic contents of dark matter haloes are studied. Merger histories are followed as a function of external density, and the different ways in which mass is assembled in galaxies and the stellar component accumulated are quantified. We find that most galaxies assemble their mass through smooth accretion, and only the most massive galaxies also grow significantly through mergers. The mean fraction of mass assembled by accretion is 77 %, and by mergers 23 %. We present typical accretion histories of hundreds of galaxies: masses of the most massive galaxies increase monotonically in time, mainly through accretion, many intermediate-mass objects also experience mass-loss events such as tidal stripping and evaporation. However, our simulations suffer from the overcooling of massive galaxies caused by the neglect of active galaxy nuclei (AGN) feedback. The time by which half of the galay mass has assembled, both in dark matter and baryons, is a decreasing function of mass, which is compatible with the observations of a so-called downsizing. At every epoch in the universe, there are low-mass galaxies actively forming stars, while more massive galaxies form their stars over a shorter period of time within half the age of the universe.Comment: A&A Accepted, 19 pages, 17 figure

Fractals vs. halos: Asymptotic scaling without fractal properties

Precise analyses of the statistical and scaling properties of galaxy distribution are essential to elucidate the large-scale structure of the universe. Given the ongoing debate on its statistical features, the development of statistical tools permitting to discriminate accurately different spatial patterns is highly desiderable. This is specially the case when non-fractal distributions have power law two-point correlation functions, which are usually signatures of fractal properties. Here we review some possible methods used in the literature and introduce a new variable called "scaling gradient". This tool and the conditional variance are shown to be effective in providing an unambiguous way for such a distinction. Their application is expected to be of outmost importance in the analysis of upcoming galaxy catalogues.Comment: 7 pages, 3 figure

Metal enrichment processes

There are many processes that can transport gas from the galaxies to their environment and enrich the environment in this way with metals. These metal enrichment processes have a large influence on the evolution of both the galaxies and their environment. Various processes can contribute to the gas transfer: ram-pressure stripping, galactic winds, AGN outflows, galaxy-galaxy interactions and others. We review their observational evidence, corresponding simulations, their efficiencies, and their time scales as far as they are known to date. It seems that all processes can contribute to the enrichment. There is not a single process that always dominates the enrichment, because the efficiencies of the processes vary strongly with galaxy and environmental properties.Comment: 18 pages, 8 figures, accepted for publication in Space Science Reviews, special issue "Clusters of galaxies: beyond the thermal view", Editor J.S. Kaastra, Chapter 17; work done by an international team at the International Space Science Institute (ISSI), Bern, organised by J.S. Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke

The probability of galaxy-galaxy strong lensing events in hydrodynamical simulations of galaxy clusters

Meneghetti et al. (2020) recently reported an excess of galaxy-galaxy strong lensing (GGSL) in galaxy clusters compared to expectations from the LCDM cosmological model. Theoretical estimates of the GGSL probability are based on the analysis of numerical hydrodynamical simulations in the LCDM cosmology. We quantify the impact of the numerical resolution and AGN feedback scheme adopted in cosmological simulations on the predicted GGSL probability and determine if varying these simulation properties can alleviate the gap with observations. We repeat the analysis of Meneghetti et al. (2020) on cluster-size halos simulated with different mass and force resolutions and implementing several independent AGN feedback schemes. We find that improving the mass resolution by a factor of ten and twenty-five, while using the same galaxy formation model that includes AGN feedback, does not affect the GGSL probability. We find similar results regarding the choice of gravitational softening. On the contrary, adopting an AGN feedback scheme that is less efficient at suppressing gas cooling and star formation leads to an increase in the GGSL probability by a factor between three and six. However, we notice that such simulations form overly massive subhalos whose contribution to the lensing cross-section would be significant while their Einstein radii are too large to be consistent with the observations. The primary contributors to the observed GGSL cross-sections are subhalos with smaller masses, that are compact enough to become critical for lensing. The population with these required characteristics appears to be absent in simulations.Comment: 13 pages, 11 figures. Submitted for publication on Astronomy and Astrophysic