Galaxy clusters

On the dynamical state of galaxy clusters: insights from cosmological simulations II (MACRO project paper II)

Cui, Weiguang; Power, Chris; Borgani, Stefano; Knebe, Alexander; Murante, Giuseppe; Lewis, Geraint F. and Poole, Gregory B., 2016, MNRAS 464, 2502

Actually, this paper should belong to the galaxy cluster page. But I put it here as it's part of the MACRO project.

As the second paper of this project, we focus on the baryonic effects on the dynamical states of galaxy clusters. Using the galaxy cluster catalogue from the first paper, we investigate four independent parameters (virial ratio η, subhalo mass fraction f_s, center of mass offset R_{cm}, Velocity dispersion deviation ζ) that are normally used to classify dynamical state. We found that

1. the virial ratio η in hydro-dynamical runs is ˜10 per cent lower than in the DM run.
2. besides the CSF run shows a higher (~20%) f_s fraction than the DM run, all there are parameters show no clear impact by baryons.
3. there is no clear separation between the relaxed and unrelaxed clusters for any parameter. With more restricted controls (0.85 < η < 1.15, ∆ < 0.04 and fs < 0.075), we are able to get a observational comparable relaxation fraction of ~35%.

Further, using the velocity dispersion deviation parameter ζ, which is defined as the ratio between cluster velocity dispersion σ and the theoretical prediction σ _t = √{G M_{total}/R}, we find that there is a linear correlation between the virial ratio η and this ζ parameter. We propose to use this ζ parameter, which can be easily derived from observed galaxy clusters, as a substitute of the η parameter to quantify the cluster dynamical state.

nIFTy Galaxy Cluster simulations IV: Quantifying the Influence of Baryons on Halo Properties

Cui, Weiguang; Power, Chris; Knebe, Alexander; Kay, Scott T.; Sembolini, Federico; Elahi, Pascal J.; Yepes, Gustavo; Pearce, Frazer; and 16 coauthors, 2016, MNRAS, 458, 4052C

Using 13 different codes modelling only gravity and non-radiative hydrodynamics (RAMSES, ART, AREPO, HYDRA and nine incarnations of GADGET), which include particle-based, moving and fixed mesh codes as well as both Eulerian and Lagrangian fluid schemes, the same galaxy cluster from identical initial condition are simulated in the nIFTy project. For each code represented, we have a dark-matter-only (DM), non-radiative (NR) version , and a full physics (FP) version (for a subset of the codes) of the cluster. This project has 5 papers published: I. Dark matter and non-radiative models; II. Radiative models; III. The similarity and diversity of galaxies and subhaloes; IV. Quantifying the influence of baryons on halo properties; V. Investigation of the cluster infall region.

As the fourth paper of this project, we focus on the baryon effects. We compare both radial mass and kinematic profiles, as well as global measures of the cluster (e.g. concentration, spin, shape), in the NR and FP runs with that in the DM runs. Our analysis reveals good consistency (⪅20 per cent) between global properties of the cluster predicted by different codes when integrated quantities are measured within the virial radius R₂₀₀. However, we see larger differences for quantities within R₂₅₀₀, especially in the FP runs.

Characterising Diffused Stellar Light in Simulated Galaxy Clusters from Two Views.

Cui, Weiguang; Murante, G.; Monaco, P.; Borgani, S.; Granato, G.L.; Killedar, M.; De Lucia, G.; Presotto, V.; Dolag, K., 2014, MNRAS, 437, 816C

We carry out a detailed analysis of the performance of two different methods to identify the diffuse stellar light with 80 zoom-in hydrodynamical simulations of galaxy clusters. One method is based on a dynamical analysis of the stellar component, which separates the brightest central galaxy (BCG) from the stellar component not gravitationally bound to any galaxy, what we call 'diffuse stellar component' (DSC). The second method mimics observational method of identifying intra-cluster light (ICL) by applying a standard surface brightness limit (SBL) to mock optical images from simulations.

We find

1. significant differences between the ICL and DSC fractions computed with the two corresponding methods, which amounts to about a factor of 2 for the AGN simulations, and a factor of 4 for the CSF set.
2. the inclusion of AGN feedback boosts the DSC and ICL fractions by a factor of 1.5-2, respectively, while leaving the BCG+ICL and BCG+DSC mass fraction almost unchanged.
3. The BCG stellar components, as identified by both methods, are slightly older and more metal-rich than the stars in the diffuse component.
4. Relaxed clusters have somewhat higher stellar mass fractions in the diffuse component. The metallicity and age of both the BCG and diffuse components in relaxed clusters are also richer in metals and older.

Lower left figure: The DSC (identified by the dynamical method, left panel) and BCG+DSC (right panel) mass fractions;

Lower right figure: The ICL (identified by the observation-like SBL method with different limits) fractions.

Properties of fossil groups in cosmological simulations and galaxy formation models.

Cui, Weiguang; Springel, Volker; Yang, Xiaohu; De Lucia, Gabriella; Borgani, Stefano, 2011, MNRAS, 416, 2997C

Using the hydrodynamical simulations of the Galaxies Intergalactic Medium Interaction Calculation (GIMIC) project as a baseline, which consists of re-simulations of five regions in the Millennium Simulation (MS) that are characterised by different large-scale densities, ranging from a deep void to a proto-cluster region, two semi-analytic models (one Munich model, one durham model) and a conditional luminosity function approach built on top of the MS are taken into account.

We compared the luminosity functions, BCG function and the luminosity gaps at different environments for these different models. Combining the optical- and X-ray-selection criteria, we also selected out fossil groups (FGs) for these environments.

We found that

1. Luminosity functions show environment dependence, but both BCG function and luminosity gap do not. Without AGN feedback, the BCGs in GIMIC simulation are far more bright than observational results.
2. The optical fossil fraction declines with increasing halo mass. There is no clear evidence that the optical fossil fraction has environmental effects in all the theoretical galaxy formation models.
3. When we investigated the properties of central galaxies in FGs and non-FGs, we have found no differences in the magnitude, X-ray luminosity, age, metallicity, concentration, colour, massto-light ratio and satellite galaxies number distributions. In particular, the mean ages of the central galaxies in FGs are not really ‘fossil’ in the sense of the word.

These results support an interpretation of fossil groups as transient phases in the evolution of ordinary galaxy groups rather than forming a physically distinct class of objects.