The absolute magnitude of the brightest galaxy of clusters varies remarkably little and is nearly independent of all other physical properties of the cluster as, e.g., its spatial extension or its richness. The question arises whether the observed small scatter is compatible with the assumption of dynamical evolution of the cluster. This is investigated with the help of statistical analysis of the results of cluster simulation. The underlying interaction process is merging (and also destruction) of smaller galaxies forming the giant galaxy. The cluster itself is supposed to be in virial equilibrium.
We find that the evolutionary importance of merger processes grows with decreasing scale. Rich clusters as well as their brightest members evolve merely slowly whereas compact groups as well as their brightest members evolve more rapidly and more violently. We also find that the number of merger processes leading to the growth of the brightest cluster galaxy (BCG) is small enough to keep the BCG mass dispersion below the measured value. Our simulations substantiate that just the combination of the initial distribution function and the following merging to form the BCG can explain the remarkably small variance of mean BCG masses between clusters of different size and different number of galaxies.
The orientations of brightest cluster galaxies (BCGs) and their host clusters tend to be aligned, but the mechanism driving this is not clear. To probe the role of cluster mergers in this process, we quantify alignments of 38 BCGs in 22 clusters undergoing major mergers (up to $\sim 1$ Gyr after first pericenter). We find alignments entirely consistent with those of clusters in general. This suggests that alignments are robust against major cluster mergers. If, conversely, major cluster mergers actually help orient the BCG, such a process is acting quickly because the orientation is in place w…
Using the data set of The Three Hundred project, i.e. a suite of 324 hydrodynamical resimulations of cluster-sized haloes, we study galaxy cluster mergers and their effect on colour and luminosity changes of their brightest cluster galaxies (BCG). We track the main progenitor of each halo at z=0 and search for merger situations based on its mass accretion history, defining mergers as very rapid increases in the halo mass. Based upon the evolution of the dynamical state of the cluster we define a pre- and post-merger phase. We create a list of all these events and statistically study their mass…
We use the IllustrisTNG simulations to explore the dynamic scaling relation between massive clusters and their central galaxies (BCGs). The Illustris TNG300-1 simulation we use includes 280 massive clusters with $M_{200} > 10^{14}$ M$_{\odot}$ enabling a robust statistical analysis. We derive the line-of-sight velocity dispersion of the stellar particles of the BCGs ($σ_{*, BCG}$), analogous to the observed BCG stellar velocity dispersion. We also compute the subhalo velocity dispersion to measure the cluster velocity dispersion ($σ_{cl}$). Both $σ_{*, BCG}$ and $σ_{cl}$ are proportional to th…
We investigate the formation of brightest cluster galaxies (BCGs) in the TNG300 cosmological simulation of the IllustrisTNG project. Our cluster sample consists of 700 haloes with $M_{200} \geq 5 \times 10^{13} \, \mathrm{M}_{\odot}$ at $z=0$, along with their progenitors at earlier epochs. This includes 280 systems with $M_{200} \geq 10^{14} \, \mathrm{M}_{\odot}$ at $z=0$, as well as three haloes with $M_{200} \geq 10^{15} \, \mathrm{M}_{\odot}$. We find that the stellar masses and star formation rates of our simulated BCGs are in good agreement with observations at $z \lesssim 0.4$, and tha…
Using the DIANOGA hydrodynamical zoom-in simulation set of galaxy clusters, we analyze the dynamics traced by stars belonging to the Brightest Cluster Galaxies (BCGs) and their surrounding diffuse component, forming the intracluster light (ICL), and compare it to the dynamics traced by dark matter and galaxies identified in the simulations. We compute scaling relations between the BCG and cluster velocity dispersions and their corresponding masses (i.e. $M_\mathrm{BCG}^{\star}$- $σ_\mathrm{BCG}^{\star}$, $M_{200}$- $σ_{200}$, $M_\mathrm{BCG}^{\star}$- $M_{200}$, $σ_\mathrm{BCG}^{\star}$- $σ_{2…
We explore the redshift evolution of the dynamical properties of massive clusters and their brightest cluster galaxies (BCGs) at $z < 2$ based on the IllustrisTNG-300 simulation. We select 270 massive clusters with $M_{200} < 10^{14}~{\rm M}_{\odot}$ at $z = 0$ and trace their progenitors based on merger trees. From 67 redshift snapshots covering $z < 2$, we compute the 3D subhalo velocity dispersion as a cluster velocity dispersion ($σ_{\rm cl}$). We also calculate the 3D stellar velocity dispersion of the BCGs ($σ_{\rm *,~BCG}$). Both $σ_{\rm cl}$ and $σ_{\rm *,~BCG}$ increase as universe ag…
Colour Gradients and the Colour-Magnitude Relation: Different Properties of Brightest Cluster Galaxies and E/S0 Galaxies in the Sloan Digital Sky Survey
In the present study, the dependences of the morphological types of the first and second ranked group galaxies on the magnitude gap were studied. It is shown that there is no increase in the relative number of elliptical galaxies among the first and second ranked group galaxies with a large magnitude gaps (in comparison with the expected, assuming that the morphological type of these galaxies does not depend on the magnitude gap). This result contradicts the merger hypothesis. The hypothesis proposed by Ambartsumian does not contradict this result.
We present a detailed study of fossil group candidates identified in "Last Journey", a gravity-only cosmological simulation covering a $(3.4\, h^{-1}\mathrm{Gpc})^3$ volume with a particle mass resolution of $m_p \approx 2.7 \times 10^9\, h^{-1}\mathrm{M}_\odot$. The simulation allows us to simultaneously capture a large number of group-scale halos and to resolve their internal structure. Historically, fossil groups have been characterized by high X-ray brightness and a large luminosity gap between the brightest and second brightest galaxy in the group. In order to identify candidate halos tha…
We want to study how the velocity segregation and the radial profile of the velocity dispersion depend on the prominence of the brightest cluster galaxies (BCGs). We divide a sample of 102 clusters and groups of galaxies into four bins of magnitude gap between the two brightest cluster members. We then compute the velocity segregation in bins of absolute and relative magnitudes. Moreover, for each bin of magnitude gap we compute the radial profile of the velocity dispersion. When using absolute magnitudes, the segregation in velocity is limited to the two brightest bins and no significant diff…
The formation and evolution of local brightest cluster galaxies (BCGs) is investigated by determining the stellar populations and dynamics from the galaxy core, though the outskirts and into the intracluster light (ICL). Integral spectroscopy of 23 BCGs observed out to 4 r_e is collected and high signal-to-noise regions are identified. Stellar population synthesis codes are used to determine the age, metallicity, velocity, and velocity dispersion of stars within each region. The intracluster light (ICL) spectra are best modeled with populations that are younger and less metal-rich than those o…
Quantified HI Morphology IV: The Merger Fraction and Rate in WHISP
螺旋星系氢原子盘的形态是受到引力相互作用（如星系合并）扰动的第一个组成部分。我们使用一个简单的参数化的形态Westerbork HI螺旋项目（WHISP）的HI柱密度图，以选择那些可能正在经历一个显着的相互作用的星系。合并星系占据了由不对称性（A）、20%最亮像素对列密度图二阶矩的相对贡献（M20）和所有像素上二阶矩的分布（GM）定义的参数空间的特定部分。基于它们的HI形态，我们发现13%的WHISP星系处于相互作用（浓度-M20）中，只有7%的星系基于数据立方体中的密切同伴。这种明显的差异可以归因于可见度时间尺度的差异：合并在0.5 Gyr时可以被识别为近对，但在~1 Gyr时可以通过它们被干扰的HI形态来识别。用体积合并率表示，两个估计值非常吻合：配对和形态扰动的HI盘分别为7 x 10^-3和6.8 x 10^-3合并Gyr^-1 Mpc^-3。我们的合并分数的一致性，如斯隆数字巡天更大的调查公布，表明HI形态可以是一个非常可行的方式来确定合并在一个大型HI调查。卷合并率的相对高的值可能是选择或WHISP卷的偏差。计划中的MeerKAT、ASKAP和WSRT/APERTIF射电天文台提供的高分辨率HI数据的预期收益将揭示合并在局部宇宙中的重要性，并随着SKA的出现，揭示整个宇宙时代的重要性。
We apply a friends-of-friends (FoF) algorithm to identify galaxy clusters and we use the catalog to explore the evolutionary synergy between BCGs and their host clusters. We base the cluster catalog on the dense HectoMAP redshift survey (2000 redshifts deg$^{-2}$). The HectoMAP FoF catalog includes 346 clusters with 10 or more spectroscopic members. We list these clusters and their members (5992 galaxies with a spectroscopic redshift). We also include central velocity dispersions ($σ_{*, BCG}$) for all of the FoF cluster BCGs, a distinctive feature of the HectoMAP FoF catalog. HectoMAP cluster…
The stellar initial mass function (IMF) of early-type galaxies is the combination of the IMF of the stellar population formed in-situ and that of accreted stellar populations. Using as an observable the effective IMF $α_{IMF}$, defined as the ratio between the true stellar mass of a galaxy and the stellar mass inferred assuming a Salpeter IMF, we present a theoretical model for its evolution as a result of dry mergers. We use a simple dry merger evolution model, based on cosmological $N$-body simulations, together with empirically motivated prescriptions for the IMF to make predictions for how…
Galaxy clusters are known to be reservoirs of Cosmic Rays (CRs), as inferred from theoretical calculations or detection of CR-derived observables. CR acceleration in clusters is mostly attributed to the dynamical activity that produces shocks. Shocks in clusters emerge out of merger or accretion, but which one is more effective in producing CRs? at which dynamical phase? and why? To this aim, we study the production or injection of CRs through shocks and its evolution in the galaxy clusters using cosmological simulations with the {\sc enzo} code. Particle acceleration model considered here is …
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 synthet…
