The dynamical evolution of poor clusters of galaxies: Growth and properties of the first-ranked galaxy

Abstract

We report N-body simulations of the dynamical evolution of isolated clusters of 50 galaxies containing a dark matter component that comprises 90% of the cluster mass. For our adopted physical scaling, the line-of- sight velocity dispersion of the cluster is 310 km s^-1^ and the initial core radius is 250 kpc. Our results are applicable to (1) present-day poor clusters, (2) the small systems that may have merged to produce present-day rich clusters, and (3) virialized subclumps within larger systems, in between major substructure merger events. We have evolved a total of 10 cluster models, using N = 40,000 particles per model. The models are fully self-consistent in that each galaxy is represented as an extended structure containing many particles and the gravitational potential arises from the particles alone. Dark matter is apportioned between the galaxy halos and a smoothly distributed common group halo, the intracluster background (ICB). The percentage of cluster mass initially in the ICB, β, is chosen to be 50, 75, or 90. Increasing β has the effect of removing mass from dark halos around galaxies and distributing it throughout the cluster. The initial conditions were constructed by randomly sampling a King distribution with W_0_ = 6. The galaxies are also King models; the masses of the galaxies follow a Schechter distribution function. The five β = 50 models all followed a similar pattern of behavior. Galaxies experience dynamical friction and undergo orbital decay, leading to an enhanced encounter rate. In ~10 Gyr, merging has resulted in the formation of a dominant, centrally located galaxy. Almost all of the subsequent merging involves this dominant galaxy accreting the others. Mass segregation is apparent, leading the largest galaxies to preferentially engage in merging. Merging produces an extension of the galaxy mass distribution to higher masses, while at the same time it reduces the characteristic mass of the distribution owing to the overall depletion of bright galaxies. Once the first-ranked galaxy (FRG) has grown to twice the size of the initially largest galaxy, its velocity has typically decreased to less than half the cluster velocity dispersion and it remains within the cluster core. The distribution of FRG peculiar velocities at this point contains no values greater than the cluster dispersion; there are no high-velocity FRGs of the sort that have been observed in ~10% of clusters. The most evident change in the cluster space density profile occurs in the inner 200 kpc, where a rise in density causes the core to be erased. If the location of the FRG is taken to define the cluster center, then the density profile is even more strongly cusped and resembles a singular isothermal sphere. The FRG-centered surface density profile can be fit by both power-law and exponential profiles. Once the FRG has assumed a central position in the cluster, multiple nuclei are seen at least 20% of the time, roughly what is expected from the projected surface density distribution. The frequency rises above this to ~40% at ~11 Gyr. The additional nuclei are on orbits which bring them into contact with the FRG. After these satellites merge with the FRG, the frequency of multiple nuclei falls back to the value expected from projection. Observations of {DELTA}M_12_, FRG luminosity, and the number of multiple nuclei can best be fit by cluster models with ages ~11 Gyr; growth in luminosity of the FRG during this amount of time is consistent with only weak cannibalism. Fitting observations of the peculiar velocities of the FRG requires younger ages of ~8 Gyr. Increasing β to 75 slows the rate of merging, but otherwise causes little change in behavior For β = 90, the onset of merging can be delayed for over 13 Gyr; thus a dominant central galaxy is not created.