THE DYNAMICAL EVOLUTION OF POOR CLUSTERS OF GALAXIES - GROWTH AND PROPERTIES OF THE 1ST-RANKED GALAXY

We report N-body simulations of the dynamical evolution of isolated cl usters of 50 galaxies containing a dark matter component that comprise s 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 init ial core radius is 250 kpc. Our results are applicable to (1) present- day poor clusters, (2) the small systems that may have merged to produ ce present-day rich clusters, and (3) virialized subclumps within larg er systems, in between major substructure merger events.We have evolve d a total of 10 cluster models, using N = 40,000 particles per model. The models are fully self-consistent in that each galaxy is represente d as an extended structure containing many particles and the gravitati onal potential arises from the particles alone. Dark matter is apporti oned between the galaxy halos and a smoothly distributed common group halo, the intracluster background (ICB). The percentage of cluster mas s initially in the ICB, beta, is chosen to be 50, 75, or 90. Increasin g beta has the effect of removing mass from dark halos around galaxies and distributing it throughout the cluster. The initial conditions we re constructed by randomly sampling a King distribution with W0 = 6. T he galaxies are also King models; the masses of the galaxies follow a Schechter distribution function. The five beta = 50 models all followe d a similar pattern of behavior. Galaxies experience dynamical frictio n and undergo orbital decay, leading to an enhanced encounter rate. In approximately 10 Gyr, merging has resulted in the formation of a domi nant, centrally located galaxy. Almost all of the subsequent merging i nvolves this dominant galaxy accreting the others. Mass segregation is apparent, leading the largest galaxies to preferentially engage in me rging. Merging produces an extension of the galaxy mass distribution t o higher masses, while at the same time it reduces the characteristic mass of the distribution owing to the overall depletion of bright gala xies. Once the first-ranked galaxy (FRG) has grown to twice the size o f the initially largest galaxy, its velocity has typically decreased t o 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 a re no high-velocity FRGs of the sort that have been observed in approx imately 10% of clusters. The most evident change in the cluster space density profile occurs in the inner 200 kpc, where a rise in density c auses the core to be erased. If the location of the FRG is taken to de fine the cluster center, then the density profile is even more strongl y cusped and resembles a singular isothermal sphere. The FRG-centered surface density profile can be fit by both power-law and exponential p rofiles. Once the FRG has assumed a central position in the cluster, m ultiple nuclei are seen at least 20% of the time, roughly what is expe cted from the projected surface density distribution. The frequency ri ses above this to approximately 40 % at approximately 11 Gyr. The addi tional 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 DELTAM12, FRG luminosity, and the number of multiple nuclei can bes t be fit by cluster models with ages approximately 11 Gyr; growth in l uminosity of the FRG during this amount of time is consistent with onl y weak cannibalism. Fitting observations of the peculiar velocities of the FRG requires younger ages of approximately 8 Gyr. Increasing beta to 75 slows the rate of merging, but otherwise causes little change i n behavior. For beta = 90, the onset of merging can be delayed for ove r 13 Gyr; thus a dominant central galaxy is not created.