A GROUP-GROUP MERGER AT A REDSHIFT OF Z=0.84

We present a dynamical study of the C1 0023 + 0423 system at a redshif t of z = 0.84. This system consists of two components separated in vel ocity space by similar to 2900 km s(-1) and on the plane of the sky by similar to 0.23 h(-1) Mpc. A kinematic analysis indicates that the tw o components are a poor cluster with a velocity dispersion of 415(-63) (+102) km s(-1) and a mass of similar to 3-6 x 10(14) h(-1) M., and a less massive group with a velocity dispersion of 158(-33)(+42) km s(-1 ) and a mass of similar to 10(13) h(-1) M.. The dynamics of galaxy gro ups at high redshift can provide important insights into the creation of present-day galaxy clusters. Therefore, we have performed a dynamic al study on this system in order to determine whether the two groups a re infalling. This analysis includes an analytic two-body calculation and N-body simulations. The results of both studies indicate that the system is most likely not bound, but simply a chance projection on the sky; however, within the observational uncertainties, there exist bou nd solutions where the two galaxy groups are currently moving toward e ach other and will eventually merge into a larger system of galaxies. We have run 1000 N-body simulations with random initial conditions bas ed on the observed parameters of the C1 0023 + 0423 groups. A statisti cal analysis of these simulations indicates that there is a 20% chance that the two groups will merge. If the Cl 0023 + 0423 system does mer ge, it will appear as a cluster on the sky, as well as in velocity spa ce, within 1-2 Gyr. The cluster will evolve dynamically for more than 3 Gyr, appearing during this time more similar to an open, irregular c luster. The final merged system has a velocity dispersion that is cons istent with a local Abell richness class 1 cluster. The morphological analysis of the galaxy populations of C1 0023 + 0423 suggests that bot h groups are largely dominated by spiral galaxies. Early-type fraction s are 33% or less. These modest early-type fractions have implications for both cluster formation and group evolution. Studies of open clust ers at z = 0.31-0.54 indicate that they have early-type fractions betw een 45%-80%. If the CI 0023 + 0423 system is the predecessor of such a cluster, the comparison may suggest that some fraction of early-type galaxies are formed between redshifts of z similar to 0.8 and z simila r to 0.3; however, the morphological fractions are still highly uncert ain. In addition, the modest early-type fractions in both groups may b e inconsistent with the strong correlation between velocity dispersion and early-type fraction observed in nearby groups of galaxies. Both g roups apparently have relatively low early-type populations, irrespect ive of their velocity dispersion. If the groups of Cl 0023 + 0423 are typical of galaxy groups at high redshift, and if high-redshift groups are the progenitors of local groups, this result may also imply that some early-type formation is occurring at redshifts of z less than or similar to 0.8. These results do not preclude the formation of early-t ype galaxies at very high redshift, as many observations suggest; howe ver, the observations of the Cl 0023 + 0423 system may imply that a fr action of the early-type population is forming and/or undergoing signi ficant evolution at redshifts of z < 1.