A Group-Group Merger at a Redshift of z = 0.84?

Abstract

We present a dynamical study of the CL0023+0423 system at a redshift of z = 0.84. This system consists of two components separated in velocity space by 2900 km/s and on the plane of the sky by 0.23 h^{-1} Mpc. A kinematic analysis indicates that the two components are a poor cluster with a velocity dispersion of 415 km/s and a less massive group with a velocity dispersion of 158 km/s (Postman, Lubin & Oke 1998). An analytic two-body calculation and N-body simulations indicate that the system is most likely not bound but simply a chance projection on the sky; however, within the observational uncertainties, there do exist bound solutions where the two galaxy groups are currently moving toward each other and will eventually merge. A statistical analysis of 1000 N-body simulations indicates that there is an 20% chance that the two groups will merge. If the CL0023+0423 system does merge, it will appear as a cluster on the sky, as well as in velocity space, within 1-2 Gyrs. The cluster will evolve dynamically for more than 3 Gyrs, appearing during this time more similar to an open, irregular cluster. The final merged system has a velocity dispersion which is consistent with a local Abell richness class 1 cluster. The morphological analysis of the galaxy populations of CL0023+0423 suggests that both groups are largely dominated by spiral galaxies with early-type fractions of 33% or less (Lubin et al. 1998). These modest early-type fractions may have implications for both cluster formation and group evolution. A comparison with nearby and intermediate-redshift groups and clusters of galaxies suggest that some fraction of early-type galaxies are formed at redshifts of z < 1.