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We present results from an extensive spectroscopic survey, carried out withVLT FORS, and from an extensive multiwavelength imaging data set from the HSTAdvanced Camera for Surveys and ground based facilities, of the cluster ofgalaxies RDCS J1252.9-2927. We have spectroscopically confirmed 38 clustermembers in the redshift range 1.22 < z < 1.25. A cluster median redshift ofz=1.237 and a rest-frame velocity dispersion of 747^{+74}_{-84} km/s areobtained. Using the 38 confirmed redshifts, we were able to resolve, for thefirst time at z > 1, kinematic structure. The velocity distribution, which isnot Gaussian at the 95% confidence level, is consistent with two groups thatare also responsible for the projected east-west elongation of the cluster. Thegroups are composed of 26 and 12 galaxies with velocity dispersions of486^{+47}_{-85} km/s and 426^{+57}_{-105} km/s, respectively. The elongation isalso seen in the intracluster gas and the dark matter distribution. This leadsus to conclude that RDCS J1252.9-2927 has not yet reached a final virial state.We extend the analysis of the color-magnitude diagram of spectroscopic membersto more than 1 Mpc from the cluster center. The scatter and slope ofnon-[OII]-emitting cluster members in the near-IR red sequence is similar tothat seen in clusters at lower redshift. Furthermore, most of the galaxies withluminosities greater than ~ K_s*+1.5 do not show any [OII], indicating thatthese more luminous, redder galaxies have stopped forming stars earlier thanthe fainter, bluer galaxies. Our observations provide detailed dynamical andspectrophotometric information on galaxies in this exceptional high-redshiftcluster, delivering an in-depth view of structure formation at this epoch only5 Gyr after the Big Bang.Comment: 29 pages. 16 figures. ApJ accepted. Tables 2,3 and 5, figure 1 and  the full figure 5 will be available in the paper and electronic editions from  ApJ. v2: minor corrections to the abstract and text to match the Journal's  versio

VLT and ACS Observations of RDCS J1252.9−2927: Dynamical Structure and Galaxy Populations in a Massive Cluster atz= 1.237