2001ApJ...552L.101V

We present a study of the galaxy population in the IR-selected cluster RX J0848+4453 at z=1.27, using deep Hubble Space Telescope (HST) NICMOS H_F160W and WFPC2 I_F814W images of the cluster core. We morphologically classify all galaxies to K_s=20.6 that are covered by the HST imaging and determine photometric redshifts using deep ground-based BRIzJK_s photometry. Of 22 likely cluster members with morphological classifications, 11 (50%) are classified as early-type galaxies, nine (41%) as spiral galaxies, and two (9%) as ``merger/peculiar.'' At HST resolution, the second brightest cluster galaxy is resolved into a spectacular merger between three red galaxies of similar luminosity, separated from each other by ~6 h<sup>–1</sup><sub>50</sub> kpc, with an integrated magnitude K=17.6 (∼3L<sub>*</sub> at z=1.27). The two most luminous early-type galaxies also show evidence for recent or ongoing interactions. Mergers and interactions between galaxies are possible because RX J0848+4453 is not yet relaxed. The fraction of early-type galaxies in our sample is similar to that in clusters at 0.5<z<1 and consistent with a gradual decrease of the number of early-type galaxies in clusters from z=0 to z~1.3. We find evidence that the color-magnitude relation of the early-type galaxies is less steep than in the nearby Coma Cluster. This may indicate that the brightest early-type galaxies have young stellar populations at z=1.27 but is also consistent with predictions of single-age ``monolithic'' models with a galactic wind. The scatter in the color-magnitude relation is ~0.04 in rest-frame U-V, similar to that in clusters at 0<z<1. Taken together, these results show that luminous early-type galaxies exist in clusters at z~1.3 but that their number density may be smaller than in the local universe. Additional observations are needed to determine whether the brightest early-type galaxies harbor young stellar populations.