Early-type galaxies are most common object in the universe, especially in the clusters. However, it is unclear how they formed, how they assembled, how their star formation stopped.
Stellar population studies using deep spectra give us hints about above-mentioned quesitons, but in fact it is difficult observation and analysis due to faint luminosity and age-metallicity degenearcy. Galaxies in clusters are relatively well observed and their luminosity-weighted age and metallicity are determined from those spectra, but there are few works in the low density region. Galaxies in low density region at high redshift are the interesting objects to know the mechanism of star formation quenching of galaxies and the formation of clusters.
We obtained deep optical spectra of about 10 red galaxies in the "infall region" (sub-clump) of a z=0.83 cluster, RX J0152.7-1357 using Gemini-S / GMOS. We found stronger Balmer absortion lines for the "group" early-type galaxies with respect to the "cluster" counter-parts, and we quantified the epoch and the sharpness of the star formation quenching in these galaxies from their spectroscopic age and metallicity.