The build-up of the red sequence in the Hercules cluster

Abstract

We present the study of the colour-magnitude diagram of the cluster Abell 2151 (A2151), with a particular focus on the low-mass end. The deep spectroscopy with AF2/WYFFOS@WHT and the caustic method enable us to obtain 360 members within 1.3 R_200 and absolute magnitude M_r < M*_r+6. This nearby cluster shows a well defined red sequence up to M_r ~ - 18.5; at fainter magnitudes only 36% of the galaxies lie on the extrapolation of the red sequence. We compare the red sequences of A2151 and Abell 85, which is another nearby cluster with similar spectroscopic data, but with different mass and dynamical state. Both clusters show similar red sequences at the bright end (M_r < -19.5), whereas large differences appear at the faint end. This result suggests that the reddening of bright galaxies is independent of environment, unlike the dwarf population (M_r > -18.0).