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

We present the study of the colour-magnitude diagram of the cluster Abell 2151 (A 2151), with a particular focus on the low-mass end. The deep spectroscopy withAF2/WYFFOS@WHT and the caustic method enable us to obtain 360 members within 1.3 R-200 and absolute magnitude M-r less than or similar to M*(r) + 6. This nearby cluster shows a well defined red sequence up to M-r similar to -18.5; at fainter magnitudes only 36 per cent of the galaxies lie on the extrapolation of the red sequence. We compare the red sequences of A 2151 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).