In the current paradigm of galaxy formation, baryons are embedded in dark matter haloes which grow through merger events across cosmic time. The properties and subsequent evolution of baryons depend on the halo they live in. In particular, clusters of galaxies which lie at the high mass end of the halo mass function, contain both the most massive galaxies in the Universe (BGCs) and the smallest ones (dwarfs).

Dwarfs (with bolometric magnitude M_b > -18) are the most abundant type of galaxies in the Universe, and their properties differ significantly between cluster and field environments. The fact that red dwarfs are largely more abundant in the former has led to the realisation that the environment preferentially acts in halting the star formation. However, it is not clear what are the main physical processes responsible for this transformation and on what time scales they operate.

Deep spectroscopy studies of galaxy cluster members allow us to simultaneously constrain their abundances, their star formation histories and their orbital properties. Thanks to recent improvements on instruments and telescopes, the study of dwarfs in nearby clusters is reliable, helping disentangle the processes that drive their formation and evolution. With my collaborators, I have undertaken a campaign of deep spectroscopic observations in the optical band of nearby clusters down to the dwarf mass. This unique data set allowed us the analysis of several properties of the dwarf galaxy population, minimising the contribution of background sources.

For this project, we selected two nearby clusters, Abell 85 (A85, z=0.055) and Abell 2151 (A2151, z=0.036). Their bright galaxies and other properties like X-ray emission were deeply studied in the literature. With our new observations, we reached r-band magnitude M_r = -16.0. Our new data set allows us to determine 460 and 360 members out to 1.4 and 1.3 R_200 for A85 and A2151, respectively.

 A85 is massive and almost dynamically relaxed, while A2151 is an irregular and spiral-rich cluster with strong evidence that it is undergoing a merger. Their dynamical state, mass and composition place them at the two extremes of cluster formation and evolution, making them the perfect candidates for this project.
 
To put constrains on the formation and evolution of cluster galaxies, we study the build-up of the galaxy red sequence, the luminosity function (LF), -- i.e. the density of the galaxies of a given luminosity -- and the orbital properties of the cluster members. In particular, we analyse them considering different galaxy populations depending on their stellar colour, their luminosity, and their dynamical properties.

The main conclusions of this work are:
1) For the bright population, we observe an invariance of the red sequence when comparing A85 to A2151. The LFs in radial bins show variations at intermediate luminosities in A85, while the same analysis for A2151 presents an excess of bright galaxies. Moreover, the anisotropy-luminosity segregation is visible for the bright (dwarfs) galaxies in A85 (A2151). Even if these results seem contradictory, for bright galaxies they all point towards a star formation history controlled by mass quenching and an evolution driven by dynamical friction, which depends on the galaxy mass, the strength of the cluster gravitational potential and the time spent in it.
2) The faint end of the LF in A85 is dominated by the red population, unlike A2151 and the field. However, the slopes of the LFs of the dominant populations are compatible within the uncertainties. Therefore, the quenching of the star formation mainly depends on external physical processes, environmental quenching. The orbits of dwarfs are isotropic in the centre of the two clusters where a selective destruction could occur. However, the fraction of orbital time spent by dwarfs near the cluster centre can modify their orbits and/or remove their gas reservoirs affecting their star formation.
3) The colour-anisotropy segregation observed in A85 and the comparison with some theoretical parameters for virialized systems suggest that blue galaxies have not been recently accreted into this cluster. Indeed, it is evident that they reached virial equilibrium within the cluster potential.

Even if a wider sample of observed clusters is needed, we confirm that dynamical friction and environmental quenching are the major drivers of the evolution of bright and low-massive galaxies, respectively, as other literature studies indicate.