Galaxy populations and evolution in clusters. I. Dynamics and the origin of low-mass galaxies in the Virgo cluster

Early-type dwarfs are the most common galaxy in the local universe, yet the ir origin and evolution remain a mystery. Various cosmological scenarios pr edict that dwarf-like galaxies in dense areas are the first to form and hen ce should be the oldest stellar systems in clusters. By using radial veloci ties of early-type dwarfs in the Virgo cluster we demonstrate that these ga laxies are not an old cluster population but have signatures of production from the infall of field galaxies. Evidence of this includes the combined l arge dispersions and substructure in spatial and kinematic distributions fo r Virgo early-type dwarfs and a velocity dispersion ratio with giant ellipt icals expected for virialized and accreted populations. We also argue that these galaxies cannot originate from accreted field dwarfs, but must have p hysically evolved from a precursor population, of different morphology, tha t fell into Virgo some time in the past.