Galaxy harassment and the evolution of clusters of galaxies

NEARBY clusters of galaxies are filled with red elliptical 'E' and lenticular 'SO' galaxies¹, while younger clusters (at redshifts of >~ 0.4) contain substantial populations of blue spiral galaxies with morphological peculiarities^2-7 (see Fig. 1). Thus, within the last 4-5 billion years, galaxies in clusters underwent strong evolution that completely changed their character. By contrast, galaxies that are not associated with clusters show far less morphological evolution⁸. Here we propose that multiple highspeed encounters between galaxies-'galaxy harassment'- drives the morphological evolution in clusters. Our simulations show that these encounters are very different from mergers; they transform small disk galaxies into dwarf elliptical or dwarf spheroidal galaxies. Harassment will leave detectable debris arcs and could provide fuel for quasars in sub-luminous host galaxies.