GALAXY HARASSMENT AND THE EVOLUTION OF CLUSTERS OF GALAXIES

NEARBY clusters of galaxies are filled with red elliptical 'E' and len ticular 'S0' galaxies(1), while younger clusters (at redshifts of grea ter than or similar to 0.4) contain substantial populations of blue sp iral galaxies with morphological peculiarities(2-7) (see Fig. 1), Thus , within the last 4-5 billion years, galaxies in clusters underwent st rong evolution that completely changed their character, By contrast, g alaxies that are not associated with clusters show far less morphologi cal evolutions(8). Here we propose that multiple highspeed encounters between galaxies-'galaxy harassment'-drives the morphological evolutio n in clusters, Our simulations show that these encounters are very dif ferent from mergers; they transform small disk galaxies into dwarf ell iptical or dwarf spheroidal galaxies, Harassment will leave detectable debris arcs and could provide fuel for quasars in sub-luminous host g alaxies.