Evolution of splashback boundaries and gaseous outskirts: insights from mergers of self-similar galaxy clusters

A self-similar spherical collapse model predicts a dark matter (DM) splashback and accretion shock in the outskirts of galaxy clusters while missing a key ingredient of structure formation - processes associated with mergers. To fill this gap, we perform simulations of merging self-similar clusters and investigate their DM and gas evolution in an idealized cosmological context. Our simulations show that the cluster rapidly contracts during the major merger and the splashback radius r(sp) decreases, approaching the virial radius r(vir). While in the self-similar model r(sp) depends on a smooth mass accretion rate parameter Gamma(s), our simulations show that in the presence of mergers, r(sp) responds to the changes in the total mass accretion rate Gamma(vir), which accounts for both mergers and smooth accretion. The scatter of the Gamma(vir) - r(sp)/r(vir) relation indicates a generally low Gamma(s) similar to 1 in clusters in cosmological simulations. In contrast to the DM, the hot gaseous atmospheres significantly expand by the merger-accelerated (MA-) shocks formed when the runaway merger shocks overtake the outer accretion shock. After a major merger, the MA-shock radius is larger than r(sp) by a factor of up to similar to 1.7 for Gamma(s) less than or similar to 1 and is similar to r(sp) for Gamma(s) greater than or similar to 3. This implies that (1) mergers could easily generate the MA-shock-splashback offset measured in cosmological simulations, and (2) the smooth mass accretion rate is small in regions away from filaments where MA-shocks reside. We further discuss the shapes of the DM haloes, various shocks, and contact discontinuities formed at different epochs of the merger, and the ram-pressure stripping in cluster outskirts.