Probing the Merger History of Red Early-Type Galaxies with Their Faint Stellar Substructures

Major and minor galaxy mergers form a large number of stellar substructures, ranging from kinematically decoupled cores in the central kpc to faint streams, shells, tails and rings in the far outer regions, as well as massive star clusters. We study the formation of these substructures using cosmological simulations with an efficient zoom‐in technique. We analyze in particular the evolution of substructures, their transient or long‐lived nature, and their survival to subsequent mergers. We also discuss which features can unambiguously probe past mergers, and which ones could have been produced by different evolutionary processes at high redshift. Faint stellar streams are found to be one of the most robust probes of past major and minor mergers. They can survive subsequent mergers, without having their morphological and photometrical properties dramatically modified. The numerous streams surrounding modern early‐type galaxies hence likely trace merger events, and constrain the past occurrence of major and minor mergers, including their age and orbit.