Formation Pathways of the Compact Stellar Systems

Abstract The formation pathways of compact stellar systems (CSSs) are still under debate. We utilize the NewHorizon simulation to investigate the origins of such objects in the field environment. We identified 55 CSS candidates in the simulation whose properties are similar to those of the observed ultracompact dwarfs and compact ellipticals (cEs). All but two most massive objects (cE candidates) are a result of a short starburst. Sixteen are formed by tidal stripping, while the other 39 are intrinsically compact from their birth. The stripped objects originate from dwarf-like galaxies with a dark halo, but most of their dark matter is stripped through their orbital motion around a more massive neighbor galaxy. The 39 intrinsically compact systems are further divided into associated or isolated groups, depending on whether they were born near a massive dark halo or not. The isolated intrinsically compact objects (seven) are born in a dark halo and their stellar properties are older and metal-poor compared to the associated counterparts (32). The stripped compact objects occupy a distinct region in the age–metallicity plane from the intrinsically compact objects. The associated intrinsically compact objects in our sample have never had a dark halo; they are the surviving star clumps of a massive galaxy.