The Role of Clustering of Subclumps in Bright Elliptical Galaxy Formation from a Low‐Spin Seed Galaxy

We reveal the role of clustering of sub-clumps, which is expected in the colddark matter (CDM) universe, in forming a bright elliptical galaxy (BEG) from alow-spin seed galaxy. This can be done by comparing the evolution of a low-spin seed galaxyincluding small-scale density fluctuations expected in the CDM universe (Model 1) with that of a completely uniform one (Model 2), using numericalexperiments. We show that Model 2 cannot reproduce the properties of BEGs and forms a diskwhich is too compact and too bright due to the conservation of theinitial-small angular momentum. In Model 1 clustering of the sub-clumps caused by initial small-scale densityfluctuations leads to angular momentum transfer from the baryon component tothe dark matter and consequently a nearly spherical system supported by randommotions is formed. Moreover the collisionless property of the stars formed in the sub-clumpsprevents the dissipative contraction of the system, leading to a large measuredhalf-light radius. As a result, the end-product is quite well reproduces the observed propertiesof BEGs, such as the de Vaucouleurs light-profile, typical color andmetallicity gradients, the large half-light radius, the small ratio of therotational velocity to the velocity dispersion (V/\sigma). We conclude that theclustering of sub-clumps, i.e., the hierarchical clustering, plays a crucialrole in the formation of BEGs from a low-spin seed galaxy.