Relaxed blue ellipticals: accretion-driven stellar growth is a key evolutionary channel for low mass elliptical galaxies

Abstract

How elliptical galaxies form is a key question in observational cosmology. While the formation of massive ellipticals is strongly linked to mergers, the low mass (M⋆/M⊙ < 10^9.5 regime remains less well explored. In particular, studying elliptical populations when they are blue, and therefore rapidly building stellar mass, offers strong constraints on their formation. Here, we study 108 blue low-mass ellipticals (which have a median stellar mass of 10^8.7 M⊙) at z < 0.3 in the COSMOS field. Visual inspection of extremely deep optical HSC images indicates that less than 3 per cent of these systems have visible tidal features, a factor of 2 less than the incidence of tidal features in a control sample of galaxies with the same distribution of stellar mass and redshift. This suggests that the star formation activity in these objects is not driven by mergers or interactions but by secular gas accretion. We combine accurate physical parameters from the COSMOS2020 catalogue, with measurements of local density and the locations of galaxies in the cosmic web, to show that our blue ellipticals reside in low-density environments, further away from nodes and large-scale filaments than other galaxies. At similar stellar masses and environments, blue ellipticals outnumber their normal (red) counterparts by a factor of 2. Thus, these systems are likely progenitors of not only normal ellipticals at similar stellar mass but, given their high star formation rates, also of ellipticals at higher stellar masses. Secular gas accretion therefore likely plays a significant (and possibly dominant) role in the stellar assembly of elliptical galaxies in the low-mass regime.