Assembly of spheroid-dominated galaxies in the EAGLE simulation

Abstract

Context. Despite the insights gained in the last few years, our knowledge about the formation and evolution scenario for the spheroid-dominated galaxies is still incomplete. New and more powerful cosmological simulations have been developed that together withmore precise observations open the possibility of more detailed study of the formation of early-type galaxies (ETGs).Aims. The aim of this work is to analyse the assembly histories of ETGs in a Λ-CDM cosmology, focussing on the archeologicalapproach given by the mass-growth histories.Methods. We inspected a sample of dispersion-dominated galaxies selected from the largest volume simulation of the EAGLE project.This simulation includes a variety of physical processes such as radiative cooling, star formation (SF), metal enrichment, and stellarand active galactic nucleus (AGN) feedback. The selected sample comprised 508 spheroid-dominated galaxies classified according totheir dynamical properties. Their surface brightness profile, the fundamental relations, kinematic properties, and stellar-mass growthhistories are estimated and analysed. The findings are confronted with recent observations.Results. The simulated ETGs are found to globally reproduce the fundamental relations of ellipticals. All of them have an innerdisc component where residual younger stellar populations (SPs) are detected. A correlation between the inner-disc fraction and thebulge-to-total ratio is reported. We find a relation between kinematics and shape that implies that dispersion-dominated galaxies withlow V/σ L (where V is the average rotational velocity and σ L the one dimensional velocity dispersion) tend to have ellipticity smallerthan ∼ 0.5 and are dominated by old stars. On average, less massive galaxies host slightly younger stars. More massive spheroidsshow coeval SPs while for less massive galaxies (stellar masses lower than ∼ 10 10 M ), there is a clear trend to have rejuvenated innerregions, showing an age gap between the inner and the outer regions up to ∼ 2 Gyr, in apparent contradiction with observationalfindings. We find evidences suggesting that both the existence of the disc components with SF activity in the inner region and theaccretion of satellite galaxies in outer regions could contribute to the outside-in formation history in galaxies with low stellar mass.On the other hand, there are non-negligible uncertainties in the determination of the ages of old stars in observed galaxies. Strongersupernova (SN) feedback and/or the action of AGN feedback for galaxies with stellar masses lower than 10 10 M could contribute toprevent the SF in the inner regions.