Formation, evolution and properties of isolated field elliptical galaxies

Abstract

[Abridged] We study the properties, evolution and formation mechanisms of isolated field elliptical galaxies. We create a mock catalogue of isolated field elliptical galaxies from the Millennium Simulation Galaxy Catalogue, and trace their merging histories. The formation, identity and assembly redshifts of simulated isolated and non-isolated elliptical galaxies are studied and compared. Observational and numerical data are used to compare age, mass, and the colour-magnitude relation. Our results, based on simulation data, show that almost seven per cent of all elliptical galaxies brighter than -19mag in B-band can be classified as isolated field elliptical galaxies. Isolated field elliptical galaxies show bluer colours than non-isolated elliptical galaxies and they appear younger, in a statistical sense, according to their mass weighted age. Isolated field elliptical galaxies also form and assemble at lower redshifts compared to non-isolated elliptical galaxies. About 46 per cent of isolated field elliptical galaxies have undergone at least one major merging event in their formation history, while the same fraction is only about 33 per cent for non-isolated ellipticals. The mean time of the last major merging is z = 0.6 or 6 Gyrs ago for isolated ellipticals, while non-isolated ellipticals experience their last major merging significantly earlier at z = 1.1 or 8 Gyrs ago. After inspecting merger trees of simulated isolated field elliptical galaxies, we conclude that three different, yet typical formation mechanisms can be identified: solitude, coupling and cannibalism. Our results also predict a previously unobserved population of blue, dim and light galaxies that fulfill observational criteria to be classified as isolated field elliptical galaxies. This separate population comprises about 26 per cent of all IfEs.