Niemi, S-M; Heinämäki, P; Nurmi, P; Saar, E; (2010) Formation, evolution and properties of isolated field elliptical galaxies. Monthly Notices of the Royal Astronomical Society , 405 (1) pp. 477-493. 10.1111/j.1365-2966.2010.16457.x.

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Abstract

We study the properties, evolution and formation mechanisms of isolated field elliptical (IfE) galaxies. We create a ‘mock’ catalogue of IfE 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 7 per cent of all elliptical galaxies brighter than −19 mag in B band can be classified as IfE galaxies. Results also show that isolated elliptical galaxies have a rather flat luminosity function; a number density of ∼3 × 10−6 h3 Mpc−3 mag−1, throughout their B-band magnitudes. IfE galaxies show bluer colours than non-isolated elliptical galaxies and they appear younger, in a statistical sense, according to their mass-weighted age. IfE galaxies also form and assemble at lower redshifts compared to non-isolated elliptical galaxies. About 46 per cent of IfE galaxies have undergone at least one major merging event in their formation history, while the same fraction is only ∼33 per cent for non-isolated ellipticals. Almost all (∼98 per cent) isolated elliptical galaxies show merging activity during their evolution, pointing towards the importance of mergers in the formation of IfE galaxies. The mean time of the last major merging is at z∼ 0.6 or 6 Gyr ago for isolated ellipticals, while non-isolated ellipticals experience their last major merging significantly earlier at z∼ 1.1 or 8 Gyr ago. After inspecting merger trees of simulated IfE 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 fulfil observational criteria to be classified as IfE galaxies. This separate population comprises ∼26 per cent of all IfEs.

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