On the origin of isophotal shapes in elliptical galaxies

Using semi-analytical models of galaxy formation, the origin of boxy and discy elliptical galaxies is investigated. We find that the simple scenario, motivated by N-body simulations, in which the isophotal shape is only dependent on the mass ratio of the last major merger, is not able to reproduce the observation that the fraction of boxy and discy ellipticals depends on galaxy luminosity. The observations can, however, be reproduced with the following reasonable assumptions: (i) equal-mass mergers lead to boxy ellipticals and unequal-mass mergers produce discy ellipticals (as motivated by N-body simulations); (ii) major mergers between bulge-dominated galaxies result always in boxy ellipticals, independent of the mass ratio; (iii) merger remnants that subsequently accrete gas leading to a secondary stellar disc with more than 20 per cent of the total stellar fraction are always discy. This scenario indicates that the isophotal shapes of merger remnants are sensitive to the morphology of their progenitors and subsequent gas infall. Boxy and discy ellipticals can be divided into two subclasses, depending on their formation history. Boxy ellipticals are formed either by equal-mass mergers of disc galaxies or by major mergers of early-type galaxies. We find that discy ellipticals are indicators of unequal-mass mergers or late gas infall. Discy ellipticals with high luminosities are preferentially one-component systems that result from unequal-mass mergers, whereas low-luminosity discy ellipticals are more likely to harbour secondary disc components. In addition, the fraction of discy ellipticals with secular disc components should increase in regions with higher galaxy densities. Taking into account the conversion of cuspy cores into flat low-density cores by black hole merging, we find that discy ellipticals should contain central density cusps whereas boxy ellipticals should in general be characterized by flat cores. Only rare low-luminosity boxy ellipticals, resulting from equal-mass mergers of disc galaxies, could have power-law cores.