Origin of E+A galaxies – I. Physical properties of E+A galaxies formed from galaxy merging and interaction

We investigate the structural, kinematical and spectrophotometric properties of ‘E+A’ galaxies – those with strong Balmer absorption lines but no significant [O ii] emission – using numerical simulations combined with stellar population synthesis codes. We particularly focus on the two-dimensional (2D) distributions of line-of-sight velocity, velocity dispersion, colour and line index in E+A galaxies formed via the interaction and merging of two gas-rich spirals. Our numerical simulations demonstrate that E+A elliptical galaxies formed by major galaxy merging have positive radial colour gradients and negative radial Hδ gradients by virtue of their central post-starburst populations. Furthermore, we show that the projected kinematical and spectroscopic properties of the simulated E+A galaxies can be remarkably different for different major merger models. For example, the simulated E+A ellipticals with kinematically decoupled cores clearly show regions of strong Hδ absorption which are very flattened, with differences in rotation and velocity dispersion between the old and young stars. E+A ellipticals are highly likely to show more rapid rotation and a smaller central velocity dispersion in young stars than in old ones. E+A galaxies formed from the strong tidal interaction between gas-rich spirals have discy morphologies with thick discs and are highly likely to be morphologically classified as barred S0 galaxies. We also provide specific predictions on the structural, kinematical and spectrophotometric properties of young globular cluster systems in E+A galaxies. Based on these results, we discuss the importance of spatially resolved, integral field unit spectroscopy on large (8–10 m) ground-based telescopes in confirming the formation of kinematically distinct cores in elliptical galaxies produced via dissipative merging and determining the most probable physical mechanism(s) for E+A formation with discy and spheroidal morphologies.