Model colour-magnitude diagrams of elliptical galaxies^*

National Astronomical Observatory of Japan (NAOJ), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan e-mail: chisato.ikuta@nao.ac.jp

Received: 30 December 2005
Accepted: 15 May 2007

Abstract

Context. Formation and evolution of elliptical galaxies are still matters of debate. Knowledge of their star formation histories (SFHs) is essential to understand the formation mechanism. Colour-magnitude (C-M) diagrams of elliptical galaxies should provide one of the most important clues to resolve the SFHs of the elliptical galaxies.

Aims. The aim of this study is to illustrate what information can be obtained from C-M diagrams of elliptical galaxies and what differences appear in theoretical C-M diagrams of different formation scenarios.

Methods. Using a C-M diagram simulator which consistently solves SFH and chemical evolution, we present and discuss six examples of model C-M diagrams. The C-M diagrams can be considered a first approach to the theoretical C-M diagrams of elliptical galaxies of which formation is described as the monolithic collapse and the hierarchical scenarios.

Results. We show that there are three main differences in the C-M diagrams between the two formation scenarios; the magnitude of main sequence turnoff (MSTO), horizontal branch (HB) morphology, and colour distribution of red giant branch (RGB) stars. Since a later epoch of the final star formation is assumed in the hierarchical model, the magnitude of the MSTO is brighter than that of the monolithic collapse. The C-M diagram predicted by the hierarchical scenario shows well-defined blue HB. On the other hand, the C-M diagram shows few blue HB stars in the monolithic collapse case. The colour distributions predicted by the former have bluer peaks than those by the latter. We find that this corresponds to a higher frequency of metal-poor stars in the former model and metal-poor populations are born in the progenitor galaxies. In addition to the model C-M diagrams, we discuss metallicity distributions and α/Fe abundance patterns predicted by the two scenarios.