Stellar population analysis of the red and blue galaxies in the distant cluster Abell 370

Abstract

In this paper, we apply a population synthesis method based on star cluster integrated spectral properties to members of the distant galaxy cluster Abell 370. Within boxes defined from magnitude vs colour diagrams, we have co-added the data corresponding to members with similar spectral characteristics, in order to achieve a signal to noise ratio suitable for the synthesis. Population synthesis of the two blue averaged objects provides the flux and mass fractions of the young components responsible for their blue colours. These are the disc component in late spiral galaxies for one group and a recent starburst for the other group. Population synthesis of the three red averaged objects confirms that no cosmological evolutionary effects are observed in this cluster at z = 0.374. In ali three cases, we find a metallicity enrichment lower than that expected on the basis of the metallicity vs luminosity relationship found for the central regions of nearby galaxies. For the less luminous objects, which correspond to present-day massive elliptical galaxies, this reflects the contamination by light from their metal-poor halo population, as a result of Abell 370 distance. For the more luminous objects, which correspond rather to cD and extremely massive galaxies, their low metallic content implies that they were formed through merging processes occurring once the acting galaxies had already achieved the bulk of their star formation process and chemical evolution.