The origin of the CMR in Virgo

Abstract

We study the origin of the colour magnitude relation (CMR) in theVirgo cluster. The CMR could be either caused by a variation of the mean stellar metallicity along the sequence (Kodama and Arimoto, 1997) or by a combination of age and metallicity variations (Ferreras, Charlot and Silk, 1999). This uncertainty is due to the fact that the integrated light of the stellar populations suffers from a degeneracy between the age and the metallicity, i.e., the two effects cannot easily be separated using colours or absorption line-strengths (Worthey, 1994).We use spectra of exceptional high S/N (obtained at the WHT 4.2m telescope, La Palma) for six elliptical galaxies well selected along the CMR of Bower, Lucey and Ellis (1992a,b) following their best-fit regression line. The data are analysed using a new evolutionary stellar population synthesis model which has the ability to study galaxy spectra at the resolution given by their velocity dispersions (Vazdekis, 1999). We estimate the mean stellar ages for these galaxies using a new Hγ age indicator which is completely insensitive to the metallicity (in the range –0.7<[Fe/H]<+0.2) for old stellar population (Vazdekis and Arimoto, 1999). We find that the luminosity weighted mean stellar ages for all these galaxies are larger than ∼8 Gyr, showing a significant spread in age but without a clear trend as a function of galaxy luminosity or colour. Taking into account these ages and analising the strengths of several metal lines we find a positive correlation of the total metallicity with luminosity, colour and velocity dispersion. However, while various metal-indicators dominated by elements such as Mg or CN yield very steep index-luminosity relations, others such as Fe or Ca show modest trends. An extensive analysis is presented in Vazdekis et al. (2000).