PROBING THE AGE OF ELLIPTIC GALAXIES

In this paper we address the question whether age and metallicity effe cts can be disentangled with the aid of the broad-band colours and spe ctral indices from absorption feature strengths, so that the age of el liptical galaxies can be inferred. The observational data under examin ation are the indices H-beta and [MgFe], and the velocity dispersion S igma for the sample of galaxies of Gonzales (1993), supplemented by th e ultra-violet data, i.e. the colour (1550-V), of Burstein et al. (198 8). The analysis is performed with the aid of chemo-spectro-photometri c models of elliptical galaxies with infall of primordial gas (aimed a t simulating the collapse phase of galaxy formation) and the occurrenc e of galactic winds. The galaxy models are from Tantalo et al. (1995). The study consists of four parts. In the first one, the aims are outl ined and the key data are presented. In the second part, we summarize the main properties of the infall models that are relevant to our purp oses. In the third part we present the detailed calculations of the sp ectral indices for single stellar populations and model galaxies. To t his aim, we use the analytical relations of Worthey et al. (1994) who give index strengths as a function of stellar parameters. In the last part, we examine the age-metallicity problem. In contrast with previou s interpretations of the H-beta and [MgFe] data as a sort of age seque nce (Gonzales 1993), we find that the situation is more complicate whe n the space of the four variables H-beta, [MgFe], (1550-V), and Sigma is examined. Galaxies in the H-beta and [MgFe] plane do not follow a p ure sequence either of age or metallicity. The observed (1550-V) colou rs are not compatible with young ages. Basically, all the galaxies in the sample are old objects (say as old as 13 divided by 15 Gyr) but ha ve suffered from different histories of star formation. Specifically, it seems that some galaxies have exhausted the star forming activity a t very early epochs with no significant later episodes. Others have co ntinued to form stars for long periods of time. This is perhaps sustai ned by the analysis of the gradients in the H-beta and [MgFe] indices across the galaxies. There are galaxies with no age difference among t he various regions. There are other galaxies in which large gradients in the mean age of the star forming activity between the central and t he peripheral regions seem to exist. The nucleus turns out to be young er and more metal-rich than the outer regions. Finally, there are gala xies in which the nucleus is older but less metal-rich than the extern al regions. All this perhaps hints not only different histories of sta r formation but also different mechanisms of galaxy formation difficul t to pin down at the present time. From the analysis of the H-beta, [M gFe], (1550-V), and Sigma space, and of the age and metallicity gradie nts in single galaxies, the suggestion is advanced that the overall du ration of the star forming activity is inversely proportional to the v elocity dispersion Sigma (and perhaps galactic mass).