The Magnesium Mg/2 Index as an Indicator of Metallicity in Elliptical Galaxies

Abstract

A quantitative calibration of the Mg_2_ index [Faber et al.,AJ,82,941(1977)] is attempted deriving a metallicity scale for elliptical galaxies. The dependence of the index on stellar temperature,gravity and metallicity has been studied through spectroscopic observations of 87 standards applying the derived calibration to models for stellar population synthesis. Buzzoni's [ApJS, 71,817(1989)] computational code has been used to explore the behavior of the index versus age, IMF, and metallicity of simple stellar populations (SSPs) inferring galactic metallicity for the Davies et al. [ApJS, 69, 581(1987)] extensive observational database. It appears that ellipticals are old metal-rich systems, with age about 15 Gyr and (Fe/HI = +0.15. A large spread of nearly one order of magnitude is however derived for metallicity among single galaxies confirming that [Fe/H] is the driving parameter inducing the color spread in the galaxy population. Evolutionary behavior of Mg_2_ is briefly discussed giving its expected variations at early epochs for comparison with high-redshift galaxies.