The MG index in old galaxy populations - A theoretical approach

The Mg index at 5175 A is studied by using the techniques of spectrum synthesis. Population models based on theoretical luminosity functions permit an abundance calibration of the index in integrated light. The index is shown to be strongly sensitive to metallicity, but only weakly sensitive to the slope of the initial mass function (IMF) and to the Mg isotope ratio. A metallicity difference of a factor of 2.5 is derived between M31 and M32 from Spinrad and Taylor's (1969) scanner observations.