The metallicity versus luminosity relationship for early-type galaxies

In the equivalent width (W) vs absolute magnitude (Mʙ) diagram corresponding to the best metallic features in the visible range, CN and Mg + MgH, field giant E and S0 galaxies are shifted towards weaker line-strengths with respect to galaxies belonging to the Virgo and Fornax clusters. We find evidence that this is a genuine stellar population effect. For part of the deviating galaxies it results from an aperture effect, galaxies at larger redshifts including more of the metal poor component outside their nucleus. For the rest, the shift is due to a variable content of intermediate age components in the range 4-10 Gyr, superimposed on the very old underlying population.