The ages and metallicities of ellipticals from continuum colors

We present the results of a narrow band imaging project of dwarf and giant ellipticals in the Fornax (z = 0.01), Coma (z = 0.02), A2218 (z= 0.17) and A2125 (z = 0.24) cluster. Differing from spectral line projects, we determine the mean age and metallicity of the underlying stellar populations in galaxies by measurement of the position of the RGB and MS turnoff through continuum colors (3500 Å, 4100 Å, 4675 Å and5500 Å in rest system, i.e. modified Strømgren colors). Our sample includes 120 galaxies between M = –16 and –23 in Fornax and Coma plus over 300 galaxies in distant clusters. We find the color-magnitude relation to be linear for only the brightest galaxies with an increasing amount of scatter for low luminosity ellipticals. Bright ellipticals are found to have a metallicity between –0.5 and +0.5, but low luminosity ellipticals have values that range from –2 to solar. Our age index finds a weak correlation between luminosity and mean stellar age in ellipticals such that bright ellipticals are 2 to 3 Gyrs younger than low luminosity ellipticals.