Metallicity gradients in early-type galaxies.

Long-slit spectra in the range 4500-6500 A have been obtained for a sample of 42 galaxies. Rotation velocities and velocity dispersions, together with radial line strength gradients of Mg_2_, Mg_1_, Hβ, NaD, TiO_1_, TiO_2_ and Fe_5270_, have been measured to, on average, half an effective radius. To a high level of significance, Mg_2_ gradients positively correlate with those of Mg_1_ and Na D, but no correlation is observed with the other indices. In addition, correlations of Mg_2_ gradients with various physical parameters are studied. For galaxies smaller than about 10^11^ M_sun_, the Mg_2_ gradient increases with increasing mass, but more massive objects show no correlation. In low- mass galaxies, the correlation of Mg_2_ gradients with mass suggests dissipative collapse as the mechanism acting during the initial star formation episode. Different formation mechanisms, such as, for example, a less dissipative collapse and/or merging of smaller systems, are discussed as a means of explaining the lack of correlation at higher masses. Even at low masses, the galactic mass seems not to influence either the position or the slope on the Fe_5270_-Mg_2_ plane, abundance ratios within galaxies depending on still-unidentified parameters.