Metallicity gradients in early-type galaxies

Abstract

Long-slit spectra in the range 4500–6500 Å have been obtained for a sample of 42 galaxies. Rotation velocities and velocity dispersions, together with radial line strength gradients of |$\text {Mg}_{2},\enspace \text {Mg}_{1},\enspace \text H \beta ,\enspace \text {Na}\enspace \text D,\enspace \text {TiO}_{1},\enspace \text {TiO}_{2}\enspace \text {and}\enspace \text {Fe}_{5270},$| have been measured to, on average, half an effective radius. To a high level of significance, Mg2 gradients positively correlate with those of Mg1 and Na D, but no correlation is observed with the other indices. In addition, correlations of Mg2 gradients with various physical parameters are studied. For galaxies smaller than about |${10}^{11}\text {M}_{\odot},\enspace \text {the}\enspace \text {Mg}_{2}$| gradient increases with increasing mass, but more massive objects show no correlation. In low-mass galaxies, the correlation of Mg2 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 |$\text {Fe}_{5270}-\text {Mg}_{2}$| plane, abundance ratios within galaxies depending on still-unidentified parameters.