2005A&A...436...57R

The stellar population of the central region of the galaxy NGC720 has been investigated through longslit optical spectroscopy. The velocity dispersion and 13 Lick indices were obtained as a function of the radius along the semimajor axis of the galaxy. The Mg₂ index presents a gradient of dMg₂/dlogr=-0.079 which behaves similarly to the FeI lines, indicating no enhancement of Mg₂ in relation to FeI. The stellar population ages and metallicities were derived by a population synthesis method using available evolutive spectrophotometric models. The synthesis indicates a strong age gradient along the semimajor axis of NGC720. In the central region a 13Gyr and solar metallicity stellar population dominates the flux at 5870Å; the contribution of this component vanishes at a radius of 0.73kpc, where the total flux is accounted for by a younger, 5Gyr and solar metallicity stellar population. For distances larger than 1kpc a 2.5Gyr component becomes very important. Moreover, NGC720 is probably overabundant in CN in the center with respect to the solar abundance. The estimated total mass (3.29x10¹¹M_☉) and the Mg₂ gradient values suggest that this galaxy might have undergone a merger event. The correspondence between the J-band brightness profile decomposition and the result of the spectral synthesis shows that this galaxy is formed by an old (13-5Gyr), bulge-like, small-scale and massive spheroid and a younger (5-2.5Gyr), large-scale disk component. We discuss our results in comparison with available numerical simulations and propose that the merger event must have occurred about 4Gyr ago.