The elliptical galaxy NGC720: An unequal-mass galaxy merger remnant

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 Mg-2 index presents a gradient of dMg(2)/dlogr = -0.079 which behaves similarly to the Fe. lines, indicating no enhancement of Mg-2 in relation to Fe I. 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 NGC 720. In the central region a 13 Gyr and solar metallicity stellar population dominates the flux at 5870 A; the contribution of this component vanishes at a radius of 0.73 kpc, where the total flux is accounted for by a younger, 5Gyr and solar metallicity stellar population. For distances larger than 1 kpc a 2.5 Gyr component becomes very important. Moreover, NGC 720 is probably overabundant in CN in the center with respect to the solar abundance. The estimated total mass (3.29 x 10(11) M circle dot) and the Mg-2 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 - 5 Gyr), 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 4 Gyr ago.