The elliptical galaxy NGC 720: An unequal-mass galaxy merger remnant

The stellar population of the central region of the galaxy NGC 720has 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 Mg2index presents a gradient of ${\rm d\,Mg}_2\,/\,{\rm d\,log}\,r=-0.079$which behaves similarly to the $\ion{Fe}{i}$lines, indicating no enhancement of Mg2in relation to $\ion{Fe}{i}$. 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 Å; the contribution of this component vanishes at a radius of 0.73 kpc, where the total flux is accounted for by a younger, 5 Gyr and solar metallicity stellar population. For distances larger than 1 kpc a 2.5 Gyr component becomes very important. Moreover, NGC 720is probably overabundant in CN in the center with respect to the solar abundance. The estimated total mass ($3.29\times10^{11}~M_\odot$) and the Mg2gradient 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 (5–2.5 Gyr), 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.