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

¹ Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, Porto Alegre, Rio Grande do Sul, Brazil e-mail: rembold@if.ufrgs.br
² Centro de Investigaciones de Astronomía, AP 264, Mérida 5101-A, Venezuela

Received: 1 December 2004
Accepted: 8 February 2005

Abstract

The stellar population of the central region of the galaxy NGC 720 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 which behaves similarly to the lines, indicating no enhancement of Mg₂ in relation to . 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 720 is probably overabundant in CN in the center with respect to the solar abundance. The estimated total mass () 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 (13–5 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.