The elliptical galaxies NGC 1052 and NGC 7796 ^*

Stellar populations and abundance ratio α/Fe

¹ Instituto Nacional de Pesquisas Espaciais (INPE), Coordenação de Cências Espaciais e Atmosféricas, Divisão de Astrofísica, Av. dos Astronautas 1758, São José dos Campos, SP 12227-010, Brazil e-mail: acmilone@das.inpe.br
² Universidade Federal do Rio Grande do Sul (UFRGS), Instituto de Física, Departamento de Astronomia, Av. Bento Gonçalves 9500, Porto Alegre, RS 91501-970, Brazil

Received: 30 November 2005
Accepted: 22 March 2007

Abstract

Context.Understanding how each early-type galaxy forms and evolves is one of the objectives of extragalactic astrophysics and cosmology. The spatial distribution of the stellar populations inside a spheroidal system and their kinematic properties supply important information about the formation process. The reconstruction of the star formation history is crucial in this context.

Aims.We have performed a detailed stellar population analysis using long slit spectroscopic observations up to almost one effective radius of two different early-type galaxies of low density regions of the local Universe: NGC 1052, an E4 Liner prototype of a loose group that has a stellar rotating disc, and NGC 7796, an E1 of the field which shows a kinematically distinct core. Their mean luminosity-weighted stellar age, metallicity, and α/Fe ratio along both photometric axes have been obtained to reconstruct the star formation history in their kinematically distinct subsystems.

Methods.We have measured Lick indices and computed their radial gradients. They were compared with the predicted ones of simple stellar population models. We have also applied a stellar population synthesis.

Results.The star characteristics are associated with their kinematics: they are older and α-enhanced in the bulge of NGC 1052 and core of NGC 7796, while they show a strong spread of α/Fe and age along the disc of NGC 1052 and an outward radial decrease outside the core of NGC 7796. The age variation is possibly connected to the α/Fe one.

Conclusions.Both galaxies were formed by processes in which the star formation occurred first at the bulge (NGC 1052) and nucleus (NGC 7796) 12-15 Gyr ago on short timescales (0.1-1 Gyr), providing an efficient chemical enrichment by SN-II. In the disc of NGC 1052, there is some spread of age and formation timescales around its stars. In NGC 7796, the star formation timescale had some outward radial increases along both axes.