The evolution of the photometric properties of Local Group dwarf spheroidal galaxies

¹ INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy e-mail: fcalura@oats.inaf.it
² Núcleo de Astrofísica Teórica, CETEC, Universidade Cruzeiro do Sul, R. Galvão Bueno 868, Liberdade, 01506-000 São Paulo, SP, Brazil
³ Dipartimento di Astronomia – Universitá di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy

Received: 10 August 2007
Accepted: 11 January 2008

Abstract

Aims. We investigate the present-day photometric properties of the dwarf spheroidal galaxies in the Local Group. From the analysis of their integrated colours, we consider a possible link between dwarf spheroidals and giant ellipticals. From the analysis of the M_V vs. () plot, we search for a possible evolutionary link between dwarf spheroidal galaxies (dSphs) and dwarf irregular galaxies (dIrrs).

Methods. By means of chemical evolution models combined with a spectro-photometric model, we study the evolution of six Local Group dwarf spheroidal galaxies (Carina, Draco, Sagittarius, Sculptor, Sextans and Ursa Minor). The chemical evolution models, which adopt up-to-date nucleosynthesis from low and intermediate mass stars as well as nucleosynthesis and energetic feedback from supernovae type Ia and II, reproduce several observational constraints of these galaxies, such as abundance ratios versus metallicity and the metallicity distributions. The proposed scenario for the evolution of these galaxies is characterised by low star formation rates and high galactic wind efficiencies.

Results. Such a scenario allows us to predict integrated colours and magnitudes which agree with observations. Our results strongly suggest that the first few Gyrs of evolution, when the star formation is most active, are crucial to define the luminosities, colours, and other photometric properties as observed today. After the star formation epoch, the galactic wind sweeps away a large fraction of the gas of each galaxy, which then evolves passively. Our results indicate that it is likely that at a certain stage of their evolution, dSphs and dIrrs presented similar photometric properties. However, after that phase, they evolved along different paths, leading them to their disparate present-day properties.