Physical properties of z~4 LBGs: differences between galaxies with and without Ly-alpha emission

Abstract

We have analysed the physical properties of z~4 Lyman Break Galaxies observed in the GOODS-S survey, in order to investigate the possible differences between galaxies where the Ly-alpha is present in emission, and those where the line is absent or in absorption. The objects have been selected from their optical color and then spectroscopically confirmed by Vanzella et al. (2005). From the public spectra we assessed the nature of the Ly-alpha emission and divided the sample into galaxies with Ly-alpha in emission and objects without Ly-alpha line (i.e. either absent or in absorption). We have then used the complete photometry, from U band to mid infrared from the GOODS-MUSIC database, to study the observational properties of the galaxies, such as UV spectral slopes and optical to mid-infrared colors, and the possible differences between the two samples. Finally through standard spectral fitting tecniques we have determined the physical properties of the galaxies, such as total stellar mass, stellar ages and so on, and again we have studied the possible differences between the two samples. Our results indicate that LBG with Ly-alpha in emission are on average a much younger and less massive population than the LBGs without Ly-alpha emission. Both populations are forming stars very actively and are relatively dust free, although those with line emission seem to be even less dusty on average. We briefly discuss these results in the context of recent models for the evolution of Lyman break galaxies and Ly-alpha emitters.