Far-UV and deep surveys: bursting dwarfs versus normal galaxies

Abstract

Galaxy counts from bright ultraviolet (UV) and deep optical spectroscopic surveys have revealed an unexpectedly large number of very blue galaxies. The colors and luminosities of these objects indicate that they are dwarf galaxies undergoing bursts of star formation. We use a galaxy evolution model (PEGASE, Fioc & Rocca-Volmerange 1997) to describe this population as galaxies undergoing cyclical bursts of star formation, thereby determining the luminosity function of these galaxies. When these bursting galaxies are added to normally evolving populations, the combination reproduces the UV number counts, color distributions and deep optical redshift distributions fairly well. Optical (including the Hubble Deep Field) and near-infrared number counts are fitted assuming an open or a flat, Lambda-dominated, Universe. The high amplitude of the angular correlation function of very blue galaxies discovered by Landy et al. (1996) is also recovered in this modelling. The number of bursting galaxies is only a small fraction of the total number of galaxies at optical and near-infrared wavelengths, even at faintest magnitudes. In our evolution modelling, normal galaxies explain most of the blue excess in a low-Omega Universe. The problem of the blue excess remains in a flat Universe without a cosmological constant.