Galaxy number counts - V. Ultra-deep counts: The Herschel and Hubble Deep Fields

Abstract

We present u,b,r & i galaxy number counts and colours both from the North and South Hubble Space Telescope Deep Fields and from the William Herschel Deep Field. The latter comprises a 7'x7' area of sky reaching b~28.5 at its deepest. We show that simple Bruzual & Charlot evolutionary models which assume exponentially increasing star-formation rates with look-back time and q_0=0.05 continue to give excellent fits to galaxy counts and colours in the deep imaging data. With q_0=0.5, an extra population of `disappearing dwarf' galaxies is required to fit the optical counts. When we compare the observed and predicted galaxy counts for UV dropouts in the range 2<z<3.5 we find excellent agreement, indicating that the space density of galaxies may not have changed much since z=0 and identifying the Lyman break galaxies with the bright end of the evolved spiral luminosity function. Making the same comparison for B dropout galaxies in the range 3.5<z<4.5 we find that the space density of intrinsically bright galaxies remains the same but the space density of faint galaxies drops by a factor of ~5 compared to the models, consistent with the idea that L* galaxies were already in place at z~4 but that dwarf galaxies may have formed later at 3<z<4.