Galaxy number counts - V. Ultradeep counts : the Herschel and Hubble deep fields.

We present u, b, r and i galaxy number counts and colours from both the North and South Hubble Space Telescope Deep Fields and the William Herschel Deep Field. The latter comprises a 7×7 arcmin2 area of sky, reaching b28.5 at its deepest. Following Metcalfe et al., we show that simple Bruzual & Charlot evolutionary models which assume exponentially increasing star formation rates with look-back time and q0=0.05 continue to give excellent fits to galaxy counts and colours in the deep imaging data. With q0=0.5, an extra population of 'disappearing dwarf' galaxies is required to fit the optical counts. We further find that the (r−i):(b−r) colour–colour diagrams show distinctive features corresponding to two populations of early- and late-type galaxies which are well fitted by features in the Bruzual & Charlot models. The (r−i):(b−r) data also suggest the existence of an intrinsically faint population of early-types at z0.1 with similar properties to the 'disappearing dwarf' population required if q0=0.5. The outstanding issue remaining for the early-type models is the dwarf-dominated initial mass function (IMF), which we invoke to reduce the numbers of z>1 galaxies predicted at K