2006ApJ...639..644E

We present U and B galaxy differential number counts from a field of ∼900 arcmin², based on GOYA Survey imaging of the HST Groth-Westphal strip. Source detection efficiency corrections as a function of the object size have been applied. A variation of the half-exposure image method has been devised to identify and remove spurious detections. Achieved 50% detection efficiencies are 24.8 mag in U and 25.5 mag in B in the Vega system. Number count slopes are dlogN/dm=0.50±0.02 for B=21.0-24.5 and dlogN/dm=0.48±0.03 for U=21.0-24.0. Simple number count models are presented that simultaneously reproduce the counts over 15 mag in U and B and over 10 mag in K_s, using a Λ-dominated cosmology and SDSS local luminosity functions. Only by setting a recent z_f∼1.5 formation redshift for early-type, red galaxies do the models reproduce the change of slope observed at K_s=17.5 in NIR counts. A moderate optical depth (τ_B=0.6) for all galaxy types ensures that the recent formation for elliptical galaxies does not leave a signature in the U or B number counts, which are featureless at intermediate magnitudes. No ad hoc disappearing populations are needed to explain the counts if number evolution is introduced using an observationally based z evolution of the merger fraction.