Goya survey:: U and B number counts in the Groth-Westphal strip

We present U and B galaxy differential number counts from a field of similar to 900 arcmin(2), 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 d log N/ dm 0: 50 +/- 0: 02 for B 21: 0 24: 5 and d log N/ 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 Lambda- dominated cosmology and SDSS local luminosity functions. Only by setting a recent z(f) similar to 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 ( tau(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.