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 ~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 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 Ks, using a Λ-dominated cosmology and SDSS local luminosity functions. Only by setting a recent zf ~ 1.5 formation redshift for early-type, red galaxies do the models reproduce the change of slope observed at Ks = 17.5 in NIR counts. A moderate optical depth (τB = 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.