GOYA Survey: U and B Number Counts in the Groth-Westphal Strip

Abstract

We present U and B galaxy differential number counts from a field of ~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 ~ 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 (tau_B = 0.6) for all galaxy types ensures that the recent formation for ellipticals 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.