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

• Published in: The Astrophysical journal 2006-03, Vol.639 (2), p.644-671
• Main Authors: Eliche-Moral, M. Carmen, Balcells, Marc, Prieto, Mercedes, García-Dabó, César E, Erwin, Peter, Cristóbal-Hornillos, David
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/499324

We present U and B galaxy differential number counts from a field of 900 arcmin super(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 c 0.02 for B = 21.0-24.5 and d log N/dm = 0.48 d c 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 sub(s), using a -dominated cosmology and SDSS local luminosity functions. Only by setting a recent z sub(f) 1.5 formation redshift for early-type, red galaxies do the models reproduce the change of slope observed at K sub(s) = 17.5 in NIR counts. A moderate optical depth ( sub(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.