Discovery of bright z ~ 7 galaxies in the UltraVISTA survey

Abstract

We have exploited the new, deep, near-infrared Y,J,H,K_s UltraVISTA imaging of the COSMOS field, in tandem with deep optical and mid-infrared imaging, to conduct a new search for luminous galaxies at redshifts z ≃ 7. We have utilised this unique multi-wavelength dataset to select galaxy candidates at redshifts z > 6.5 by searching first for Y+J-detected objects which are undetected in the CFHT and HST optical data. This sample was then refined using a photometric redshift fitting code, enabling the rejection of lower-redshift galaxy contaminants and cool galactic M, L, T dwarf stars. The final result of this process is a small sample of (at most) ten credible galaxy candidates at z > 6.5 (from over 200,000 galaxies detected in the year-one UltraVISTA data). The new z ≃ 7 galaxies reported here are the first credible z ≃ 7 Lyman-break galaxies discovered in the COSMOS field and, as the most UV-luminous discovered to date at these redshifts, are prime targets for deep follow-up spectroscopy. We explore their physical properties, and briefly consider the implications of their inferred number density for the form of the galaxy luminosity function at z ≃ 7.