We have exploited the new , deep , near-infrared 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 \simeq 7 .
The year-one UltraVISTA data provide contiguous Y,J,H,K _ { s } imaging over 1.5 deg ^ { 2 } , reaching a 5 \sigma detection limit of Y + J \simeq 25 ( AB mag , 2-arcsec diameter aperture ) .
The central \simeq 1 deg ^ { 2 } of this imaging coincides with the final deep optical ( u ^ { * } ,g,r,i ) data provided by the Canada France Hawaii Telescope ( CFHT ) Legacy Survey and new deep Subaru Suprime-Cam z ^ { \prime } -band imaging obtained specifically to enable full exploitation of UltraVISTA .
It also lies within the Hubble Space Telescope ( HST ) I _ { 814 } -band and Spitzer IRAC imaging obtained as part of the COSMOS survey .
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 ) which we present in this paper .
The first four of these appear to be robust galaxies at z > 6.5 , and fitting to their stacked spectral energy distribution yields z _ { phot } = 6.98 \pm 0.05 with a stellar mass M _ { * } \simeq 5 \times 10 ^ { 9 } { M _ { \odot } } and rest-frame UV spectral slope \beta \simeq - 2.0 \pm 0.2 ( where f _ { \lambda } \propto \lambda ^ { \beta } ) .
The next three are also good candidates for z > 6.5 galaxies , but the possibility that they are dwarf stars can not be completely excluded .
Our final subset of three additional candidates is afflicted not only by potential dwarf-star contamination , but also contains objects likely to lie at redshifts just below z = 6.5 .
We show that the three even-brighter z \gtrsim 7 galaxy candidates reported in the COSMOS field by are in fact all lower-redshift galaxies at z \simeq 1.5 - 3.5 .
Consequently the new z \simeq 7 galaxies reported here are the first credible z \simeq 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 \simeq 7 .