We report on the discovery of three especially bright candidate z _ { \mathrm { phot } } \gtrsim 8 galaxies .
Five sources were targeted for follow-up with HST /WFC3 , selected from a larger sample of 16 bright ( 24.8 \lesssim H \lesssim 25.5 Â mag ) candidate z \gtrsim 8 LBGs identified over the 1.6 degrees ^ { 2 } of the COSMOS/UltraVISTA field .
These were identified as Y and J dropouts by leveraging the deep ( Y -to- K _ { \mathrm { S } } \sim 25.3 - 24.8 Â mag , 5 \sigma ) NIR data from the UltraVISTA DR3 release , deep ground based optical imaging from the CFHTLS and Subaru Suprime Cam programs and Spitzer /IRAC mosaics combining observations from the SMUVS and SPLASH programs .
Through the refined spectral energy distributions , which now also include new HyperSuprime Cam g,r,i,z, and Y band data , we confirm that 3/5 galaxies have robust z _ { \mathrm { phot } } \sim 8.0 - 8.7 , consistent with the initial selection .
The remaining 2/5 galaxies have a nominal z _ { \mathrm { phot } } \sim 2 .
However , if we use the HST data alone , these objects have increased probability of being at z \sim 9 .
Furthermore , we measure mean UV continuum slopes \beta = -1.91 \pm 0.26 for the three z \sim 8 - 9 galaxies , marginally bluer than similarly luminous z \sim 4 - 6 in CANDELS but consistent with previous measurements of similarly luminous galaxies at z \sim 7 .
The circularized effective radius for our brightest source is 0.9 \pm 0.2 kpc , similar to previous measurements for a bright z \sim 11 galaxy and bright z \sim 7 galaxies .
Finally , enlarging our sample to include the six brightest z \sim 8 LBGs identified over UltraVISTA ( i.e. , including three other sources from Labbé et al . 58 ) we estimate for the first time the volume density of galaxies at the extreme bright ( M _ { \mathrm { UV } } \sim - 22  mag ) end of the z \sim 8 UV LF .
Despite this exceptional result , the still large statistical uncertainties do not allow us to discriminate between a Schechter and a double power-law form .