HST imaging of the brightest z~8-9 galaxies from UltraVISTA: the extreme
  bright end of the UV luminosity function
  We report on the discovery of three especially bright candidate $z_{phot}
\gtrsim 8$ galaxies. Five sources were targeted for follow-up with HST/WFC3,
selected from a larger sample of 16 bright ($24.8 \lesssim H\lesssim25.5$~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-$K_{S} \sim 25.3-24.8$~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 $z_{phot}\sim8.0-8.7$,
consistent with the initial selection. The remaining 2/5 galaxies have a
nominal $z_{phot}\sim2$. However, if we use the HST data alone, these objects
have increased probability of being at $z\sim9$. Furthermore, we measure mean
UV continuum slopes $\beta=-1.91\pm0.26$ for the three $z\sim8-9$ galaxies,
marginally bluer than similarly luminous $z\sim4-6$ in CANDELS but consistent
with previous measurements of similarly luminous galaxies at $z\sim7$. The
circularized effective radius for our brightest source is $0.9\pm0.2$ kpc,
similar to previous measurements for a bright $z\sim11$ galaxy and bright
$z\sim7$ galaxies. Finally, enlarging our sample to include the six brightest
$z\sim8$ LBGs identified over UltraVISTA (i.e., including three other sources
from Labbe et al. 2017, in prep.) we estimate for the first time the volume
density of galaxies at the extreme bright ($M_{UV}\sim-22$~mag) end of the
$z\sim8$ 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.
