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We determine the bright end of the rest-frame UV luminosity function (UVLF)at $z=8-10$ by selecting bright $z\gtrsim 8$ photometric candidates from thelargest systematic compilation of HST (pure-)parallel observations to date, theSuper-Brightest-of-Reionizing-Galaxies (SuperBoRG) data set. The data setincludes $\sim300$ independent sightlines from WFC3 observations, totalling$800-1300$ arcmin$^2$ (depending on redshift). We identify 31 $z\gtrsim8$candidates via colour selection and photo-$z$ analysis with observed magnitude($24.1< H_{160} <26.6$) and $1\sigma$ range probability of $58-95\%$ of beingtrue high-z galaxies. Following detailed completeness and source recoverysimulations, as well as modelling of interloper contamination, we deriverest-frame UVLFs at $z=8-10$ down to $M_{UV}\simeq-23$. We find that the brightend of the galaxy luminosity function can be described both by a Schechter andby a double power-law function, with our space-based large area determinationshowing some tentative discrepancies with the luminosity functions derived fromground-based observations at the same redshifts. We identify two possibleexplanations. The first is that the UVLF includes a substantial contributionfrom AGNs at magnitudes $M_{UV}<-22$. The UVLF we derive at $z=8$ is consistentwith no evolution of the bright end from $z=6-7$ UVLFs that include AGNs. Analternative (and non-exclusive) possibility is that the dust content of theobserved galaxies decreases with redshift, thereby countering the naturallyexpected decrease in the UV luminosity functions because of a decreased starformation rate with increasing redshift. Both scenarios raise interestingprospects to further understand galaxy formation in extreme objects during theepoch of reionisation.

The UV luminosity functions of Bright z>8 Galaxies: Determination from ~0.41 deg2 of HST Observations along ~sim 300 independent sightlines