The Brightest $z\gtrsim8$ Galaxies over the COSMOS UltraVISTA Field.

We present 16 new ultrabright $H_{AB}\lesssim25$ galaxy candidates at z~8 identified over the COSMOS/UltraVISTA field. The new search takes advantage of the deepest-available ground-based optical and near-infrared observations, including the DR3 release of UltraVISTA and full-depth Spitzer/IRAC observations from the SMUVS and SPLASH programs. Candidates are selected using Lyman-break color criteria, combined with strict optical non-detection and SED-fitting criteria, designed to minimize contamination by low-redshift galaxies and low-mass stars. HST/WFC3 coverage from the DASH program reveals that one source evident in our ground-based near-IR data has significant substructure and may actually correspond to 3 separate z~8 objects, resulting in a total sample of 18 galaxies. The UV-continuum slope $\beta$ for the bright z~8 sample is $\beta=-2.2\pm0.6$, bluer but still consistent with that of similarly bright galaxies at z~6 ($\beta=-1.55\pm0.17$) and z~7 ($\beta=-1.75\pm0.18$). Their typical stellar masses are 10$^{9.1^{+0.5}_{-0.4}}$ $M_{\odot}$, with SFRs of $32^{+44}_{-32}M_{\odot}$/year, specific SFR of $4^{+8}_{-4}$ Gyr$^{-1}$, stellar ages of $\sim22^{+69}_{-22}$ Myr, and low dust content A$_V=0.15^{+0.30}_{-0.15}$ mag. Using this sample we constrain the bright end of the z~8 UV luminosity function (LF). When combined with recent empty field LF estimates at similar redshifts, the resulting z~8 LF can be equally well represented by either a Schechter or a double power-law (DPL) form. Assuming a Schechter parameterization, the best-fit characteristic magnitude is $M^*=-20.95^{+0.30}_{-0.35}$ mag with a very steep faint end slope $\alpha=-2.15^{+0.20}_{-0.19}$. These new candidates include amongst the brightest yet found at these redshifts, 0.5-1.0 magnitude brighter than found over CANDELS, providing excellent targets for spectroscopic and longer-wavelength follow-up studies.