The Brightest z ≳ 8 Galaxies over the COSMOS UltraVISTA Field

We present 16 new ultrabright H-AB less than or similar to 25 galaxy candidates at z similar to 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/JRAC 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 similar to 8 objects, resulting in a total sample of 18 galaxies, 10 of which seem to be fairly robust (with a >97% probability of being at z > 7). The UV-continuum slope beta for the bright z similar to 8 sample is beta = -2.2 +/- 0.6, bluer but still consistent with that of similarly bright galaxies at z similar to 6 (3 = -1.55 +/- 0.17) and z similar to 7 (3 = -1.75 +/- 0.18). Their typical stellar masses are 10(-0.4)(9.1)(+0.5) M-circle dot, with the SFRs of 32(-32)(+44)M(circle dot) yr(-1), specific SFR of 4(-4)(+8) Gyr(-1), stellar ages of similar to 22(-22)(+69) Myr, and low dust content A(V) = 0.15(-0.15)(+0.30) mag. Using this sample we constrain the bright end of the z similar to 8 UV luminosity function. When combined with recent empty field luminosity function estimates at similar redshifts, the resulting z similar to 8 luminosity function can be equally well represented by either a Schechter or a double-power-law form. Assuming a Schechter parameterization, the best-fit characteristic magnitude is M* = -20.95(-0.35)(+0.30) mag with a very steep faint-end slope alpha = 2.15(-0.19)(+0.20) . These new candidates include some of the brightest objects found at these redshifts, 0.5-1.0 magnitude brighter than those found over CANDELS, and providing excellent targets for spectroscopic and longer-wavelength follow-up studies.