The Brightest $z\gtrsim 8$ Galaxies over the COSMOS UltraVISTA Field

Abstract

We present 16 new ultrabright $H_{AB}\lesssim 25$ 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 criteria, combined with strict optical non-detection and SED-fitting criteria, minimizing 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 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 ${\hat{I}}^2$ for the bright z~8 sample is ${\hat{I}}^2=-2.2\pm 0.6$, bluer but still consistent with that of similarly bright galaxies at z~6 (${\hat{I}}^2=-1.55\pm 0.17$) and z~7 (${\hat{I}}^2=-1.75\pm 0.18$). Their typical stellar masses are 10${}^{9.1_{-0.4}^{+0.5}}{M}_{\odot }$, with the SFRs of $32_{-32}^{+44}{M}_{\odot }$/year, specific SFR of $4_{-4}^{+8}$ Gyr${}^{-1}$, stellar ages of $\sim 22_{-22}^{+69}$\,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~8 UV luminosity function (LF). When combined with recent empty field LF estimates at z~8-9, 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 $M^{\ast }=-20.95_{-0.35}^{+0.30}$ mag with a very steep faint end slope $\hat{I}\pm =-2.15_{-0.19}^{+0.20}$. These new candidates include amongst the brightest yet found at these redshifts, 0.5-1.0 mag brighter than found over CANDELS, providing excellent targets for follow-up studies.

This version reflects the accepted manuscript