SuperBoRG: Exploration of Point Sources at z ∼ 8 in HST Parallel Fields*

Abstract

To extend the search for quasars in the epoch of reionization beyond the tip of the luminosity function, we explore point-source candidates at redshift z  ∼ 8 in SuperBoRG, a compilation of ∼0.4 deg 2 archival medium-deep ( m F160 W  ∼ 26.5 ABmag, 5 σ ) parallel infrared (IR) images taken with the Hubble Space Telescope (HST). Initial candidates are selected by using the Lyman-break technique. We then carefully analyze source morphology, and robustly identify three point sources at z  ∼ 8. Photometric redshift analysis reveals that they are preferentially fit by extragalactic templates, and we conclude that they are unlikely to be low- z interlopers, including brown dwarfs. A clear IRAC ch2 flux excess is seen in one of the point sources, which is expected if the source has strong H β +[O iii ] emission with a rest-frame equivalent width of ∼3000 Å. Deep spectroscopic data taken with Keck/MOSFIRE, however, do not reveal Ly α emission from the object. In combination with the estimated H β +[O iii ] equivalent width, we place an upper limit on its Ly α escape fraction of . We estimate the number density of these point sources as ∼1 × 10 −6 Mpc −3 mag −1 at M UV  ∼ −23 mag. The final interpretation of our results remains inconclusive: extrapolation from low- z studies of faint quasars suggests that ≳100× survey volume may be required to find one of this luminosity. The James Webb Space Telescope will be able to conclusively determine the nature of our luminous point-source candidates, while the Roman Space Telescope will probe ∼200 times the area of the sky with the same observing time considered in this HST study.