SuperBoRG: Exploration of Point Sources at z similar to 8 in HST Parallel Fields

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 deg2 archival medium-deep (mF160W ~ 26.5 ABmag, 5σ) 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 Hβ+[O iii] emission with a rest-frame equivalent width of ~3000 A. Deep spectroscopic data taken with Keck/MOSFIRE, however, do not reveal Lyα emission from the object. In combination with the estimated Hβ+[O iii] equivalent width, we place an upper limit on its Lyα escape fraction of ${f}_{\mathrm{esc},\mathrm{Ly}\alpha }\lesssim 2 \% $. We estimate the number density of these point sources as ~1 × 10−6 Mpc−3 mag−1 at MUV ~ −23 mag. The final interpretation of our results remains inconclusive: extrapolation from low-z studies of faint quasars suggests that gsim100× 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.