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

November 2020 • 2020ApJ...904...50M

Authors • Morishita, T. • Stiavelli, M. • Trenti, M. • Treu, T. • Roberts-Borsani, G. W. • Mason, C. A. • Hashimoto, T. • Bradley, L. • Coe, D. • Ishikawa, Y.

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 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 Å. 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 ≳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. * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555 (doi:10.17909/t9-m7tx-qb86).


IPAC Authors


Takahiro Morishita

Assistant Scientist