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 deg2 archival medium-deep (m F160W ∼ 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 . 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.

The Physical Properties of Luminous $z\gtrsim8$ Galaxies and Implications for the Cosmic Star Formation Rate Density From $\sim$0.35 deg$^{2}$ of (Pure-)Parallel HST Observations

“…To construct our photometric data set, we make use of a subset of the SuperBoRG catalogs, which act as a compilation of several, homogeneously-reduced extragalactic HST /WFC3 (pure-)parallel surveys. For full details on the SuperBoRG compilation and data reduction, we refer the reader to Morishita et al (2020) and Morishita (2021), however we also provide a short description here, for convenience. Briefly, the compilation of HST photometry includes pure-parallel observations from the BoRG survey (specifically, Cycles 17, 19, 22 and 25;Trenti et al 2011;Bradley et al 2012;Schmidt et al 2014;Calvi et al 2016;Morishita 2021), Cycle 17 of the Hubble Infrared Pure Parallel Imaging Extragalactic Survey (HIPPIES; Yan et al 2011) and the COS-GTO data set, as well as coordinated-parallel observations from the Cluster Lensing And Supernonva survey with Hubble (CLASH; Postman et al 2012) and the Reionization Lensing Cluster Survey (RELICS; Salmon et al 2018;Coe et al 2019).…”

“…Furthermore, we point out that the F160W filter generally affords lower signal-tonoise (S/N) than the F125W filter for the same exposure time of a flat spectral energy distribution (SED) in F ν . For these reasons, and to remain as complete as possible, we re-run SExtractor over the F125W images in the exact same manner as detailed in Morishita et al (2020) in order to create F125W-derived photometric catalogs which will be publicly-released in a forthcoming data release. All detection images are convolved to the F160W resolution prior to being used with SExtractor and where duplication of sources (for z ∼ 8 dropout searches only) occurs we assume the F125W-detected photometry.…”

“…In some cases, the HST observations of Super-BoRG fields overlap with observations taken with Spitzer /IRAC (140/288 fields, or ∼49% of the search area), providing useful additional constraints with which to constrain the high-z nature of candidates and their rest-frame optical properties. To extract the IRAC photometry, we adopt the same procedure described in Morishita et al (2020): given the lower spatial resolution of the IRAC images, we apply the GALFIT (Peng et al 2002(Peng et al , 2010 tool to the high-resolution HST detection images in order to obtain the structural parameters (radius, Sérsic index, axis ratio) of the high-z sources and use these as fixed parameters to infer the galaxy profiles on the low-resolution IRAC images (magnitude and central position are left as free parameters). Neighbouring sources within a 150 pixel (i.e., 12 arcsec) radius from the high-z source and with > 20% of its brightness are fitted simultaneously, while all other sources are masked.…”

“…Furthermore, an additional consideration is contamination by brown dwarfs, which can mimick the NIR photometry of bright, high redshift galaxies. While such contamination is expected to be relatively low at the redshifts we probe (Morishita et al 2020), some minor contributions remain possible as we do not explicitly exclude point sources here -i.e., we limit the exclusion of point sources to the SExtractor stellarity parameter cut (i.e., exclude sources with stellarity > 0.95).…”

“…The two sources selected for follow up were 0853+0310 112 and 2203+1851 1071, with apparent magnitudes of H 160 ∼ 25 − 25.5 AB and photometric redshifts of z phot = 7.66 and z phot = 7.84, respectively. As mentioned in Section C, the observations of 0853+0310 112 have already been discussed in Morishita et al (2020) and we refer to that paper for details. As a brief summary, however, ∼ 4 hrs of Y -band did not reveal any Lyα emission.…”