The Sizes of z ∼ 9-10 Galaxies Identified in the Brightest of Reionizing Galaxies (BoRG) Survey

Abstract

Redshift z = 9-10 object selection is the effective limit of Hubble Space Telescope (HST) imaging capability, even when confirmed with Spitzer. If only a few photometry data points are available, it becomes attractive to add criteria based on their morphology in these J- and H-band images. One could do so through visual inspection, a size criterion, or alternate morphometrics. We explore a vetted sample of Brightest of Reionizing Galaxies (BoRG) z ∼ 9 and z ∼ 10 candidate galaxies and the object rejected by Morishita+ to explore the utility of a size criterion in z = 9-10 candidate selection. A stringent, point-spread function (PSF)-corrected effective radius criterion ( ${r}_{e}\lt 0\buildrel{\prime\prime}\over{.} 3$ ) would result in the rejection of 65%-70% of the interlopers visually rejected by Morishita et al. It may also remove up to ∼20% of bona fide brightest ( $L\gg {L}^{* }$ ) z = 9 or 10 candidates from a BoRG selected sample based on the Mason et al. luminosity functions, assuming the Holwerda et al. $z\sim 9$ size-luminosity relation. We argue that including a size constraint in lieu of a visual inspection may serve in wide-field searches for these objects in, e.g., Euclid or HST archival imaging with the understanding that some brightest ( $L\gg {L}^{* }$ ) candidates may be missed. The sizes of the candidates found by Morishita et al. follow the expected size distribution of z ∼ 9 for bright galaxies, consistent with the log normal in Shibuya et al. and single objects. Two candidates show high star formation surface density ( ${{\rm{\Sigma }}}_{\mathrm{SFR}}\gt 25{M}_{\odot }\,{\mathrm{kpc}}^{-2}$ ) and all merit further investigation and follow-up observations.