The Bright-end Galaxy Candidates at z ∼ 9 from 79 Independent HST Fields

Abstract

We present a full data analysis of the pure-parallel Hubble Space Telescope (HST) imaging observations in the Brightest of Reionizing Galaxies Survey (BoRG[z9]) in Cycle 22. The medium-deep exposures with five HST/WFC3IR+UVIS filter bands from 79 independent sightlines (∼ 370 arcmin 2 ) provide the least biased determination of number density for z ∼ > 9 bright galaxies against cosmic variance. After a strict twostep selection for candidate galaxies, including dropout color and photometric redshift analyses, and revision of previous BoRG candidates, we identify one source at z ∼ 10 and two sources at z ∼ 9. The z ∼ 10 candidate shows evidence of line-of-sight lens magnification (µ ∼ 1.5), yet it appears surprisingly luminous (M UV ∼ −22.6 ± 0.3 mag), making it one of the brightest candidates at z > 8 known (∼ 0.3 mag brighter than the z = 8.68 galaxy EGSY8p7, spectroscopically confirmed by Zitrin and collaborators). For z ∼ 9 candidates, we include previous data points at fainter magnitudes and find that the data are well fitted by a Schechter luminosity function with α = −2.1 +0.3 −0.3 , M * UV = −21.0 +0.7 −1.4 mag, and log φ * = −4.2 +0.6 −0.9 Mpc −3 mag −1 , for the first time without fixing any parameters. The inferred cosmic star formation rate density is consistent with unaccelerated evolution from lower redshift.

Probing the Bright End of the Rest-frame Ultraviolet Luminosity Function at z = 8–10 with Hubble Pure-parallel Imaging

“…In the right panel of Figure 10 we present the higher redshift bin with the predicted LF shaded from z = 9-10 along with the comparison data from (Calvi et al 2016;Livermore et al 2018;Morishita et al 2018;Bouwens et al 2019). Note that in this figure the redshift of comparative data points from previous results is subdivided in galaxies at redshift z ∼ 9 and z ∼ 10 so that the reader can better observe the distribution of redshifts compared to the luminosity function.…”

“…Therefore we prefer to assume a flat luminosity prior. Here we note that Morishita et al (2018) used a prior in EAZY for their galaxy selections in BoRG[z910]. The impact of including or not including a prior for finding galaxies can be better evaluated in the future and ideally with spectroscopically confirmed galaxies at high-redshift from different studies.…”

“…From M18, we miss their highest redshift source, in field 2140+0241 and at z = 10.0, as we calculate the p(z) of this source to peak at z ∼ 6 with our photometry. We tried an additional EAZY run including the IRAC 3.6µm photometry analyzed and reported in Morishita et al (2018) with a mag 3.6µm = 23.8, where we find a best redshift solution at an even lower redshift of z ∼ 2. In this case we note that our use of EAZY is different compared to M18 as we do not utilize a prior to calculate the P (z).…”

“…In this study we make use of the Cycle 22 BoRG program, known as BoRG [z910], which includes filter coverage that allows for more robust selection of z > 9 galaxies. Previous analyses of BoRG[z910] reported in Calvi et al (2016) and Morishita et al (2018) both find high-redshift galaxies using first a Lyman break color selection, and then apply a photometric redshift measurement. However, the color selection box might exclude potential candidates and therefore in this study we instead rely exclusively on the photometric redshift technique aiming to produce a more complete sample of high-redshift galaxy candidates.…”

A lack of evolution in the very bright end of the galaxy luminosity function from z ≃ 8 to 10

“…With filter-sets especially designed for selection z 8 and z 9 sources, the BoRG and BoRG[z9] surveys have covered an area of approximately 350 arcmin 2 each, allowing constraints on the bright-end of the LF. At z = 9, Morishita et al (2018) used the full BoRG[z9] survey to search for galaxy candidates, finding two sources consistent with being at this redshift. As shown in Fig.…”