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Register a new userThe Sizes of $zsim9-10$ Galaxies Identified in the BoRG Survey
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Benne W. Holwerda
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Redshift $z=9--10$ object selection is the effective limit of Hubble Space Telescope 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 BoRG $zsim9$ and $zsim10$ candidate galaxies and the object rejected by Morishita+ (2018) to explore the utility of a size criterion in z=9-10 candidate selection. A stringent, PSF-corrected effective radius criterion ($r_e<0farcs3$) would result in the rejection of 65-70% of the interlopers visually rejected by Morishita+. It may also remove up to $sim20$% of bona-fide brightest ($L>>L^*$) z=9 or 10 candidates from a BoRG selected sample based on the Mason+ (2015) luminosity functions, assuming the Holwerda+ (2015) $zsim9$ 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 ($L>>L^*$) candidates may be missed. The sizes of the candidates found by Morishita+ (2018) follow the expected size distribution of $zsim9$ for bright galaxies, consistent with the lognormal in Shibuya+ (2015) and single objects. Two candidates show high star-formation surface density ($Sigma_{SFR} > 25 M_odot/kpc^2$) and all merit further investigation and follow-up observations.
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The Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) enabled the search for the first galaxies observed at z ~ 8 - 11 (500 - 700 Myr after the Big Bang). To continue quantifying the number density of the most luminous galaxies (M_AB ~ -22.0) at the earliest epoch observable with HST, we search for z ~ 10 galaxies (F125W-dropouts) in archival data from the Brightest of Reionizing Galaxies (BoRG[z8]) survey, originally designed for detection of z ~ 8 galaxies (F098M-dropouts). By focusing on the deepest 293 arcmin^2 of the data along 62 independent lines of sight, we identify six z ~ 10 candidates satisfying the color selection criteria, detected at S/N > 8 in F160W with M_AB = -22.8 to -21.1 if at z = 10. Three of the six sources, including the two brightest, are in a single WFC3 pointing (~ 4 arcmin^2), suggestive of significant clustering, which is expected from bright galaxies at z ~ 10. However, the two brightest galaxies are too extended to be likely at z ~ 10, and one additional source is unresolved and possibly a brown dwarf. The remaining three candidates have m_AB ~ 26, and given the area and completeness of our search, our best estimate is a number density of sources that is marginally higher but consistent at 2{sigma} with searches in legacy fields. Our study highlights that z ~ 10 searches can yield a small number of candidates, making tailored follow-ups of HST pure-parallel observations viable and effective.
Until now, investigating the early stages of galaxy formation has been primarily the realm of theoretical modeling and computer simulations, which require many physical ingredients and are challenging to test observationally. However, the latest Hubble Space Telescope observations in the near infrared are shedding new light on the properties of galaxies within the first billion years after the Big Bang, including our recent discovery of the most distant proto-cluster of galaxies at redshift z~8. Here, I compare predictions from models of primordial and metal-enriched star formation during the dark ages with the latest Hubble observations of galaxies during the epoch of reionization. I focus in particular on the luminosity function and on galaxy clustering as measured from our Hubble Space Telescope Brightest of Reionizing Galaxies (BoRG) survey. BoRG has the largest area coverage to find luminous and rare z~8 sources that are among the first galaxies to have formed in the Universe.
We present the first results and design from the redshift z~9-10 Brightest of the Reionizing Galaxies {it Hubble Space Telescope} survey BoRG[z9-10], aimed at searching for intrinsically luminous unlensed galaxies during the first 700 Myr after the Big Bang. BoRG[z9-10] is the continuation of a multi-year pure-parallel near-IR and optical imaging campaign with the Wide Field Camera 3. The ongoing survey uses five filters, optimized for detecting the most distant objects and offering continuous wavelength coverage from {lambda}=0.35{mu}m to {lambda}=1.7{mu}m. We analyze the initial ~130 arcmin$^2$ of area over 28 independent lines of sight (~25% of the total planned) to search for z>7 galaxies using a combination of Lyman break and photometric redshift selections. From an effective comoving volume of (5-25) $times 10^5$ Mpc$^3$ for magnitudes brighter than $m_{AB}=26.5-24.0$ in the $H_{160}$-band respectively, we find five galaxy candidates at z~8.3-10 detected at high confidence (S/N>8), including a source at z~8.4 with mAB=24.5 (S/N~22), which, if confirmed, would be the brightest galaxy identified at such early times (z>8). In addition, BoRG[z9-10] data yield four galaxies with $7.3 lesssim z lesssim 8$. These new Lyman break galaxies with m$lesssim26.5$ are ideal targets for follow-up observations from ground and space based observatories to help investigate the complex interplay between dark matter growth, galaxy assembly, and reionization.
We present followup imaging of two bright (L > L*) galaxy candidates at z > 8 from the Brightest of Reionizing Galaxies (BoRG) survey with the F098M filter on HST/WFC3. The F098M filter provides an additional constraint on the flux blueward of the spectral break, and the observations are designed to discriminate between low- and high-z photometric redshift solutions for these galaxies. Our results confirm one galaxy, BoRG 0116+1425 747, as a highly probable z ~ 8 source, but reveal that BoRG 0116+1425 630 - previously the brightest known z > 8 candidate (mAB = 24.5) - is likely to be a z ~ 2 interloper. As this source was substantially brighter than any other z > 8 candidate, removing it from the sample has a significant impact on the derived UV luminosity function in this epoch. We show that while previous BoRG results favored a shallow power-law decline in the bright end of the luminosity function prior to reionization, there is now no evidence for departure from a Schechter function form and therefore no evidence for a difference in galaxy formation processes before and after reionization.
(Abridged) Our sensitive ($sigma_{rm n} = 572,{rm nJy,beam}^{-1}$), high-resolution (FWHM $theta_{1/2} = 220,{rm mas} approx2mathrm{,kpc~at~}zgtrsim1$) 10$,$GHz image covering a single Karl G.~Jansky Very Large Array (VLA) primary beam (FWHM $Theta_{1/2} = 4.25$) in the GOODS-N field contains 32 sources with $S_{rm p}gtrsim2,mu{rm Jy~beam}^{-1}$ and optical and/or near-infrared (OIR) counterparts. Most are about as large as the star-forming regions that power them. Their median FWHM major axis is $langletheta_{rm M} rangle=167pm32,{rm mas} approx 1.2pm0.28,{rm kpc}$ with rms scatter 91 was $approx$ 0.79 kpc. In units of the effective radius $r_{rm e}$ that encloses half their flux, these radio sizes are $langle r_{rm e}rangle = 69pm13{rm mas} approx pm114mathrm{,pc}$ and have rms scatter $38mathrm{,mas}approx324mathrm{,pc}$. These sizes are smaller than those measured at lower radio frequencies, but agree with dust emission sizes measured at mm/sub-mm wavelengths and extinction-corrected H$alpha$ sizes. We made a low-resolution ($theta_{1/2}=1.0$) image with $approx10times$ better brightness sensitivity to detect extended sources and measure matched-resolution spectral indices $alpha_{1.4}^{10}$. It contains 6 new sources with $S_{rm p}gtrsim3.9,mu{rm Jy~beam}^{-1}$ and OIR counterparts. The median redshift of all 38 sources is $1.24pm0.15$. The 19 sources with 1.4$,$GHz counterparts have median spectral index $-0.74pm0.10$ with rms scatter $0.35$. Including upper limits on $alpha$ for sources not detected at 1.4$,$GHz flattens the median to $gtrsim-0.61$, suggesting that the $mu$Jy radio sources at higher redshifts, and hence selected at higher rest-frame frequencies, may have flatter spectra. If the non-thermal spectral index is -0.85, the median thermal fraction at rest-frame frequency 20$,$GHz is $gtrsim$48%.
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