THE LUMINOSITY FUNCTION ATz∼ 8 FROM 97Y-BAND DROPOUTS: INFERENCES ABOUT REIONIZATION

Abstract

We present the largest search to date for Y-band dropout galaxies (z ∼ 8 Lyman break galaxies, LBGs) based on 350 arcmin 2 of HST observations in the V-, Y-, J-and H-bands from the Brightest of Reionizing Galaxies (BoRG) survey. In addition to previously published data, the BoRG13 dataset presented here includes approximately 50 arcmin 2 of new data and deeper observations of two previous BoRG pointings, from which we present 9 new z ∼ 8 LBG candidates, bringing the total number of BoRG Y-band dropouts to 38 with 25.5 m J 27.6 (AB system). We introduce a new Bayesian formalism for estimating the galaxy luminosity function, which does not require binning (and thus smearing) of the data and includes a likelihood based on the formally correct binomial distribution as opposed to the often used approximate Poisson distribution. We demonstrate the utility of the new method on a sample of 97 Y-band dropouts that combines the bright BoRG galaxies with the fainter sources published in Bouwens et al. (2011) from the Hubble Ultradeep Field (HUDF) and Early Release Science (ERS) programs. We show that the z ∼ 8 luminosity function is well described by a Schechter function over its full dynamic range with a characteristic magnitude M = −20.15 +0.29 −0.38 , a faint-end slope of α = −1.87 +0.26 −0.26 , and a number density of log 10 φ [Mpc −3 ] = −3.24 +0.25 −0.24 . Integrated down to M = −17.7 this luminosity function yields a luminosity density, log 10 [erg/s/Hz/Mpc 3 ] =25.52 +0.05 −0.05 . Our luminosity function analysis is consistent with previously published determinations within 1σ. The error analysis suggests that uncertainties on the faint-end slope are still too large to draw firm conclusion about its evolution with redshift. We use our statistical framework to discuss the implication of our study for the physics of reionization. By assuming theoretically motivated priors on the clumping factor and the photon escape fraction we show that the UV luminosity density from galaxy samples down to M = −17.7 can ionize only 10-50% of the neutral hydrogen at z ∼ 8. Full reionization would require extending the luminosity function down to M = −15. The data are consistent with a substantial fraction of neutral hydrogen at z > 7, in agreement with recent suggestions based on deep spectroscopy of redshift 8 LBGs.

The Galaxy UV Luminosity Function Before the Epoch of Reionization

“…The best-fit parameters are in good agreement with observations (Schmidt et al 2014;Bowler et al 2015;Oesch et al 2010;Finkelstein et al 2015b;Bouwens et al 2015b) given the large degeneracies in Schechter function parameters. Encouragingly, we find the evolution of the derived Schechter parameters is in excellent agreement with the observed evolution (Bouwens et al 2015b;Bowler et al 2015): we find dα/dz ∼ −0.1, dM * /dz ∼ 0.1, and d log(Φ * )/dz ∼ −0.3 between z ∼ 4 and z ∼ 8.…”

“…The picture emerging from these observations is that the number density of galaxies decreases with increasing redshift, while the LF remains consistent with a Schechter (1976) form, Φ(L) = Φ * (L/L * ) α exp (−L/L * )/L * out to z ∼ 8, albeit the uncertainty and co-variance in the best fitting parameters grows at high redshift (Schmidt et al 2014;Bouwens et al 2015b). However, other studies find either an equally good fit with a double-power law at z ∼ 8 (Finkelstein et al 2015b), or a preference for the double power law at z ∼ 7 (Bowler et al 2014;Bowler et al 2015; but note that the z ∼ 7 investigations are based on ground, rather than space observations).…”

“…All of the parameters involved are difficult to estimate, and may evolve with redshift as reionization progresses and the IGM evolves (Furlanetto & Oh 2005;Shull et al 2012). In this work, we follow Schmidt et al (2014) and use a distribution of parameters. For two limiting magnitudes (M ab = −17, corresponding to currently observable galaxies, and M ab = −12, corresponding to atomic cooling halos) we assume the escape fraction is uniformly distributed between f esc = 0.1 − 0.3 (Ouchi et al 2009), and we use a uniform distribution between C = 1 − 6 for the clumping factor.…”

“…Transformational results on the rest-frame UV light from the epoch of reionization, when the universe was less than 0.8 Gyr old, have been made possible by large and dedicated community efforts that have identified a sample of more than 1000 galaxy candidates at z > 7, spanning a large range in luminosities. This progress is thanks to a variety of surveys, including ultradeep observations in blank fields with the HUDF09 and HUDF12 campaigns Dunlop et al 2013;Illingworth et al 2013), the use of cluster-scale lensing to probe intrinsically faint objects (e.g., Yue et al 2014;, the large area, panchromatic CANDELS survey (Grogin et al 2011;Koekemoer et al 2011), and wide-field randompointing surveys to identify more luminous but rarer obcmason@physics.ucsb.edu jects at the bright end of the LF (e.g., Trenti et al 2011;Bradley et al 2012;Schmidt et al 2014).…”

The photometric properties of galaxies in the early Universe

“…As of 2016 there are now ∼1000 candidate objects, encompassing a wide range of luminosities, identified (e.g. Bouwens et al 2010Bouwens et al , 2011Bouwens et al , 2014aBouwens et al ,b, 2015aOesch et al 2010Oesch et al , 2013Oesch et al , 2014Oesch et al , 2015Oesch et al , 2016Bunker et al 2010;Wilkins et al 2010Wilkins et al , 2011aFinkelstein et al 2010Finkelstein et al , 2012Finkelstein et al , 2015Lorenzoni et al 2011Lorenzoni et al , 2013McLure et al 2011McLure et al , 2013Ellis et al 2013;Laporte et al 2014Laporte et al , 2015Laporte et al , 2016Schmidt et al 2014;McLeod et al 2015;McLeod, McLure & Dunlop 2016;Atek et al 2015a, Atek et al2015b) with the first small samples now identified at z ∼ 10 and beyond.…”