Abstract
ABSTRACT We study the extent to which very bright ( ) Lyman-break-selected galaxies at redshifts display detectable Lyα emission. To explore this issue, we obtained follow-up optical spectroscopy of 9 galaxies from a parent sample of 24 galaxy candidates selected from the 1.65 deg2 COSMOS-UltraVISTA and SXDS-UDS survey fields using the latest near-infrared public survey data, and new ultra-deep Subaru z′-band imaging (which we also present and describe in this paper). Our spectroscopy yielded only one possible detection of Lyα at z = 7.168 with a rest-frame equivalent width . The relative weakness of this line, combined with our failure to detect Lyα emission from the other spectroscopic targets, allows us to place a new upper limit on the prevalence of strong Lyα emission at these redshifts. For conservative calculation and to facilitate comparison with previous studies at lower redshifts, we derive a 1σ upper limit on the fraction of UV-bright galaxies at that display , which we estimate to be . This result may indicate a weak trend where the fraction of strong Lyα emitters ceases to rise, and possibly falls between z ≃ 6 and . Our results also leave open the possibility that strong Lyα may still be more prevalent in the brightest galaxies in the reionization era than their fainter counterparts. A larger spectroscopic sample of galaxies is required to derive a more reliable constraint on the neutral hydrogen fraction at based on the Lyα fraction in the bright galaxies.
Highlights
Understanding the epoch of cosmological reionization has been a key challenge in modern observational cosmology
This paper aims to study the Lyα fraction based on Lyman-break galaxies (LBGs) candidates at z = 7 in the two common fields: COSMOS/ UltraVISTA and Subaru/XMM-Newton Deep Survey (SXDS)/UKIDSS-UDS
If the active galactic nucleus (AGN) feedback is weak in the bright galaxies at z = 7, the destruction of the H II regions associated with star formation would be reduced, which supports higher luminosities of Lyα emission
Summary
Understanding the epoch of cosmological reionization has been a key challenge in modern observational cosmology. The Gunn–Peterson test with spectra of high-redshift (high-z) quasars is an observational tool for detecting the reionization epoch, and suggests that the intergalactic medium (IGM) was reionized at z > 6 (Fan et al 2006). There is observational evidence that neutral hydrogen still remains in the IGM of the universe at z ~ 6 (e.g., studies with quasars, Becker et al 2015, and with gamma-ray bursts, Totani et al 2014; Hartoog et al 2015). Observational studies indicated that the luminosity functions (LFs) of the Lyman-break galaxies (LBGs) show a monotonic decline in number density from z = 3 to z = 7 (Bowler et al 2012, 2014, 2015; McLure et al 2013; Bouwens et al 2015)
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