The ionizing properties of two bright Ly α emitters in the Bremer Deep Field reionized bubble at z = 7

Abstract

Aims. We investigate the ionizing properties of the pair of bright Lyα emitting galaxies BDF521 and BDF2195 at z = 7.012 in order to constrain their contribution to the formation of the Bremer Deep Field (BDF) 'reionized bubble'in which they have been shown to reside. Methods. We obtain constraints on four UV emission lines (the CIVλ1548 doublet, HeIIλ1640, the OIII]λ1660 doublet, and the CIII]λ1909 doublet) from deep VLT X-shooter observations and compare them to those available for other high-redshift objects, and to models with mixed stellar and active galactic nucleus (AGN) emission. We use this spectroscopic information, together with the photometry available in the field, to constrain the physical properties of the two objects using the spectro-photometric fitting code BEAGLE. Results. We do not detect any significant emission at the expected position of the UV lines, with 3ρ upper limits of equivalent width (EW) ≤2;2-7 A rest-frame. We find that the two objects have a lower CIII] emission than expected on the basis of the correlation between the Lyα and CIII] EWs. The EW limits on CIV and HeII emission exclude pure AGN templates at ∼2-3ρ significance, and only models with a ≤2;40% AGN contribution are compatible with the observations. The two objects are found to be relatively young (∼20-30 Myrs) and metal-poor (≤0.3Z⊙), with stellar masses of a few 109M⊙. Their production rate of hydrogen ionizing photons per intrinsic UV luminosity is log(ξion∗/Hz erg-1) = 25.02-25.26, consistent with values typically found in high-redshift galaxies, but more than twice lower than values measured in z > 7 galaxies with strong CIII] and/or optical line emission (=25.6-25.7). Conclusions. The two BDF emitters show no evidence of higher-Than-Average ionizing capabilities and are not capable of reionizing their surroundings by their own means, under realistic assumptions of the escape fraction of ionizing photons. Therefore, a dominant contribution to the formation of the reionized bubble must have been provided by fainter companion galaxies. The capabilities of the James Webb Space Telescope will be needed for spectroscopic confirmation of these objects.