The ionizing properties of two bright Ly<i>α</i> emitters in the Bremer Deep Field reionized bubble at <i>z</i> = 7
M. Castellano, L. Pentericci, G. Cupani, Emma Curtis-Lake, E. Vanzella, R. Amorín, Davide Belfiori, Antonello Calabrò, Stefano Carniani, S. Charlot, Jacopo Chevallard, Pratika Dayal, Mark Dickinson, Andrea Ferrara, A. Fontana, E. Giallongo, Anne Hutter, E. Merlin, D. Paris, P. Santini
Abstract
Aims. We investigate the ionizing properties of the pair of bright Ly α 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 CIV λ 1548 doublet, HeII λ 1640, the OIII] λ 1660 doublet, and the CIII] λ 1909 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 3 σ upper limits of equivalent width (EW) ≲2–7 Å rest-frame. We find that the two objects have a lower CIII] emission than expected on the basis of the correlation between the Ly α and CIII] EWs. The EW limits on CIV and HeII emission exclude pure AGN templates at ∼2 − 3 σ significance, and only models with a ≲40% AGN contribution are compatible with the observations. The two objects are found to be relatively young (∼20–30 Myrs) and metal-poor (≲0.3 Z ⊙ ), with stellar masses of a few 10 9 M ⊙ . Their production rate of hydrogen ionizing photons per intrinsic UV luminosity is log( ξ ion * /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.