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

¹ INAF – OAR, Via Frascati 33, 00078 Monte Porzio Catone (Roma), Italy
e-mail: marco.castellano@inaf.it
² INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
³ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
⁴ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁵ Departamento de Astronomía, Universidad de La Serena, Av. Juan Cisternas 1200 Norte, La Serena, Chile
⁶ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Raúl Bitrán 1305, La Serena, Chile
⁷ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁸ Sorbonne Université, CNRS, UMR7095, Institut d’Astrophysique de Paris, 75014 Paris, France
⁹ Astrophysics, The Denys Wilkinson Building, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹⁰ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹¹ NSF’s NOIRLab, Tucson, AZ 85719, USA

Received: 17 February 2022
Accepted: 6 May 2022

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 3σ 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 Lyα and CIII] EWs. The EW limits on CIV and HeII emission exclude pure AGN templates at ∼2 − 3σ 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 10⁹ 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.