The production of ionizing photons in UV-faint z ∼ 3–7 galaxies^⋆

¹ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
e-mail: gonzalo.prieto@nbi.ku.dk
³ Technische Universität München, Physik-Department, James-Franck Str. 1, 85748 Garching, Germany
⁴ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁵ Dipartimento di Fisica “E.R. Caianiello”, Università Degli Studi di Salerno, Via Giovanni Paolo II, 84084 Fisciano, (SA), Italy
⁶ INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
⁷ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
⁸ INAF – IASF Milano, Via A. Corti 12, 20133 Milano, Italy
⁹ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁰ Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
¹¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Rome, Italy
¹² School of Physics, University of Melbourne, Parkville, 3010 VIC, Australia
¹³ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Mount Stromlo Rd, Australia
¹⁴ IPAC, California Institute of Technology, MC 314-6, 1200 E. California Boulevard, Pasadena, CA 91125, USA
¹⁵ Department of Physics and Astronomy, University of California, Los Angeles, 430 Portola Plaza, Los Angeles, CA 90095, USA
¹⁶ Center for Astrophysical Sciences, Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
¹⁷ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 22 November 2022
Accepted: 23 January 2023

Abstract

Aims. The demographics of the production and escape of ionizing photons from UV-faint early galaxies is a key unknown that has hindered attempts to discover the primary drivers of reionization. With the advent of JWST, it is finally possible to observe the rest-frame optical nebular emission from individual sub-L^*z > 3 galaxies to measure the production rate of ionizing photons, ξ_ion.

Methods. Here we study a sample of 370 z ∼ 3 − 7 galaxies spanning −23 < M_UV < −15.5 (median M_UV ≈ −18) with deep multiband HST and JWST/NIRCam photometry that covers the rest-UV to the optical from the GLASS and UNCOVER JWST surveys. Our sample includes 102 galaxies with Lyman-alpha emission detected in MUSE spectroscopy. We used Hα fluxes inferred from NIRCam photometry to estimate the production rate of ionizing photons that do not escape these galaxies, ξ_ion(1 − f_esc).

Results. We find median log₁₀ξ_ion(1 − f_esc) = 25.33 ± 0.47, with a broad intrinsic scatter of 0.42 dex, which implies a broad range of galaxy properties and ages in our UV-faint sample. Galaxies detected with Lyman-alpha have ∼0.1 dex higher ξ_ion(1 − f_esc), which is explained by their higher Hα equivalent width distribution; this implies younger ages and higher specific star formation rates and, thus, more O/B stars. We find significant trends of increasing ξ_ion(1 − f_esc) with increasing Hα equivalent width, decreasing UV luminosity, and decreasing UV slope; this implies that the production of ionizing photons is enhanced in young galaxies with assumed low metallicities. We find no significant evidence for sources with very high ionizing escape fractions (f_esc > 0.5) in our sample based on their photometric properties, even amongst the Lyman-alpha-selected galaxies.

Conclusions. This work demonstrates that considering the full distribution of ξ_ion across galaxy properties is important for assessing the primary drivers of reionization.