Closing in on the sources of cosmic reionization: First results from the GLASS-JWST program

¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
e-mail: sara.mascia@inaf.it
² Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica, 1, 00133 Roma, Italy
³ Department of Physics and Astronomy, University of California, 430 Portola Plaza, Los Angeles, CA 90095, USA
⁴ Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Kashiwa 277-8583, Japan
⁵ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
⁶ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁷ INAF – IASF Milano, Via A. Corti 12, 20133 Milano, Italy
⁸ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
⁹ INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁰ School of Physics, University of Melbourne, Parkville, 3010 VIC, Australia
¹¹ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Stromlo, ACT 2611, Australia
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ Center for Astrophysical Sciences, Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
¹⁴ Cosmic Dawn Center (DAWN), Denmark
¹⁵ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 København N, Denmark
¹⁶ IPAC, California Institute of Technology, MC 314-6, 1200 E. California Boulevard, Pasadena, CA 91125, USA
¹⁷ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218 Hawthorn, VIC 3122, Australia
¹⁸ Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455, USA
¹⁹ School of Astronomy and Space Science, University of Chinese Academy of Sciences (UCAS), Beijing 100049, PR China
²⁰ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
²¹ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, PR China
²² School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
²³ Department of Mathematics and Physics, University of Ljubljana, Jadranska ulica 19, 1000 Ljubljana, Slovenia
²⁴ Department of Physics and Astronomy, University of California, Davis, 1 Shields Ave, Davis, CA 95616, USA

Received: 9 January 2023
Accepted: 16 February 2023

Abstract

The escape fraction of Lyman-continuum (LyC) photons (f_esc) is a key parameter for determining the sources of cosmic reionization at z ≥ 6. At these redshifts, owing to the opacity of the intergalactic medium, the LyC emission cannot be measured directly. However, LyC leakers during the epoch of reionization could be identified using indirect indicators that have been extensively tested at low and intermediate redshifts. These include a high [O III]/[O II] flux ratio, high star-formation surface density, and compact sizes. In this work, we present observations of 29 4.5 ≤ z ≤ 8 gravitationally lensed galaxies in the Abell 2744 cluster field. From a combined analysis of JWST-NIRSpec and NIRCam data, we accurately derived their physical and spectroscopic properties: our galaxies have low masses (log(M_⋆)∼8.5), blue UV spectral slopes (β ∼ −2.1), compact sizes (r_e ∼ 0.3 − 0.5 kpc), and high [O III]/[O II] flux ratios. We confirm that these properties are similar to those characterizing low-redshift LyC leakers. Indirectly inferring the fraction of escaping ionizing photons, we find that more than 80% of our galaxies have predicted f_esc values larger than 0.05, indicating that they would be considered leakers. The average predicted f_esc value of our sample is 0.12, suggesting that similar galaxies at z ≥ 6 have provided a substantial contribution to cosmic reionization.