Issue |
A&A
Volume 675, July 2023
|
|
---|---|---|
Article Number | A121 | |
Number of page(s) | 11 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/202346069 | |
Published online | 07 July 2023 |
The ionizing photon production efficiency of bright z ∼ 2 − 5 galaxies
1
INAF – OAR, Via Frascati 33, 00078 Monte Porzio Catone, Roma, Italy
e-mail: marco.castellano@inaf.it
2
Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
3
Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
4
Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
5
University of Bologna – Department of Physics and Astronomy “Augusto Righi” (DIFA), Via Gobetti 93/2, 40129 Bologna, Italy
6
INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
7
Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
8
Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100 Copenhagen, Denmark
9
INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Via A. Corti 12, 20133 Milan, Italy
10
Instituto de Astrofisica, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago RM, Chile
11
Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
12
European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago de Chile, Chile
13
Department of Astronomy, University of Geneva, 51 Chemin Pegasi, 1290 Versoix, Switzerland
14
Sub-department of Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
15
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
16
IBEX Innovations, Sedgefield, Stockton-on-Tees TS21 3FF, UK
17
Department of Physics & Astronomy, University of California, Los Angeles, 430 Portola Plaza, Los Angeles, CA 90095, USA
Received:
2
February
2023
Accepted:
22
May
2023
Aims. We investigate the production efficiency of ionizing photons (ξion∗) of 1174 galaxies with secure redshift at z = 2 − 5 from the VANDELS survey to determine the relation between ionizing emission and physical properties of bright and massive sources.
Methods. We constrained ξion∗ and galaxy physical parameters by means of spectrophotometric fits performed with the BEAGLE code. The analysis exploits the multi-band photometry in the VANDELS fields and the measurement of UV rest-frame emission lines (CIII]λ1909, HeIIλ1640, and OIII]λ1666) from deep VIMOS spectra.
Results. We find no clear evolution of ξion∗ with redshift within the probed range. The ionizing efficiency slightly increases at fainter MUV and bluer UV slopes, but these trends are less evident when the analysis is restricted to a complete subsample at log(Mstar/M⊙) > 9.5. We find a significant trend of increasing ξion∗ with increasing EW(Lyα), with an average log(ξion∗/Hz erg−1) > 25 at EW > 50 Å and a higher ionizing efficiency for high-EW CIII]λ1909 and OIII]λ1666 emitters. The most significant correlations are found with respect to stellar mass, specific star formation rate (sSFR), and SFR surface density (ΣSFR). The relation between ξion∗ and sSFR increases monotonically from log(ξion∗/Hz erg−1)∼24.5 at log(sSFR) ∼ −9.5 yr−1 to ∼25.5 at log(sSFR) ∼ −7.5 yr−1. This relation has a low scatter and only a weak dependence on mass. The objects above the main sequence of star formation consistently have higher than average ξion∗. A clear increase in ξion∗ with ΣSFR is also found, with log(ξion∗/Hz erg−1) > 25 for objects at ΣSFR > 10 M⊙ yr−1 kpc−2.
Conclusions. Bright (MUV ≲ 20) and massive (log(Mstar/M⊙)≳9.5) galaxies at z = 2 − 5 have a moderate ionizing efficiency. However, the correlation between ξion∗ and sSFR, together with the known increase in the average sSFR with redshift at fixed stellar mass, suggests that similar galaxies in the epoch of reionization can be efficient sources of ionizing photons. The availability of sSFR and ΣSFR as proxies for ξion∗ can be fundamentally important in determining the role of galaxy populations at z ≳ 10 that were recently discovered by the James Webb Space Telescope in the onset of reionization.
Key words: galaxies: high-redshift / galaxies: evolution / dark ages, reionization, first stars
© ESO 2023
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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