RESEARCH
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Elevated ionizing photon production efficiency in faint high-equivalent-width Lyman-α emitters

M. V. Maseda, R. Bacon, D. Lam, J. Matthee, J. Brinchmann, J. Schaye, I. Labbe, K. B. Schmidt, L. A. Boogaard, R. J. Bouwens, S. Cantalupo, M. Franx, T. Hashimoto, H. Inami, H. Kusakabe, G. Mahler, T. Nanayakkara, J. Richard, L. Wisotzki

Abstract
While low-luminosity galaxies dominate number counts at all redshifts, their contribution to cosmic reionization is poorly understood due to a lack of knowledge of their physical properties. We isolate a sample of 35 z ≈ 4–5 continuum-faint Lyman-α emitters from deep VLT/MUSE spectroscopy and directly measure their H α emission using stacked Spitzer/IRAC Ch. 1 photometry. Based on Hubble Space Telescope imaging, we determine that the average UV continuum magnitude is fainter than −16 (≈ 0.01 L), implying a median Lyman-α equivalent width of 259 Å. By combining the H α measurement with the UV magnitude, we determine the ionizing photon production efficiency, ξion, a first for such faint galaxies. The measurement of log10ion [Hz erg−1]) = 26.28 (⁠+0.28−0.40⁠) is in excess of literature measurements of both continuum- and emission line-selected samples, implying a more efficient production of ionizing photons in these lower luminosity, Lyman-α-selected systems. We conclude that this elevated efficiency can be explained by stellar populations with metallicities between 4 × 10−4 and 0.008, with light-weighted ages less than 3 Myr.

Keywords
Galaxies: evolution; Galaxies: high-redshift; Galaxies: ISM; Astrophysics - Astrophysics of Galaxies

Monthly Notices of the Royal Astronomical Society
Volume 493, Issue 4, Page 5120
2020 April

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia