The Lyman-continuum photon production efficiency ξ{sub ION} OF z ∼ 4–5 galaxies from IRAC-based Hα measurements: implications for the escape fraction and cosmic reionization
- Leiden Observatory, Leiden University, NL-2300 RA Leiden (Netherlands)
- Yale Center for Astronomy and Astrophysics, Yale University, New Haven, CT 06520 (United States)
- Institute of Space Sciences and Astronomy, University of Malta, Msida MSD 2080 (Malta)
Galaxies represent one of the preferred candidate sources to drive the reionization of the universe. Even as gains are made in mapping the galaxy UV luminosity density to z>6, significant uncertainties remain regarding the conversion to the implied ionizing emissivity. The relevant unknowns are the Lyman-continuum (LyC) photon production efficiency ξ{sub ion} and the escape fraction f {sub esc}. As we show here, the first of these unknowns is directly measurable in z = 4–5 galaxies based on the impact the Hα line has on the observed IRAC fluxes. By computing a LyC photon production rate from the implied Hα luminosities for a broad selection of z = 4–5 galaxies and comparing this against the dust-corrected UV-continuum luminosities, we provide the first-ever direct estimates of the LyC photon production efficiency ξ{sub ion} for the z⩾4 galaxy population. We find log{sub 10} ξ{sub ion}/[Hz erg{sup −1}] to have a mean value of 25.27{sub −0.03}{sup +0.03} and 25.34{sub −0.02}{sup +0.02} for sub-L* z = 4–5 galaxies adopting Calzetti and SMC dust laws, respectively. Reassuringly, both derived values are consistent with the standard assumed ξ{sub ion}'s in reionization models, with a slight preference for higher ξ{sub ion}'s (by ∼0.1 dex) adopting the SMC dust law. High values of ξ{sub ion} (∼25.5–25.8 dex) are derived for the bluest galaxies (β<−2.3) in our samples, independent of dust law and consistent with results for a z = 7.045 galaxy. Such elevated values of ξ{sub ion} would have important consequences, indicating that f {sub esc} cannot be in excess of 13% for standard assumptions about the faint-end cut-off to the LF and the clumping factor.
- OSTI ID:
- 22868488
- Journal Information:
- Astrophysical Journal, Vol. 831, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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