tokens: et al. ( 2009 ). Similar to the conclusions from Bouwens et al. ( 2009 ), we find that most of the SFR density at z ≳ 3.5 appears to be directly observable in the rest-frame UV (see also Burgarella et al. 2013 ; Bourne et al. 2016 ; Dunlop et al. 2016 ). 5.3. Implications for the Lyman Continuum Photon Production Efficiencies Another consequence of our new ALMA results is for the interpretation of the prominent H α emission lines inferred in galaxies at z ∼ 4–5 based on the observed Spitzer /IRAC 3.6 μ m and 4.5 μ m fluxes (e.g., Schaerer de Barros 2009 ; Shim et al. 2011 ; Stark et al. 2013 ). As first noted by Shim et al. ( 2011 ), the total H α luminosities of galaxies at z ∼ 4–5 are in excess of what one might expect based on their luminosities in the rest-frame UV. The excess is conservatively as large as a factor of 2 using the general z ∼ 4–5 LBG selections (Smit et al. 2015 ) but was earlier reported to be a factor of 6 (Shim et al. 2011 ) using samples selected based on their emission line properties. One explanation for these high luminosities would be if the selected galaxies predominantly had young stellar ages (given that the Ly α emission was prominent in the spectroscopic sample considered by Shim et al. 2011 ) or if dust extinction preferentially had a larger impact on the observed UV-continuum fluxes than it did on the observed H α fluxes (Marmol-Queralto et al. 2016 ; Smit et al. 2015 ). Neither mechanism appears to provide a fully satisfactory explanation for the high H α fluxes in z ∼ 5 galaxies. Dust extinction falls off rapidly toward lower masses, but the H α EWs in these sources remain essentially unchanged (or possibly increase toward lower mass: see Smit et al. 2015 ). Similarly, young ages for the z ∼ 4–5 galaxy population cannot provide an explanation, because a high ratio of H α to UV continuuum is observed for typical star-forming galaxies at z ∼ 4–5 (Marmol-Queralto et al. 2016 ; Smit et al. 2015 ). 31 31 Shim et al. ( 2011 ) had speculated that young ages might explain the high ratios of H α to UV continuum observed in z ∼ 4–5 galaxies they studied that showed strong Ly α emission. However, it is now clear that such ratios apply both to sources that show Ly α in emission and to those that do not (Smit et al. 2015 ). This essentially forces us to conclude that the observed ratio of H α to UV-continuum luminosity must be intrinsic and that star-forming galaxies indeed have very high H α luminosities relative to their luminosities in the UV continuum, particularly in comparison with conventional stellar population models (i.e., Bruzual Charlot 2003 ). It has been speculated that such could be achieved due to binarity or rapid rotation in massive stars, allowing them to output large amounts of ionizing radiation tens of million of years after an initial burst of star formation (e.g., Yoon et al. 2006 ; Eldridge Stanway 2009 , 2012 ; Levesque et al. 2012 ; de Mink et al. 2013 ; Kewley et al. 2013 ; Leitherer et
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