THE LYMAN-CONTINUUM PHOTON PRODUCTION EFFICIENCY ξ_ION OF z ∼ 4–5 GALAXIES FROM IRAC-BASED Hα MEASUREMENTS: IMPLICATIONS FOR THE ESCAPE FRACTION AND COSMIC REIONIZATION

Abstract

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 x ion and the escape fraction f 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 x ion for the  z 4 galaxy population. 25.34 0.02 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 x ion ʼs in reionization models, with a slight preference for higher x ion ʼs (by ∼0.1 dex) adopting the SMC dust law. High values of x ion (∼25.5-25.8 dex) are derived for the bluest galaxies (b < -2.3) in our samples, independent of dust law and consistent with results for a z=7.045 galaxy. Such elevated values of x ion would have important consequences, indicating that f esc cannot be in excess of 13% for standard assumptions about the faint-end cut-off to the LF and the clumping factor.

INFERRED Hα FLUX AS A STAR FORMATION RATE INDICATOR AT z ∼ 4–5: IMPLICATIONS FOR DUST PROPERTIES, BURSTINESS, AND THE z = 4–8 STAR FORMATION RATE FUNCTIONS

“…The first supposes thatz 4 galaxies can be described using a Meurer et al (1999) dust calibration with  = A A VV ,stars ,gas and that the UVbased SFR estimates need to be corrected by ∼0.1 dex to correct the measured, time-averaged values to the instantaneous ones. The second supposes thatz 4 galaxies can be described using an SMC dust calibration with  = A A VV ,stars ,gas and thatz 4 galaxies are more efficient at producing ionizing photons thangalaxies in standard stellar population models (and thus the a L H /SFR ratio is high;see Bouwens et al 2016b;andStark et al 2015andStark et al , 2016 for a discussion of how this may affect thegalaxies' role in driving the reionization of the universe).…”

“…In particular, comparing the low infrared excess as a function the UV-continuum slope in a sample of stacked galaxies below a stellar mass of  M 10 9. 75 , Bouwens et al (2016b) find that the slope of the IRX-β relationship has to be <1.22 and <0.97 forz 2 3 -and z 41 0 -galaxies, respectively (Figure 7). In conclusion, the first high-resolution FIR/submm observations taken by ALMA would indicate that the dust attenuation law is closer to that found for the Small Magellanic Cloud (SMC; e.g., Prevot et al 1984).…”

“…really much more efficient producers of ionizing photons, this would significantly impactgalaxies' possible role in reionizing the universe (Bouwens et al 2016b;Stark et al 2015Stark et al , 2016. We illustrate this in the right panel of Figure 9, where we show stellar population tracks for a constant star formation history for stars with zero rotation (at solar metallicity) and for stars that rotate at 40% of the break-up velocity (at =  ZZ 0.6 ) from the models described in Leitherer et al (2014).A t 100 Myr, the low-metallicity models that includestellar rotation are∼0.15 dex offsetin the Possible evolution in the SFR-a L H relationship therefore offers us a way to explain the observed high Hα fluxeseven in the scenario where the typical high-refshift UV-selected galaxy has low dust masses,as hasbeen argued by Schaerer et al (2015) and Capak et al (2015).…”

The photometric properties of galaxies in the early Universe

“…As discussed in more depth in Wilkins et al (2016b) the production efficiency is also sensitive to the choice of SPS model with models predicting higher values generally being favoured by recent observations (e.g. Stark et al 2015;Bouwens et al 2015d). The production efficiency will also be affected by the contribution of nebular (mostly continuum) emission, however this effect is relatively small, being at most ≈10 per cent when the escape fraction is zero.…”

“…Because direct observations of the LyC photon production efficiency are difficult to obtain (though see Stark et al 2015;Bouwens et al 2015d, for recent constraints at high-redshift) the LyC photon production rateṄ LyC is often inferred from the intrinsic rest-frame UV continuum luminosity L ν, fuv assuming a production efficiency ξ LyC ,…”