2015ApJ...800...97T

We compute the production rate of photons in the ionizing Lyman continua (LyC) of H I (λ ≤ 912 Å), He I (λ ≤ 504 Å), and He II (λ ≤ 228 Å) using recent stellar evolutionary tracks coupled to a grid of non-LTE, line-blanketed WM-basic model atmospheres. The median LyC production efficiency is Q_LyC= (6±2)x10⁶⁰ LyC photons per M_☉ of star formation (range [3.1-9.4]x10⁶⁰), corresponding to a revised calibration of 10^53.3±0.2 photons/s per M_☉/yr. Efficiencies in the helium continua are Q_HeI ≈ 10⁶⁰ photons M_☉^–1 and Q_HeII ≈ 10⁵⁶ photons M_☉^–1 at solar metallicity and larger at low metallicity. The critical star formation rate needed to maintain reionization against recombinations at z ~ 7 is {dot}ρ_SFR = (0.012 M_☉/yr/Mpc3)[(1+z)/ 8]³(C_H/3)(0.2/f_esc) for fiducial values of the intergalactic medium (IGM) clumping factor C_H~ 3 and LyC escape fraction f_esc~ 0.2. The boost in LyC production efficiency is an important ingredient, together with metallicity, C_H, and f_esc, in assessing whether IGM reionization was complete by z ~ 7. Monte Carlo sampled spectra of coeval starbursts during the first 5 Myr have intrinsic flux ratios of F(1500)/F(900) ~ 0.4-0.5 and F(912^–)/F(912⁺) ~ 0.4-0.7 in the far-UV (1500 Å), the LyC (900 Å), and at the Lyman edge (912 Å). These ratios can be used to calibrate the LyC escape fractions in starbursts.