2018ApJ...869..123S

We present results of a deep spectroscopic survey quantifying the statistics of the escape of ionizing radiation from star-forming galaxies at z ∼ 3. We measure the ratio of ionizing to non-ionizing UV flux density <f₉₀₀/f₁₅₀₀>_obs, where f₉₀₀ is the mean flux density evaluated over the range [880, 910] Å. We quantify the emergent ratio of ionizing to non-ionizing UV flux density by analyzing high signal-to-noise ratio composite spectra formed from subsamples with common observed properties and numbers sufficient to reduce the statistical uncertainty in the modeled IGM+CGM correction to obtain precise values of <f₉₀₀/f₁₅₀₀>_out, including a full-sample average <f₉₀₀/f₁₅₀₀>_out = 0.057 ± 0.006. We show that <f₉₀₀/f₁₅₀₀>_out increases monotonically with W_λ(Lyα), inducing an inverse correlation with UV luminosity as a by-product. We fit the composite spectra using stellar spectral synthesis together with models of the ISM in which a fraction f_c of the stellar continuum is covered by gas with column density N_HI. We show that the composite spectra simultaneously constrain the intrinsic properties of the stars (L₉₀₀/L₁₅₀₀)_int along with f_c, N_HI, E(B-V), and f_esc,abs, the absolute escape fraction of ionizing photons. We find a sample-averaged f_esc,abs = 0.09 ± 0.01, with subsamples falling along a linear relation <f_esc,abs>≃0.75[W_λ(Lyα)/110Å]. Using the far-UV luminosity function, the distribution function n(W(Lyα)), and the relationship between W_λ(Lyα) and <f₉₀₀/f₁₅₀₀>_out, we estimate the total ionizing emissivity of z ∼ 3 star-forming galaxies with M_uv <= -19.5, which exceeds the contribution of quasi-stellar objects by a factor of ∼3, and accounts for ∼50% of the total ε_LyC at z ∼ 3 estimated using indirect methods.