The Keck Lyman Continuum Spectroscopic Survey (KLCS): The Emergent Ionizing Spectrum of Galaxies at z ∼ 3

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 , where f ₉₀₀ is the mean flux density evaluated over the range [880, 910] A. 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 , including a full-sample average = 0.057 ± 0.006. We show that increases monotonically with , 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 . We show that the composite spectra simultaneously constrain the intrinsic properties of the stars (L ₉₀₀/L ₁₅₀₀)_int along with f _c, 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 . Using the far-UV luminosity function, the distribution function n(W(Lyα)), and the relationship between and , 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 _LyC at z ∼ 3 estimated using indirect methods.