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

Abstract

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 < {f}₉₀₀{/f}₁₅₀₀{> }_obs}, where f ₉₀₀ 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 < {f}₉₀₀{/f}₁₅₀₀{> }_out}, including a full-sample average < {f}₉₀₀{/f}₁₅₀₀{> }_out} = 0.057 ± 0.006. We show that < {f}₉₀₀{/f}₁₅₀₀{> }_out} increases monotonically with {W}_λ ({Ly}α ), 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}_{{H}{{I}}}. We show that the composite spectra simultaneously constrain the intrinsic properties of the stars (L ₉₀₀/L ₁₅₀₀)_int along with f _c , {N}_{{H}{{I}}}, 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 ({Ly}α )/110 \mathringA ]. Using the far-UV luminosity function, the distribution function n(W(Lyα)), and the relationship between {W}_λ ({Ly}α ) and < {f}₉₀₀{/f}₁₅₀₀{> }_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 ɛ _LyC at z ∼ 3 estimated using indirect methods.

Based on data obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W.M. Keck Foundation.