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 〈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(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_(H1). We show that the composite spectra simultaneously constrain the intrinsic properties of the stars (L_(900)/L_(1500))_(int) along with f_c , N_(H1), 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)/110 Å]. Using the far-UV luminosity function, the distribution function n(W(Lyα)), and the relationship between W_ λ(Lyα) 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 ϵ_(LyC) at z ~ 3 estimated using indirect methods.
© 2018 The American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2018 May 16; revised 2018 October 18; accepted 2018 October 27; published 2018 December 18. 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. This work has been supported in part by the US National Science Foundation through grants AST-0606912, AST-0908805, and AST-1313472 (M.B., C.C.S., G.C.R., R.F.T., A.L.S.). C.C.S. acknowledges additional support from the John D. and Catherine T. MacArthur Foundation, the David and Lucile Packard Foundation, and the JPL/Caltech President's and Director's program. We thank J. J. Eldridge and Elizabeth Stanway for their continued work developing the BPASS SPS models and for many illuminating conversations. We are grateful to the dedicated staff of the W.M. Keck Observatory who keep the instruments and telescopes running effectively. We wish to extend thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests.
Published - Steidel_2018_ApJ_869_123.pdf
Submitted - 1805.06071.pdf