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

Charles C. Steidel, Milan Bogosavljević, Alice E. Shapley, Naveen A. Reddy, Gwen C. Rudie, Max Pettini, Ryan F. Trainor, Allison L. Strom

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish (US)
Article number123
JournalAstrophysical Journal
Issue number2
StatePublished - Dec 20 2018


  • cosmology: observations
  • galaxies: evolution
  • galaxies: high-redshift
  • intergalactic medium
  • ultraviolet: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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