Q1549-C25: A Clean Source of Lyman-Continuum Emission at z = 3.15

Abstract

We present observations of Q1549-C25, an ∼ {L}^* star-forming galaxy at z = 3.15 for which Lyman-continuum (LyC) radiation is significantly detected in deep Keck/LRIS spectroscopy. We find no evidence of contamination from a lower-redshift interloper close to the line of sight in the high signal-to-noise spectrum of Q1549-C25. Furthermore, the morphology of Q1549-C25 in V ₆₀₆, J ₁₂₅, and H ₁₆₀ Hubble Space Telescope (HST) imaging reveals that the object consists of a single, isolated component within 1″. In combination, these data indicate Q1549-C25 as a clean spectroscopic detection of LyC radiation, only the second such object discovered to date at z ∼ 3. We model the spectral energy distribution of Q1549-C25, finding evidence of negligible dust extinction, an age (assuming continuous star formation) of ∼1 Gyr, and a stellar mass of {M}_* =7.9× {10}⁹ {M}_⊙ . Although it is not possible to derive strong constraints on the absolute escape fraction of LyC emission, f _esc(LyC), from a single object, we use simulations of intergalactic and circumgalactic absorption to infer {f}_{{esc}}({{LyC}})≥slant 0.51 at 95% confidence. The combination of deep Keck/LRIS spectroscopy and Hubble Space Telescope imaging is required to assemble a larger sample of objects like Q1549-C25, and obtain robust constraints on the average f _esc(LyC) at z ∼ 3 and beyond.

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 the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W.M. Keck Foundation.