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 ) ≥ 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.

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