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We present observations of Q1549-C25, an ~L* star-forming galaxy at z=3.15for which Lyman-continuum (LyC) radiation is significantly detected in deepKeck/LRIS spectroscopy. We find no evidence for contamination from alower-redshift interloper close to the line of sight in the highsignal-to-noise spectrum of Q1549-C25. Furthermore, the morphology of Q1549-C25in V_606, J_125, and H_160 Hubble Space Telescope (HST) imaging reveals thatthe object consists of a single, isolated component within 1". In combination,these data indicate Q1549-C25 as a clean spectroscopic detection of LyCradiation, only the second such object discovered to date at z~3. We model thespectral energy distribution (SED) of Q1549-C25, finding evidence fornegligible dust extinction, an age (assuming continuous star formation) of ~1Gyr, and a stellar mass of M_*=7.9x10^9 M_sun. Although it is not possible toderive strong constraints on the absolute escape fraction of LyC emission,f_esc(LyC), from a single object, we use simulations of intergalactic andcircumgalactic absorption to infer f_esc(LyC)>=0.51 at 95% confidence. Thecombination of deep Keck/LRIS spectroscopy and HST imaging is required toassemble a larger sample of objects like Q1549-C25, and obtain robustconstraints on the average f_esc(LyC) at z~3 and beyond.

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