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

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 x 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.