MUSE Spectroscopic Identifications of Ultra-faint Emission Line Galaxies with M UV ∼ −15

Using an ultra-deep blind survey with the MUSE integral field spectrograph on the European Southern Observatory Very Large Telescope, we obtain spectroscopic redshifts to a depth never before explored: galaxies with observed magnitudes m AB 30-32. Specifically, we detect objects via Lyα emission at 2.9<z<6.7 without individual continuum counterparts in areas covered by the deepest optical/near-infrared imaging taken by the Hubble Space Telescope, the Hubble Ultra Deep Field. In total, we find 102 such objects in 9 square arcminutes at these redshifts. Detailed stacking analyses confirm the Lyα emission as well as the 1216 A breaks and faint ultraviolet continua (M UV ∼−15). This makes them the faintest spectroscopically confirmed objects at these redshifts, similar to the sources believed to reionize the universe. A simple model for the expected fraction of detected/undetected Lyα emitters as a function of luminosity is consistent with these objects being the high-equivalent width tail of the normal Lyα-emitter population at these redshifts.