Filamentary infall of cold gas and escape of Lyα and hydrogen ionizing radiation from an interacting high-redshift galaxy★

Abstract

We present observations of a peculiar Lyα-emitting galaxy at redshift z = 3.344, discovered in a deep, blind spectroscopic survey for faint Lyα emitters with the Magellan II telescope in the Hubble Ultra Deep Field. The galaxy exhibits complex Lyα emission, including an extended, asymmetric component that is partially suppressed by damped Lyα absorption, and two spatially elongated, narrow emission features. Archival Hubble Space Telescope Advanced Camera for Surveys imaging shows evidence for tidal disruption of the stellar component. This V = 27 galaxy appears to give us unprecedented insight into two fundamental stages in the formation of structure at high redshift: the inflow of gas into ordinary galaxies, and the escape of ionizing radiation into the intergalactic medium. Neutral hydrogen, falling in partly in the form of a narrow filament, appears to emit fluorescent Lyα photons induced by the stellar ionizing flux escaping from the disturbed galaxy. The in-falling material may represent primary cold accretion or an interaction-triggered inflow. The rate of ionizing photons required by the observed Lyα emission is consistent with the rate of photons produced by the observed stellar population, with roughly 50 per cent of ionizing photons escaping from the immediate galaxy and encountering the in-falling gas. We briefly discuss cooling radiation and large-scale shocks as additional sources for Lyα and ionizing radiation in high-redshift galaxies, but find that stellar radiation is likely to be the dominant source of ionizing photons for most faint galaxies. The observational properties of the galaxy lend support to a picture where galaxy interactions facilitate the escape of both Lyα and ionizing radiation. We argue that galaxies like the present object may be common at high redshift. This galaxy may therefore be a late example of an interacting population of dwarf galaxies that contribute significantly to the reionization of the universe.