Observational aspects of galactic accretion at redshift 3.3

We investigate the origin of extragalactic continuum emission and its relation to the stellar population of a recently discovered peculiar z = 3.344 Ly alpha emitter. Based on an analysis of the broad-band colours and morphology, we find further support for the idea that the underlying galaxy is being fed by a large-scale (L >= 35 kpc) accretion stream. ArchivalHST images show small-scale (similar to 5 kpc) tentacular filaments converging near a hotspot of star formation, possibly fueled by gas falling in along the filaments. The spectral energy distribution of the tentacles is broadly compatible with either (1) non-ionizing rest-frame far-UV continuum emission from stars formed in a 60 million-year-old starburst; (2) nebular two-photon continuum radiation, arising from collisional excitation cooling; or (3) a recombination spectrum emitted by hydrogen fluorescing in response to ionizing radiation escaping from the galaxy. The latter possibility simultaneously accounts for the presence of asymmetric Ly alpha emission from the large-scale gaseous filament, and the nebular continuum in the smaller scale tentacles as caused by the escape of ionizing radiation from the galaxy. Possible astrophysical explanations for the nature of the tentacles include: a galactic wind powered by the starburst; infalling gas during cold accretion, or tails of interstellar medium dragged out of the galaxy by satellite haloes that have plunged through the main halo. The possibility of detecting extragalactic two-photon continuum emission in space-based, broad-band images suggests a tool for studying the gaseous environment of high-redshift galaxies at much greater spatial detail than possible with Lya or other resonance line emission.