Are we seeing accretion flows in a 250 kpc Lyα halo at z=3

Using MUSE on the ESO-VLT, we obtained a four-hour exposure of the z = 3.12 radio galaxy MRC0316-257. We detect features down to similar to 10(-19) erg s(-1) cm 2 arcsec(-2), with the highest surface brightness regions reaching more than a factor of 100 higher. We find Ly ff emission out to similar to 250 kpc in projection from the active galactic nucleus (AGN). The emission shows arc-like morphologies arising at 150 250 kpc from the nucleus in projection, with the connected filamentary structures reaching down into the circumnuclear region. The most distant arc is offset by similar to 700 km s(-1) relative to circumnuclear HeII lambda 1640 emission, which we assume to be at the systemic velocity. As we probe emission closer to the nucleus, the filamentary emission narrows in projection on the sky, the relative velocity decreases to similar to 250 km s(-1), and the line full-width at half maximum ranges from similar to 300 700 km s(-1). From UV line ratios, the emission on scales of 10s of kpc from the nucleus along a wide angle in the direction of the radio jets is clearly excited by the radio jets and ionizing radiation of the AGN. Assuming ionization equilibrium, the more extended emission outside of the axis of the jet direction would require 100% or more illumination to explain the observed surface brightness. High-speed (greater than or similar to 300 km s(-1)) shocks into rare gas would provide sufficiently high surface brightness. We discuss the possibility that the arcs of Ly alpha emission represent accretion shocks and the filamentary emission represents gas flows into the halo, and compare our results with gas accretion simulations.